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DiscoverCircuits.com -- Hobby Corner
Last Updated on: Wednesday, October 18, 2017 05:04 AM


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I have compiled a number of circuits, projects and ideas for you, which I think will fall into a general “hobby” category.  See Introduction to the Hobby Corner for the philosophy behind creating the Hobby Corner.    Please feel free to make suggestions.

0.5v to 6v Voltage Converter  -  Conventional silicon transistors just can't operate at voltages less than about 0.7v.  Old germanium transistors could be used, but those are hard to find these days and most are rather large in size.  Some new n-channel MOSFET devices with very . . . Hobby Circuit designed by David Johnson P.E.-May, 2007

0VAC to 280VAC Variable Isolated AC Voltage  -  I designed and built this circuit about 25 years ago.  It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC.  It provides complete isolation from the power line.  It uses two 1:1 isolation transformers that are wired in parallel . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000

1 Second One Shot Circuit  -  This timer circuit draws only 3ua of current, and can be powered by DC supplies ranging from 3v to 15v.  The circuit produces accurate pulses whose time can easily be adjusted, based on the R1 x C1 time constant.  The time should not change,  . . . Hobby Circuit designed by David A. Johnson P.E.-September, 2010

10 Turn Pot Forms Frequency Readout  -  Let’s say you are designing a signal generator for a test fixture.  To adjust the frequency you would like to use a ten turn pot with a nice dial mechanism attached as a frequency display.  However, a display range of 0 to 9.99 would be impractical since . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2008

100Khz Low Power Light Receiver  -  By starving a high speed logic inverter for current, this circuit can produce a sensitive 100KHz light receiver circuit, which is immune to ambient light, but only drawing 100 microamps from a 3 volt supply . . . Hobby Circuit designed by Dave Johnson P.E.-April, 2005

12 Volt Lite/Dark Lamp Switch  -  Often, for certain low voltage lighting systems; you would like to turn off the lights during the bright light of the day.  Most commercial day/night switches are designed for AC lighting.  The circuit below was designed for a 12v DC system.  But, it could be modified for other voltage as well.  It uses an inexpensive phototransistor as the light detector.   . . . Hobby Circuit designed by David Johnson P.E.-June, 2008

120 VAC / 240 VAC Indicator  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

125KHZ Medium Power Oscillator  -  This circuit is similar to MEDiUM POWER 125KHZ OSCiLLATOR but adds even more inverters in parallel to deliver yet more power.  The values shown are for 125KHz . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

125khz Oscillator with Medium Power  -  This circuit adds even more inverters in parallel to deliver yet more power.  The values shown are for 125KHz . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

125Khz Ultra Pure Sine Wave Signal Source  -  For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed.  The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01% . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

12kv High Voltage Generator  -  The hobby circuit below uses an unusual method to generate about 12, 000 volts with about 5uA of current.  Two SCRs form two pulse generator circuits.  The two SCRs discharge a 0.047uF a 400v capacitor through a xenon lamp trigger coil at 120 times a second . . . Hobby Circuit designed by David A. Johnson P.E.-March, 1999

12v Battery Voltage Monitor-Expanded Scale   -  The circuit below expands the battery voltage monitoring scale for a 12v battery so a more accurate voltage reading can be made.  The numbers printed onto the face of a standard 0v to 5v meter are changed so the scale ranges from 10v to 15v . . . Hobby Circuit designed by Dave Johnson P.E.-October, 2005

12v Brush Motor Speed Controller  -  in many applications, you would like to hold the speed of a motor constant, even as variations in the power supply voltage or mechanical load try to change its speed.  In other applications, the average current to the motor needs to be limited, so the . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2007

12v DC Motor with Logic Controlled Automatic Brake  -  There are times when you would like to control a DC motor using a simple +5v logic swing signal.  The circuit below performs this feat and also includes an automatic motor braking circuit.  When the input logic signal swings from 0v to +5v, the n-channel . . . Hobby Circuit designed by David Johnson P.E.-February, 2010

12v_To_24v_Solar_Battery_Charger  -  A while back I received an email from a Discover Circuits visitor.  He wanted to know how he could use a single 12v solar panel to charge a 24v lead acid battery bank.  He said he used the 24v battery to run an emergency 120vac power system.   By using the higher 24 volts instead of 12v, his DC to AC inverter could pump out more peak power, for starting things like well pumps and window air conditioners. . . . Hobby Circuit by David Johnson P.E.-July, 2017

12v-24v AC & DC Day/Night Switch  -  The circuit uses a bridge rectifier circuit to convert AC into DC.  A 12v zener diode limits the operating voltage of the circuit to 12v.  A tiny Schmitt trigger inverter and a phototransistor detects the ambient light intensity.  If there is sufficient light, the inverter disables a push-pull oscillator circuit . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2012

14v LED AuxiliaryLamp with a 60 Watt Lamp  -  This circuit controls power to a standard 60 watt lamp plus generates an auxiliary 14v DC supply which can power an automotive style 12 LED array . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

175Khz Inductive Pulse Transmitter  -  This circuit is discussed in more detail in the Experimenters Journal.  The transmitter’s six-inch diameter coil launches powerful magnetic 175KHz ring pulses that can be detected by the circuit below . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

1Khz to Over 70Mhz Light Receiver  -  This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response.  It is designed around a PIN photo diode that is packaged for use with plastic optical fibers.  It can be used as an optical fiber receiver.  By using the open end of the optical fiber it can "sniff" out any modulated light signals . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

1-Nimh Cell powers White LED  -  There are a lot of white LED driver circuits floating around.  A popular circuit called the “Joule Thief” can drive one white LED from a single 1.2v or 1.5v battery cell.  Most of these circuits use one or two transistors to form a voltage boost circuit . . . Hobby Circuit designed by David Johnson P.E.-September, 2009

1us LED Pulse Driver Voltage to Frequency Converter  -  This circuit receives the signal from the above amplifier and launches powerful 1uS infrared light pulses from a low cost LED that are frequency modulated by the audio information.  The 10KHz center frequency of the pulse stream is low enough so a standard . . . Hobby Circuit designed by David A. Johnson P.E.-March, 1999

1us Light Pulse Discriminator  -  This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit.  The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter.  The circuit is designed to process a pulse frequency . . . Hobby Circuit designed by Dave Johnson P.E.-March, 1999

1us Light Pulse Receiver  -  This circuit is designed to detect very weak light pulses lasting 1uS.  It uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity.  A post voltage amplifier is included with a gain of about X20 . . . Hobby Circuit designed by Dave Johnson P.E.-March, 1999

1W White LED Night Light  -  As many of you know, I have a pet peeve with poorly made LED night lights.  Often, the light from the LED quickly fades, so within months, the light is useless.  I have posted several versions of modified night lights using higher quality components.  This circuit is yet another version, which produces much more light than those other designs.   . . . Hobby Circuit designed by Dave Johnson P.E.-November, 2008

2 Pushbutton Motor Controller  -  Two small pushbutton switches, a few diodes and two relays form a method to control on/off power to a brush motor as well as the motor direction.  The circuit was originally designed for a motorized lifting platform . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

2-12v Battery Isolator Circuit with a LTC4412  -  Has just announced a neat little chip (LTC4412).  It has been designed to be used in conjunction with an external P-channel power FET, to form an ideal diode function with a very low 0.05v voltage drop.  The chip monitors the voltage on eit . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2006

24 Volt Day Night Switch  -  Many low voltage outdoor lighting systems use 24vac to power the devices.  The low voltage is much safer to use outside than standard 120vac or 240vac power systems.  The circuit below is designed to turn off power to outdoor lights during daylight and turn them back on at night . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2010

24 Volts AC to 24 Volts DC Power Supply  -  The idea for the circuit below came from a company needing to power several 24v DC LED assemblies from an existing 24vac supply.  They did not need a rock solid and regulated 24 volts but a voltage which hovered around that voltage.  Running the AC through . . . Hobby Circuit designed by David Johnson P.E.-February, 2011

24V DC Powered Beeper with 4 Separate Inputs  -  24vac is a popular voltage.  It is often used in low voltage lighting systems and industrial controls.  It is much less dangerous than 120vac/240vac line voltages.  This hobby circuit below is one method to efficiently power a cluster of 7 white LEDs from 24vac.   . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2009

24v NiMH Battery Charger using a 12v Solar Panel   -  A while back I received an email from a guy in the UK.  He wanted to use a small solar panel, designed to charge 12v lead acid batteries, but use it to charge a 24v battery pack.  His 24v battery pack used 20 AA size NiMH cells.  He went on to explain that his 20 cell NiMH battery pack was used very infrequently . . . Hobby Circuit designed by David Johnson P.E.-October, 2011

24-Volt AC Powered Isolated +5V Supply  -  24vac is a very popular power supply in many industrial systems.    it is often used in systems that involve water.    it is also popular with many inside and outside lighting systems.    The lower voltage means the danger of human electrocution is much less.     . . . Hobby Circuit designed by David Johnson P.E.-July, 2009

2Mhz Broad Band Optical Fiber Receiver  -  if you need more sensitivity than the above circuit this circuit provides about ten times more gain.  It too is designed around an inexpensive plastic optical fiber detector . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

3 Minute LED Closet Light  -  The circuit below is powered by three 1.5v alkaline AA cells.  With a finger tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned on. The lights remain on for about 3 minutes, then will turn off.  The circuit’s standby current . . . Hobby Circuit designed by David Johnson P.E.-November, 2010

30 LED Night Light  -  This Hobby Circuit uses a capacitor to limit the current through a series-parallel string of 30 LEDs.15 LEDs are wired in series in one direction and 15 more in the other direction.  The capacitor limits the average current to about 20ma.  The 100 ohm metal oxide resistor acts as a fuse.   . . . Hobby Circuit designed by David A. Johnson P.E.-January, 1900

3Khz Filter Audio Amplifier  -  This circuit is the audio amp section for a complete optical transmitter.  The circuit amplifies and filters the voice audio signals from an electret microphone.  The circuit is described in more detail in . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

3mw Laser Test-20Mhz VCSEL  -  This circuit takes advantage of some new vertical cavity surface emitting lasers (VCSEL) that don’t require light output control circuits.  The circuit shows how to drive the device from a single high speed CMOS IC  . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

3v Powered Super Bright LED Flasher  -  I have posted several kinds of LED flashers over the years.   This is yet one more flasher, designed to flash any high power LED of any color.  It hits the LED with a 25ms 250ma pulse once every 2 seconds.   This works out to an average current about 3ma.  It . . . Hobby Circuit designed by David Johnson P.E.-April, 2009

3v Universal LED Flasher  -  This low power LED flasher circuit draws about 100ua from any DC power supply ranging from 3 volts to 24 volts.  The 1.0Hz 10ms LED pulsed current is held to a constant 10ma . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

4 to 20ma Current Loop Senses Temperature  -  I designed a circuit similar to this one years ago to accurately measure the air temperature inside a building 1000s of feet from a control room.  The circuit uses a very robust current loop method.  It uses a highly accurate semiconductor temperature sensor . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

4_D-Cell_LED_Lantern_Modified  -  While shopping at a sporting goods store one day, I spotted a compact LED lantern.  It looked like the kind of lantern I could modify. The lantern LEDs were grouped into three sections of 7 LEDs each, oriented into 120 degree angles.  The LEDs were standard T 1-3/4 epoxy types.  The lantern had a three mode switch which selected between off, full power and half power modes.  At the half power mode, only half of the 21 LEDs were illuminated.  At full power all LEDs were illuminated. . . . Hobby Circuit by David Johnson P.E.-September, 2017

40Khz Burst Laser Diode Driver  -  Some laser tag or simulated combat games can use this circuit to send short bursts of modulated laser light at the opponent's vest, equipped with a matching light receiver.  The circuit operates from three 1.5v cells (4.5v) that should provide enough energy . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

40KHz Light Detector with Sunlight Immunity  -  This hobby circuit below was designed to turn on an external 12v relay, whenever it detects light from a nearby LED light source, modulated at 40KHz to 50KHz.  This circuit was originally designed to operate from a fast moving vehicle . . . Hobby Circuit designed by David Johnson P.E.-February, 2007

40Khz Precision Triangle Generator  -  This circuit generates a precision 40KHz triangle waveform . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

40Khz Ultrasound Transducer Driver uses Medium Power  -  This crystal controlled circuit drives a 40KHz piezo transducer with a 30v peak to peak signal . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

5+V Classic Linear Supply using 6.3VAC Transformer  -  A classic method for producing a regulated +5v DC supply is shown below.  This circuit consists of an iron core transformer, a bridge rectifier, a filter capacitor and a voltage regulator.  Many people are tempted to use a very popular 6.3v transformer . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2009

50ua Current Drives Solid State Relay Requires Only  -  Most solid state relays require at least 5ma and often more input control current, to fully turn on the device.  But such current levels often force battery powered circuits to use excessively large batteries.  The relay hobby circuit shown below demands only 50uA of input current . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

5v Line-Powered Isolated Supply  -  Often a circuit requires a 5v DC supply to power some logic circuits.  The conventional method is to use an AC wall adapter.  But, many systems, which bring AC power onto a circuit board, need a small AC to DC power supply right on the circuit board.  The . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2009

5v Touch Switch  -  This simple circuit uses a single IC to form a nice touch switch circuit.    A single transistor forms the remote active switch sensor.    Multiple switches can be wired in parallel.    The switch circuit can be located about 500 feet from the control circuit.   . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

60hz Clock Generator is Line Powered  -  This circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit. The optoisolator used provides 5000 volts of isolation between the power line and the logic side of the circuit . . . Hobby Circuit designed by David Johnson P.E.-February, 1999

A Current Transformer used to Harvest Energy  -  Energy harvesting is all the rage these days.  With some modern electronics, information from low power sensors can be sent to a distant data collection point using a low power RF transmitter.  To power these remote sensors, various energy sources can be . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2010

AC Current Indicator Light  -  This circuit could be wired into a 120vac power line, which feeds power to any load, ranging from 40 watts to 250 watts.  It will turn on a LED light whenever current is being drawn by a load.  It is especially useful for remote lights, where you may not . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

AC Current Monitor  -  This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.  The transformer contains a single turn primary with a low 0.001-ohm resistance.  It can easily handle 30 . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

AC Indicator-Self Powered  -  Often there is a need to monitor when AC current flows into a load.  The circuit below works well for this application.  It uses an off-the-shelf torrid type AC-1020 current transformer from Triad, available from Mouser.  A wire carrying the AC current is . . . Hobby Circuit designed by David Johnson P.E.-December, 2007

AC Isolated Current Monitor  -  The hobby circuit below shows how a small current transformer from Magnetek can be used to monitor AC current to a load.  The AC voltage developed at the secondary of the transformer can be scaled so it can be read by any digital multimeter.  The transformer . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

AC Line Powered LED Flasher  -  Some of the latest LEDs are amazing.  They can produce a surprising amount of light with very little current.  In the light flasher application shown below I take advantage of a nice surface mounted LED from King Bright.  This flasher circuit draws only about 3ua of average current from the AC power line to flash this green LED.  This flasher might be used as a power indicator light . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2008

AC Line Powered LED Pilot Light  -  This simple circuit can be used to light a LED indicator lamp, powered from the 120vac line.  I have included component values for 240vac lines as well . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

AC Line Under/Over Voltage Alarm  -  Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  The circuit below sounds an alarm whenever the line voltage is higher or lower than normal.  I set the alarm limits at about +-15% from standard levels . . . Hobby Circuit designed by Dave Johnson P.E.-March, 2009

AC Peak Detector that is Precise  -  This unique circuit uses a very inexpensive voltage comparator to form a peak detector.  The DC voltage produced tracks the positive peak of the input signal.  It works from about ten millivolts to about 10 volts peak to peak.  The maximum frequency is about . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

AC to DC (240v to 5v)Power Supply  -  This is simple way to power some 5v logic from a 240vac source.  If a 120vac power adapter is used, the circuit will also work for 120vac power lines . . . Hobby Circuit designed by David A. Johnson P.E.-February, 2002

AC to DC 18V Power Supply  -  This is a classic linear power supply which produces a regulated 18v, rated at about 1 amp . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

AC to DC Power Supply requires Low Power  -  Last month I showed how to use a small common mode choke as a fully isolated transformer and a 130v sidac to generate about 100mw of DC power from the AC line.  This second circuit still uses the same inexpensive tiny off-the-shelf 5mH common mode filter coil with a split bobbin as a high frequency transformer but uses two SCRs instead of a sidac to discharge a capacitor . . . Hobby Circuit designed by David Johnson P.E.-November, 2011

AC to DC Power Supply, Non-Isolated Off-Line   -  This compact efficient circuit can provide up to 100ma of a regulated 5 volts from an AC power source ranging from 20vac to 120vac . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

AC/DC relay from Charge Coupled MOSFET  -  This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current.  The differential oscillator supplies voltage to the gates of the two FETs with good isolation will drawing only 1.5ma of current . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Accurate Low Capacitance Meter  -  The circuit measures the unknown capacitance by applying a clean 20KHz 5v peak to peak triangle signal to the capacitor.  The current routed through the capacitance is sent to a current to voltage converter.  The voltage that emerges is therefore a 20KHz square wave signal, whose amplitude is proportional to the capacitance.  The signal’s peak amplitude is rectified and filtered to produce a DC voltage.   . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2004

Active High Pass Filters  -  This is a collection of inverting and non-inverting active high pass filter circuits.  I included one, two three, four and six pole filter circuits.  You can change the component value ratios shown to achieve any frequency cut-off you may need . . . Hobby Circuit designed by David A. Johnson P.E.-February, 2002

Alarm Discriminator Battery Monitor of the Electric Field Disturbance Monitor  -  This schematic is the alarm discriminator plus the batterry monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Hobby Circuit designed by Dave Johnson P.E.-March, 2002

Alarm for a Chain Link Fence  -  Long stretches of steel fence can be monitored using the simple circuit below.  An insulated wire is woven through the links in the fence.  Where the fence begins, the circuit is connected to an earth ground and the security wire.  At the end of the fence,  . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2009

Alarm Sounds At Freezing Temperatures  -  This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).  A 9v battery powers the hobby circuit.  The average 9ua current is so low that the battery should last for many years . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Alarm Sounds when Phone Line is Cut  -  Burglars, rapists and other criminals will often cut a phone line before they break into a home.  The cut phone line will prevent many alarm systems from alerting the police.  The battery powered circuit below will sound an alarm when the unit . . . Hobby Circuit designed by David Johnson P.E.-October, 2005

Alarm sounds when Power Cord is Unplugged  -  Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries.  However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries.  So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2000

Ambient Light Dectector with Compensation  -  These circuits were taken from a few application notes on infrared remote control devices.  They use a current compensation method to separate the modulated light pulses from ambient light.  They appear to have limited bandwidth and may only work at the 30K . . . Hobby Circuit designed by David A. Johnson P.E.-February, 2002

Appliance Unplugged Alarm  -  I received a call from an administrator at a large prison.  He told me that he was having problems with inmates who keep unplugging refrigerators, vending machines and microwave ovens in a commons area.  He wanted a battery-powered device, which could be mounted to the outside of the power cord of an appliance.   . . . Hobby Circuit designed by Dave Johnson P.E.-April, 2010

Astable Multivibrator with Very Low Power  -  This classic circuit draws only 200 nanoamps from a 1.5v supply . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Astable Ultra Low Power Multivibrator  -  Taking advantage of some new voltage comparators, this circuit can produce a nice square wave signal while drawing only 1.6 micro amps.  With the inclusion of a diode, the circuit can also produce short pulses instead of a square wave signal . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Automatic DC Motor Brake  -  in many DC motor powered systems you would like the motor to quickly come to a full stop.   This circuit provides an automatic electronic braking action to any DC motor ranging from 6v to 24v up to 1 amp of current.  Some of the component values could be changed . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Automatic Timeout included with this On/Off Flip/flop Circuit  -  This circuit is ideal when a device needs to be turned on and off with a single pushbutton switch, but also needs to turn itself off after some period of time.  With the components shown, the output will stay on for only about 20 seconds . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Auxiliary 12v Automotive Style LED Light Makes Table Lamp  -  This circuit provides about one watt of non-isolated DC power for an automotive type 12v LED array lamp in addition to a standard incandescent table lamp . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Band Drummer’s Microphone Switch  -  I received an email from guy who plays drums for a small band.  He wanted a simple way to turn on and off his microphone with just a tap of his drum stick onto a small box.  The circuit below performs this task with the aid of a piezo device as a shock sensor and a small dual coil latching relay with two sets of contacts.  The contacts can be wired any number of ways to a microphone circuit . . . Hobby Circuit designed by David A. Johnson P.E.-October, 2011

Basement Remote Doorbell  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Battery (12v) Load Cutout Circuit  -  Lead-acid batteries should not be discharged below a certain point if they are to last.  Also, you don’t want certain 12v powered devices, which might be plugged into a car’s cigarette lighter outlet, to drain the car battery down to a point where the car . . . Hobby Circuit designed by Dave Johnson P.E.-August, 2009

Battery Powered Plus-Minus 15v Supply  -  Many classic operational amplifier circuits call for a split positive and negative 15 volt supply.  The circuit below makes the job of generating such a supply much easier by using an off-the-shelf DC to DC converter, powered from a cheap 6v battery made . . . Hobby Circuit designed by David Johnson P.E.-August, 2008

Battery-Powered LED Flashing Advertising #2  -  This circuit is similar to flashing LED advertising badge #1.    it uses a CD4013 dual D Flip/Flop IC .    The 74HCT74 IC in #81 does not always work.    As in #81, a single lithium battery will provide months of continuous LED flashing.   . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

Beeper indicates Check Engine  -  When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel.  You could drive for many miles before noticing the light.  To provide an audible indication of trouble, the hobby . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Bi-Directional Charge Coupled Powers MOSFET Relay  -  The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply.  Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Bidirectional DC Current Indicator  -  This circuit is handy for battery powered systems.  The circuit is self-powered using a single 3v lithium coin cell.  The current draw is so low that the battery should last for years.  The hobby circuit flashes a red LED when DC current is drawn from the battery and flashes a green LED whenever current flows into the battery.  Two “probe” input wires connect across a section of cable, which acts as a shunt resistor.  A voltage of only 5mv in either direction will trigger a LED indication . . . Hobby Circuit designed by David Johnson P.E.-March, 2011

Boost Infrared TV/VCR Remote Signal  -  This circuit will boost the signal from any infrared TV or VCR remote, extending the range by a factor of 3X.   . . . Hobby Circuit designed by Dave Johnson P.E.-October, 2005

Broad Band 50Mhz Optical Fiber Receiver Version A  -  if the above circuit it still too slow, you can try this circuit.  What it lacks in sensitivity it makes up for in speed.  The circuit attaches a plastic fiber optic PIN photodiode assembly to a small box containing a small 3v battery and a standard . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Broken Solar Panels  -  As illustrated below, one way to salvage a large number of broken solar cells is to wire them in parallel, not in the more conventional series connection.  Broken cells of various sizes and shapes are grouped together into panels of about one quarter to on . . . Hobby Circuit designed by David Johnson P.E.-January, 2012

Brownout Voltage Tester  -  I used this circuit years ago to test AC line powered devices under 95vac line voltage conditions.    it has a rating of 250 watts.   . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Call-for-Service Beeper  -  This device functions like a lobby desk service request bell.  A short beep is sounded whenever the large pushbutton on top of the device is pressed.  Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed . . . Hobby Circuit designed by David Johnson P.E.-September, 2005

Camera Shutter Time Measurement  -  A while back I received an email from someone who needed a simple way to measure the speed of a photographic camera.  I thought about this and suggested the method below.  It relies on an oscilloscope and a cheap photodiode to measure the speed . . . Hobby Circuit designed by David Johnson P.E.-August, 2011

Candle Power  -  A single lit candle in conjunction with an inexpensive thermopile voltage generator provides enough power to charge a small battery when used with a small low voltage DC to DC converter . . . Hobby Circuit designed by David A. Johnson P.E.-February, 2014

Capacitance Proximity Switch-3 volts  -  This circuit was designed to provide a touch activated switch function without an external power supply.  It draws so little power that a single 3v battery will operate the circuit for many years . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

Capacitance Touch Activated Momentary Switch-5v  -  This circuit is discussed in more detail in the section on Capacitance Proximity Switch Technology.  The circuit is powered from a standard +5v supply.  It has both a source and sink output that change state whenever a metal button connected to the circuit . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000

Capacitance Touch Switch  -  A single 1.5v silver oxide button cell powers this complete touch activated switch circuit for 5 years.  It features both a normally open and a normally closed set of solid state switch thermals.  It also has an adjustable sensitivity, which can be set . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2002

Capacitance Type Liquid Level Monitor  -  This circuit was originally designed to monitor the level of liquid natural gas in a tank but it can be used to also measure almost any liquid.  Two custom insulated metal tubes form a capacitor plate.  The capacitance between the two tubes increases as the level of the liquid rises . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Capacitance-Based Touch Switch  -  This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Car Ignition 12v Switch  -  A Discover Circuits visitor needed a way to redirect +12v DC in his car to some auxiliary loads whenever the car’s engine was running.  Many cars have cigarette lighter outlet jacks, which are either live all the time or become live when the car’s engine is running . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2012

Cell Phone Charger  -  it seems that as cell phones become more sophisticated, their operating times become shorter and shorter.  If you are on the go all the time and have difficulty finding AC or DC outlets to charge your phone, you might consider this simple scheme . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Cell Phone Solar Powered Charger  -  The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas instruments . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2011

Characteristics of CMOS Logic Inverter Amplifiers  -  This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . Hobby Circuit designed by David Johnson P.E.-February, 2006

Charge Pump Battery Charger-+5 Volts  -  The circuit below will trickle charge a four cell pack of AA or AAA NiMH batteries.  The circuit draws current from the +5v available a USB connection and pumps about 70ma of current into the battery.  This should be enough current to fully charge a pack of . . . Hobby Circuit designed by David Johnson P.E.-February, 2010

Cheap Close-Up Camera Lens  -  I often use my digital camera to take photos of some electronic projects.  At times, I need to get in close to capture some details.  But, without some additional optics, my camera just could not maintain focus, as I received closer to an object.  Being a cheap . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2012

Cheap Light Power Meter  -  Some time ago I was experimenting with the use of some low leakage photovoltaic solar cells, to harvest some energy from the diffuse lighting inside a room.  The plan was to use a small array of solar cells (the type often used on solar powered calculators)  . . . Hobby Circuit designed by Dave Johnson P.E.-January, 1900

Cheap Low Power 3v 2hz Oscillator  -  This is yet another method to generate low frequency output pulses.  The circuit uses an inexpensive under-voltage monitor IC and a couple of cheap MOSFETs.  It could easily be modified to produce pulse frequencies from about one cycle per minute to a maxim . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Circuit Eliminates 9v Battery  -  if you use 9v battery powered devices for long periods of time, you may get tired of constantly changing batteries.  The circuit below can be wired into any 9v battery powered device, drawing power from an external AC to DC power adapter.  I designed the c . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2010

Circuit Indicates Battery Charge Current  -  This circuit was inspired by a company needing a DC current indicator light which was small enough to be molded into a medium size 28v rechargeable battery assembly.  They specified that they wanted the circuit to turn on the LED indicator light with 500ma or more of battery charge current . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2011

Circuit Indicates Time to Dust  -  I have used the circuit below for many different applications, which require ultra low power drain.  It uses a single CA4013 flip/flop.  One section forms a pulse generator.  The generator produces 100uS pulses at a rate of about 1 per second . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2006

Circuit Monitors 120 Power Line  -  This hobby electronic circuit produces a short beep once each minute.  It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on.  It is simple enough to be packaged inside a small plastic box . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Circuit Monitors 12v Lamp Current   -  This circuit monitors DC current and turns on a LED indictor light whenever DC current flows into a 12v lamp . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Circuit Monitors Power Loss  -  For some medical equipment it is important for an operator to know when power is lost to the machine.  The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts . . . Hobby Circuit designed by David A. Johnson P.E.-March, 1999

Circuit tests the self-resonance frequency  -  This circuit tests the self-resonance frequency is of an inductor.  During the test, only a few picofarads of parallel capacitance are added to the inductor.  When connected to an oscilloscope and a signal generator, you can measure resonant frequencies . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Classic Linear 5v Supply using 6.3VAC Transformer  -  There are times when you wish to control a pulse width modulation (PWM) signal with a DC voltage, ranging from 0v to 5v.  This voltage is often created by a computer interface.  The circuit below performs this voltage to PWM conversion.  PWM signals are often . . . Hobby Circuit designed by David Johnson P.E.-October, 2010

Clip-on AC Current Indicator Circuit  -  Several times over the last few years I have received calls from people looking for a simple circuit that would tell them when a sump pump or water well pump motor was running.  These people told me that the electrical circuits that turn on and off . . . Hobby Circuit designed by David A. Johnson P.E.-November, 2004

CMOS IC Flashes LED  -  Judging from the numerous references I have received, this electronic circuit, hobby circuit seems to be rather popular.  Many published circuits which flash LEDs need 3 or more volts.  This electronic circuit uses only a single inexpensive C-MOS IC and . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

CMOS Inverter forms a Resonant LD Oscillator   -  This circuit uses a single CMOS inverter to form a series resonant LC oscillator.  The values shown set the oscillation at about 125KHz but the other frequencies are possible by changing the main LC values . . . Hobby Circuit designed by Dave Johnson P.E.-March, 2002

CMOS Inverters Form 125Khz Oscillator  -  This circuit inverts the LC components so the inductor is grounded.  Two inverters are needed to produce the needed oscillation.  Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

CMOS Oscillator, Draws Only 0.5ua  -  if truly low power oscillators interest you, this circuit draws a mere 2 microwatts (500nA) from a 6v battery.  It uses a very inexpensive C-MOS IC to produce a frequency of 2Hz.  However, by changing the component values you can push it to 300Hz . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Colman_Lantern3_Modified  -  While shopping at a sporting goods store one day, I spotted a compact LED lantern.  It looked like the kind of lantern I could modify. The lantern LEDs were grouped into three sections of 7 LEDs each, oriented into 120 degree angles.  The LEDs were standard T 1-3/4 epoxy types.  The lantern had a three mode switch which selected between off, full power and half power modes.  At the half power mode, only half of the 21 LEDs were illuminated.  At full power all LEDs were illuminated. . . . Hobby Circuit by David Johnson P.E.-September, 2017

Computer Controlled Power Relay  -  Often there is a need to turn on and off a 120vac device using a low power +5v logic signal from a computer.  The circuit below shows how this could be done.  The circuit uses a power relay with a 120vac coil and 10 amp contacts.  A small solid state relay turns on and off the relay.  The solid state relay provides 5KV isolation from the computer . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2011

Computer, Long Period, Watch Dog Timer  -  This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer. Should the computer fail to send a pulse, the output changes state. The time can easily be set from seconds to hours. . . Hobby Circuit designed Dave Johnson P.E.

Constant Current Drives LEDs  -  LEDs are current driven devices.  The light they produce is proportional to the current flowing though them.  One way to maintain a fairly steady light output is to keep the current through the LED fixed, even as the supply voltage may change.  A classic . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2010

Constant Current LED Driver  -  Bob Pease from National Semiconductor came up with this circuit and I have used it many times.  It maintains a constant current through one or more LEDs.  A single resistor controls the desired current.   . . . Hobby Circuit designed by David A. Johnson P.E.-November, 2004

Control both Power & Direction of a DC Motor  -  TTL type Q and inverted Q inputs control a classic H-bridge circuit, rated at 50 volts and about 10 amps.  The circuit can control power and direction of a DC motor . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Control Variable LED Intensity  -  The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.  A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity of the LEDs from 0% to 100%.  The beauty of the circuit is that when the . . . Hobby Circuit designed by Dave Johnson P.E.-September, 2009

Controlled Up/Down Ramping Light  -  There are times when you want a lamp to gradually change from dark to light and from light to dark, with each push of a pushbutton switch.  The circuit below performs this control function using a flip/flop and a classic pulse width modulation or PWM . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2009

Controller for a 9v Powered Xenon Photoflash  -  This 9v battery powered circuit is designed for remote control flash needs.  A charge control circuit turns off the high voltage generator when the photoflash capacitor is fully charged.  A neon lamp is included to indicate when the system is ready to flash . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Convert AC to DC signal with is Full Wave Rectifier  -  This is a classic circuit that can accurately convert an AC signal to DC.  Using a LF412 high speed dual op amp, this circuit will rectify any signal up to about 100KHz with a minimum amplitude of about 50mv peak to peak.  At 40KHz the input signal can be . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Converts a Square Wave input signal into a Voltage  -  This is a test circuit converts a square wave input signal into a voltage.  But, the voltage produced is proportional to the time between edges (period) of the signal, not the frequency.  The range is from 100uS to to 10mS, which produces a voltage from 100mV to 10 volts.  Other scale factors are also possible . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Cooling Fan Thermostat (12v)  -  This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level.  Both the high and low temperatures are both adjustable . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2011

Count Cars with Laser  -  This is an illustration how a laser could be used to count traffic and measure the speed of each car passing through the sensor area . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

CT Hour Meter  -  Many systems require routine maintenance based on machine operation time.  The circuit below is a simple way to turn on a hour meter whenever AC power is supplied to the machine.  An inexpensive snap-on current transformer from Magnetics inc, is used to . . . Hobby Circuit designed by David Johnson P.E.-February, 2010

Current Source is Computer Controlled  -  Often in industrial control systems a constant current source is needed, which is controlled by a computer and referenced to circuit ground.  The circuit below converts a zero to 5v signal from a computer’s analog output into a current, with a full scale of 100ma.  The circuit shown requires a 9v DC supply . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2008

CW & CCW Alternating Motor Driver  -  A while back a company needed a motor controller for a custom door latching system.    The system used a small brush motor with a speed reduction gear to both latch and unlatch the door.    When pulsed for about 250ms in the clockwise direction, the motor would unlatch the door . . . Hobby Circuit designed by Dave Johnson P.E.-September, 2010

Dark Activated 12v Power Supply Switch  -  Using an inexpensive phototransistor, this circuit switches 12v to 24v power to a load, whenever the sun goes down . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

DC Brushless Motor Monitor  -  The circuit below was requested by a person needing a way to keep track of the number of hours a 24v DC brushless motor was in operation.  The circuit taps into a bit of the power going into the motor using a bridge rectifier and filter capacitor.  The voltage produced feeds the input of a small solid state relay.  The output of the relay routes AC power to an hour meter, every time the brushless motor is turned on.  This same circuit could be used with other DC motors with some simple modifications . . . Hobby Circuit designed by Dave Johnson P.E.-April, 2005

Demo of a Wave Power Generator System  -  An inexpensive stepper motor is used as a voltage generator in this wave generator demo.   . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2003

Detect Clear Fluid  -  This circuit detects when a clear fluid is inside a piece of clear plastic tubing.  It can be used for fluid level control and monitoring . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Detect RF Signal at 2.45GHz  -  Many cordless telephones, computer networks and cell phones use a RF transmitter lefted on the 2.45GHz band.  Using a microwave diode detector and 100uA meter, this passive circuit will display the strength this signal.  The drawing shows how a bent piece of wire can be used as the antenna.   . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Detect Short Xenon Lamp Flashes  -  This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect the short light flashes from a xenon lamp.  The coil makes the circuit immune to normal room lights.  Its 10mv sensitivity can detect light flashes from a range of over 100 feet.   . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2002

Detector for Human Footsteps  -  This circuit uses a surplus geophone to detect human footsteps from 100 feet away.  It was designed for monitoring the Mexico/USA border.  This preamp part of the system draws about 50 micro amps from a 12 volt supply . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Detector/Switch for Audio Signal  -  For some audio systems it is desirable to have an automatic switch circuit turn on other devices, such as an amplifier, if an audio signal is detected on one line.  The hobby circuit below will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal.  The switch could be used to turn on a relay, routing the signal to where it is needed.  A single 3v lithium cell will power the electronic circuit for about 5 years . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Detects Low Voltage 120VAC Power Line   -  This hobby circuit will turn on a beeper when the line voltage drops below 100 volt AC . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Detects Xenon Lamp Flash  -  This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp.  When connected to a flip/flop it can serve as an on on/off Controller . . . Hobby Circuit designed by David Johnson P.E.-December, 2004

Detects Xenon Lamp Flash  -  This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp.  When connected to a flip/flop it can serve as an on on/off Controller . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Digital Counter using Pedometer  -  There are many occasions when you may want to count something electronically.  Perhaps it is car traffic on a street or items moving down an assembly line.  It might be the number of times a machine is activated or maybe you want to count the number of . . . Hobby Circuit designed by David Johnson P.E.

Digital noise generator  -  When you need to test an audio circuit with broadband noise, this circuit works great.  It uses just three inexpensive C-MOS ICs  that generate a series of output pulses whose widths vary randomly.  I included a level control pot . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2000

Divide a TTL Clock Signal by 1.5  -  Two inexpensive ICs  divide a TTL clock signal by 1.5.  By following the circuit with another flip/flop, you could also generate a divide by three function . . . Hobby Circuit designed by David Johnson P.E.-July, 2000

Doorbell heard in Basement  -  if you can't hear your doorbell when you are in your basement try this circuit.  This circuit takes advantage of the 24vac power source located near the furnace.  Using a simple current transformer technique, the circuit sounds a beeper whenever the . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Drive a LED with an Intensity Ramping Mode  -  The circuit below was designed to drive a LED with an intensity ramping mode.  Two 555 timers generate two different triangle waveforms.  The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal.  The two signals are fed to a . . . Hobby Circuit designed by David Johnson P.E.-December, 2007

DVD Recorder TV Camera Make Cheap Data Logger  -  Sometimes recording the results of a test requires as much ingenuity as designing the product.  Consider a situation which requires the collection of many different variables over a long period of time . . . Hobby Circuit designed by David A. Johnson P.E.-April, 2005

Earthquake Alarm Circuit  -  Geophones are dandy motion sensors.  They are metal cans containing a powerful magnet, suspended with springs inside a coil of wire.  The magnet functions as a floating mass.  The slightest vertical motion of the device causes the coil to move relative to . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2005

ELC signal between 200Mhz-400Mhz  -  if you need a clean emitter coupled logic (ECL) type signal between 200MHz and 400MHz this circuit works fine.  It uses four voltage-controlled capacitors to change the frequency . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2002

Exciter Circuit-12v Touch Switch  -  This circuit generates a 20KHz pseudo sine wave signal that can power approximately 50 remote touch activated switch circuits.  This hobby circuit can support a cable length of approximately 2500 feet.  A typical remote switch circuit is also shown as . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

FETs & Logic Devices Divides Supply Voltage on Charge Pump Efficiently   -  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2000

Finger Tap Piezo Relay  -  This circuit uses a flat piezo wafer, glued inside a plastic box, as a finger tap sensor.  With each tap of a finger to the box holding the wafer, the circuit turns on and off AC or DC power to an external device.  The circuit is powered by a 9v 9v battery . . . Hobby Circuit designed by Dave Johnson P.E.-November, 2010

Fireflies_In_Jar_Solar_Light  -  When I was a kid, growing up in Illinois, I remember catching fireflies and putting them into a jar.  I loved watching them flashing away at night.  Later, I would let them out to live another day. . . . Hobby Circuit by David Johnson P.E.-September, 2017

Flasher_&_Beeper_for_Well_Pump  -  Several years ago I installed a box on my water well pump head.  At the well head, the solar powered box flashed a very bright LED light and chirped a very load siren, whenever the pump motor was in operation.  After many years of service, the box finally became corroded from rain water and died.  So, I thought it was time to redesign the circuit and build up a replacement. . . . Hobby Circuit by David Johnson P.E.-June, 2017

Flashing LED Advertising Badge #1  -  I have seen numerous flashing light badges at trade shows and conventions.  They are often handed out as gifts to promote some business.  The devices often use inefficient circuits, which cause the battery power source to be quickly depleted . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

Flashing Light to simulate Car Alarm  -  Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Flexible Outputs from Square Wave Driver  -  This circuit can produce an output signal ranging from DC to 100KHz.  It can source a voltage ranging from 1v to 30v.  It can sink a voltage ranging from zero volts to –30v.  It can drive up to 200ma of current and can even be switched to a floating tristate . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000

Fluorescent Lamp Intensity Modulator  -  This hobby circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light.  This hobby circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz.  Each pulse launches about 10 watts of visible light.  The lamp starting method is a bit crude but the circuit does work.   . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Fluorescent Lamp Night Light Needs Only 1 Watt  -  if you are tired of replacing small night light lamps, try this circuit.  The line powered circuit uses a long life cold cathode fluorescent lamp.  A simple charge pump technique runs the lamp without any transformer . . . Hobby Circuit designed by David Johnson P.E.-March, 1999

Form Push-Pull Driver using N-Ch & P-Ch Transistors   -  This circuit can produce high speed output signals with fast rise and full times.  The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts.  The output current is only limited by the rating of . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Front End Power Supply Circuits of the Electric Filed Disturbance Monitor  -  This schematic is the power supply and front-end sections of my field monitor, "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

Front-End Circuit-Modulated Light Receiver  -  This circuit uses a unique cascode amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network.  It is very stable and very sensitive.  This hobby circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm.  I included a simple JFET post-amplifier with a gain of about 20.   . . . Hobby Circuit designed by David Johnson P.E.-March, 1999

Fully Isolated 50/60hz Sync Generator  -  This circuit will produce a single pulse at the zero voltage cross points of the power line voltage.  An opto-coupler provides a very safe 5KV isolation . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Gated 125Khz Oscillator  -  This circuit is uses a NAND gate as an inverter.  The gate allows the oscillator to be gated on and off.  Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

Gated Crystal Oscillator with Very Low Power  -  The circuit gates the output of a continuously operating 32KHz crystal oscillator to the input of a C-MOS buffer when clock pulses are needed.  The technique gets around the problem of a slow starting crystal oscillator by keeping the oscillator going and switching on a transistor power stage only as needed . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Generate 0.5v Negative Supply  -  A while back someone needed a simple way to generate a negative 0.5v supply from a conventional transformer type AC to DC power supply.  The circuit below uses a couple schottky diodes and a filter capacitor to perform the feat . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2011

Generate Short Delayed Pulse  -  This circuit generates a short 10ms pulse 15 minutes after a “start” pushbutton switch is activated . . . Hobby Circuit designed by David Johnson P.E.-December, 2004

Ground Resistance Tester  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1ohms.    This resistance must be tested with actual line current in excess or 10 amps.    The circuit shown uses a couple of LEDs to indicat . . . Hobby Circuit designed by Dave Johnson P.E.-April, 2001

H Bridge Motor Controller uses Low Power  -  This circuit uses a few logic gates & a 74AC240 to control power to a small gear motor . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Harvest Earthworms with this Electrical Device  -  When I was a kid my engineer uncle built a circuit similar to the one below.  My cousin and I used it to force earth worms out of the ground to be used for fishing.  It worked like a champ.  Please be careful since there is high voltage at the probe tips . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

High Ambient Light Immunity Light Detector   -  The circuit below was designed to turn on an external 12v relay, whenever it detects light from a nearby LED light source, modulated at 40KHz to 50KHz.   This circuit was originally designed to operate from a fast moving vehicle . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

High DC Current Indicator  -  A while back, someone requested a simple method to indicate when 20 to 50 amps of DC current was flowing through a wire.  The circuit below works well and is about as simple as you can get.  A sensitive reed relay . . . Hobby Circuit designed by David A. Johnson P.E.-April, 2005

High Impedance JFET Buffers  -  I have used these circuits many times.  They are great when you need a low gain AC signal amplifier with a very high input impedance.  It is good to beyond 50MHz . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

High Speed Light Detector from FET Input  -  This circuit is yet another design that converts current from a PIN photo diode to a voltage.  It has a bandwidth that extends beyond 50MHz . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

High Temperature Thermocouple activates Relay  -  Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

High/Low Voltage Tester on AC Power Line   -  if you wish to test a line-powered device under both a 15% high and a 15% low voltage condition, you can use the circuit below.  The circuit uses an 18v transformer with a 3 amp rating.  A double pole, double throw toggle switch then switches in the . . . Hobby Circuit designed by David Johnson P.E.-March, 2010

Highly Stable Lower Power Oscillator  -  This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a 3 volt battery.  It is highly stable under varying temperature and supply voltages . . . Hobby Circuit designed by David Johnson P.E.-May, 2000

HO Train Lighthouse Flasher Revisited  -  HO train sets often have authentic looking scale model homes and buildings.    A while back someone asked me to design a flashing LED light, which he could mount inside a model lighthouse and have it operate so it would appear to rotate and flash.    I designed a working circuit but I was never pleased with the results.    i’m still not 100% pleased but the revised circuit below seems to operate a bit better than the older circuit.  … [Hobby Circuit designed by Dave Johnson P.  E.  , May 2012] . . . Hobby Circuit designed by David Johnson P.E.-December, 2012

Home Made Geophone Detects Foot Steps  -  A home made geophone is made from a strong magnet, a coil of wire and a rubber band.  The circuit is sensitive enough to detect the vibrations of a nearby foot stomp.  It could be used as an earthquake detector.   . . . Hobby Circuit designed by David Johnson P.E.-June, 2006

Homemade Corner Cube Reflector  -  Corner cubes are optical devices which have a unique property.  Light sent into the front face will be sent back to the source in a parallel path.  As long as light can strike the reflective surfaces the it will always be sent back to the source.  Tiny co . . . Hobby Circuit designed by Dave Johnson P.E.-January, 1900

Hot AC Power Line Indicator  -  The circuit below is designed to detect when a power cable is active (hot).  When the flat sensor end of the device is held near an active AC line, the bright green LED on the top of the housing will glow.  It is powered by a 3v CR2032 lithium coin cell.  I . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2005

Hour Meter triggered by Machine Vibration  -  This hobby circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine . . . Hobby Circuit designed by David Johnson P.E.-December, 1998

Ideal Diode for PV Panels  -  Solar panels usually include a diode at their outputs.  The diode prevents current from back feeding into the panel at night from the battery being charged by the panel.  But, this diode does waste some power and does reduce the available panel voltage . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2009

Ideal Diode Function  -  Most photovoltaic solar panels are used to charge a battery during the daytime.  Nearly all panels come equipped with a blocking diode.  The diode prevents DC current from flowing backwards from the battery bank into the panel at night.  The usual blocking . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2008

Illuminated 16vAC Doorbell Lamp Replacement    -  A while back I noticed that the lamp inside the illuminated pushbutton switch for my doorbell burned out.  I carefully removed the doorbell, opened the switch housing and then replaced the incandescent lamp inside with the circuit shown below.  The circuit uses two small but very bright green LEDs from King Bright.  The resistor sets the average LED current at about 3ma.  The diode prevents reverse biasing the LEDs.  The two lamps emit plenty of light and should last much longer than the incandescent lamp they replaced . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2011

Improve Response Time of a Photo Transistor Circuit  -  This page shows different ways to improve the response time of a photo transistor circuit . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Improve the quality of standard TV images  -  I designed this circuit many years ago, based on the claims that the technique would improve the quality of standard TV images.  The circuit adds information to the edges of the objects and was reported to bring out more detail.  After building and testing . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2000

Improved Solar Path Light  -  The circuit below was designed to replace the guts of an existing path light assembly with one which has a much better performance.  I suggest using a quality 10 cell solar panel with at least a 4 square inch area.  Buy a panel with solar panels that fills . . . Hobby Circuit designed by David Johnson P.E.-March, 2011

Increased power for LED Flasher  -  Power LEDs are becoming more popular these days.  The DC input power to these devices ranges from 1 watt to 5 watts.  Normal LEDs only receive about 0.05 watts.  At this higher power, these devices can emit a lot of light . . . Hobby Circuit designed by David A. Johnson P.E.-8/0/2008

Indicate DC Current Flow  -  The circuit below is a simple way to indicate when DC current is flowing through a wire.  The circuit is designed so it will turn on a LED indicator light, whenever the voltage drop across a shunt resistor exceeds about 0.1volts.  The value of the resist . . . Hobby Circuit designed by David A. Johnson P.E.-October, 2005

Indicates 120 VAC or 240 VAC  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Indicates Automotive Electrical Voltage  -  When troubleshooting the 12v electrical system of an automobile, it is nice to have a simple voltage indicator tool instead of a voltmeter.  This electronic circuit, hobby circuit is powered by two AA or even two N cells, which will provide enough energy . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Indicates Battery Charge Current  -  This circuit turns on a LED whenever it detects at least 25ma of battery charge current . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Indicator for Acceptable Voltage  -  I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed.  The circuit can also be used to adjust a particular voltage be within specific high or low limits.  The three LEDs will indicate if the voltage is . . . Hobby Circuit designed by David A. Johnson P.E.-February, 2002

Inductive Pulse Receiver (175Khz)  -  This circuit is discussed in more detail in the Experimenters Journal.  The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provided enough amplification for the signal to be easily viewed on an oscilloscope . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Inductive Pulse Receiver (175Khz)  -  This circuit is discussed in more detail in the Experimenters Journal. The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provided enough amplification for the signal to be easily viewed on an oscilloscope . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Infrared Remote Extender Circuit  -  This circuit is designed to be placed directly in front of a standard TV or VCR remote.  The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Inrush Current Limiter for a Incandescent Lamp   -  This circuit limits the large inrush current often associated with large incandescent lamps.  With the components shown the current is limited to 1 amp, but it could be scaled to any desired current . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Insolution_Meter_Solar  -  You don’t need a fancy and expensive meter to measure the sun’s intensity.  With some inexpensive parts you can build your own meter.  I used a 1ma meter movement and a single solar cell I got on eBay with an area of about one square inch.  I calibrated the meter on a very clear bright summer day when the sun was directly overhead. . . . Hobby Circuit by David Johnson P.E.-July, 2017

Isolated Miniature AC/DC Power Supply  -  This circuit uses a novel approach to produce a fully isolated and regulated 5 volts @30ma from the 120vac power line.  It uses two tiny SCRs that alternately discharge two capacitors through a miniature high frequency transformer . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

It's Time to Dust Indicator  -  I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough.  I have not yet built and tested this circuit completely but in concept it should work . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2000

Knocking rings Doorbell  -  in some apartment buildings and homes, not equipped with a door bell, it may be hard to hear someone knocking on the front door.  The circuit below provides a means to activate a loud beeping sound inside, whenever someone knocks on the door . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2009

Laptop Power Supply  -  Concept Only.  I have a big screen HP laptop computer.  The large brick size AC to DC power supply is rated at 19.2v and 9.5 amps, which is about 180 watts.  I’d like to be able to use the thing in my car.  I looked into a 12v car adapter for it but never really found one that I liked.   . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2009

Laser Light Detyector-10Mhz to 20Mhz  -  This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system.  It has good ambient light immunity . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Latching_Relay_Dusk_To_Dawn  -  Over the years I have had to replace many dusk to dawn switches for outside lights.  These switches turn on a light during the night, then switch them off during daylight hours.  Some commercial switches are not sensitive enough to detect when the sun is out.  Others flutter badly during the dawn and dusk switching transitions, burning out compact fluorescent lamps, which do not like rapid power fluctuations. . . . Hobby Circuit by David Johnson P.E.-July, 2017

LC Oscillator Medium Power 125Khz  -  This circuit is similar to MEDiUM POWER 125KHZ OSCiLLATOR but adds even more inverters in parallel to deliver yet more power.  The values shown are for 125KHz . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

LED Camping Lantern with Dimmer  -  A while back I received an email from a Discover Circuits visitor who wanted to build his own LED camping lantern.  He wanted to use a single bright LED powered by 3 large alkaline D cells.  He also wanted to be able to dim the light intensity down to a low night light level, to extend the battery life.  The circuit below performs this task . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2013

LED Driver-Wireless  -  The circuit solves this problem by sending power to the LED through the plastic, using a magnetic coupling technique.  The circuit below can route power through plastic enclosures as thick as ¼ inch.  The circuit will not work through metal boxes . . . Hobby Circuit designed by David A. Johnson P.E.-November, 2008

LED Flasher needs only 1.5 VOLTS  -  To squeeze even more energy from a alkaline battery cell, this circuit adds two transistors to a circuit similar to the above design to boost the efficiency.  A small 1.5 volt alkaline N cell should flash the LED for a full year.  It too uses a "charge pump" technique to provide a LED the needed voltage . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

LED Flasher that is Battery Free  -  LED flashers have been used for a wide variety of applications.  Some flasher units have been used in food and hardware stores, to draw attention to sale items and are often attached to boxes which dispense product discount coupons . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2010

LED Flasher Universal 3v  -  The electronic circuit below will flash any LED color with fixed current pulses powered by a 3v battery.  It uses a charge pump approach, which routes a constant current pulse through the LED, regardless of the LED voltage requirement.  Using this approach . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2008

LED Flasher-Wig/Wag  -  This simple circuit will flash two LEDs in an alternating fashion . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2004

LED Flasher-Wireless Circuit  -  When you would like to flash a LED through a thick plastic case without wires, use this hobby circuit.  It performs this feat.  It draws about 300ua of average current at 5v.  It can be powered from a supply ranging from 3v to 6v . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2011

LED Strings powered by AC Line  -  This shows 4 different ways to string white LEDs that are powered by a 120vac or 240vac power line. __ Designed by Ken Schultz, drawn Dave Johnson 12.18.2004-December, 2004

LED Test Signal Generator-40Khz  -  This 40KHz crystal controlled oscillator circuit drives an infrared LED with powerful 40ma pulses.  The circuit can be used to test optical communications circuits, designed to receive 40KHz modulated light signals . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2002

LED/Laser Light Output Intensity Meter  -  This circuit uses a large 1cm X 1cm silicon PIN photo diode and a transimpedance amplifier to measure the light power output of infrared and visible LEDs and laser diodes.  It can be modified to produce almost any milliwatts to volts scale factor.  It can b . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000

LED_Light_8_Solar_Cell  -  I have been modifying a lot of solar garden and path lights for the last several years.  I buy some units from home improvement stores, then I gut them completely.  I take out the battery, electronic control board, LED and solar panel.  I then replace those components with new units.  I increase the size of the solar panel and battery.  I use a more efficient and brighter LED . . . Hobby Circuit by David Johnson P.E.-September, 2017

LED_Light_for_Tool_Shed  -  I have a lean-to 8ft by 12ft shed next to my house.  It is full of lawn chemicals and garden supplies.  There were times when I needed to get something from inside the shed during the dead of night.  Early on I had to use a flashlight.  But later, I installed a series of five 12v LED light strips on the ceiling.  Each strip contained 15 3v white LEDs.  The strip had 5 strings of three LEDs, wired in series, with a 150 ohm current limiting resistor. . . . Hobby Circuit by David Johnson P.E.-July, 2017

Light Intensity Monitor  -  For an on-going consulting project, I had to determine the light output decay time of different fluorescent lamps.  I wanted to know how fast the light from a lamp dropped to a low level, after current was interrupted in the lamp.  I used a high voltage . . . Hobby Circuit designed by David Johnson P.E.-April, 2005

Light Monitor Circuit  -  The circuit below takes advantage of a low power op amp with a “rail to rail” feature.  The single resistor R1 defines the light sensitivity of the circuit.  The output voltage is directly proportional to the intensity of light striking the small photodiode . . . Hobby Circuit designed by David A. Johnson P.E.-September, 2010

Light Pulse Receiver (2uS)  -  Although the LF357 is an obsolete part, this circuit gives you an idea how to build a sensitive modulated light detector with high ambient light immunity.  The first section uses a 100mH inductor as an efficient photodiode current to voltage conversion circuit . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2002

Light Receiver 30Khz Amp  -  This circuit uses NPN darlington transistor to amplify the signal produced from short light flashes, as detected by a PIN photo diode.  The circuit draws only about 330uA from a 6v battery . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

Light Receiver Amp-40Khz  -  This circuit is similar to 30KHZ LIGHT RECEIVER AMP  but provides more gain and operates up to 40KHz.  However it draws more power supply current.   . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2002

Light Receiver Front-End Circuit  -  if you want even more sensitivity than the above circuit, try this design.  When used with a one centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module.  Note: The LF357 operational amplifier is no longer available this circuit is for reference only.   . . . Hobby Circuit designed by David A. Johnson P.E.-March, 1999

Light Receiver-10MHz to 20MHz  -  Light power to volts conversion = APPROX.15mV per microwatt at 850nM . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Light to Frequency Converter  -  This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter.  A small PIN photo diode is used as the light detector.  The pulses produced are short, so in some applications you may want to stretch them or . . . Hobby Circuit designed by Dave Johnson P.E.-August, 2006

Light_Pathway_1w_Solar_Cob_LED  -  Chip on board (COB) LEDs are all the rage.  These devices assemble multiple tiny LED chips onto a metal backed circuit board, then cover the assembly with a plastic cover.  The metal back allows the assembly to be mounted to heat sink to keep the device cool.  The multiple LED chips can be wired into any number of series/parallel configurations.  Some are wired for 12v operation while other COB LEDs in flashlights are all wired in parallel, with an operating voltage of about 3 volts. . . . Hobby Circuit by David Johnson P.E.-May, 2017

Lighthouse_Solar_Powered  -  While at a discount store one day, I spotted an interesting tall lighthouse.  It had a tiny solar panel and an equally tiny LED light.  The LED was pointing downward, at the center of a rotating light reflector.  I thought it might be turned into an interesting addition to my wife’s garden. . . . Hobby Circuit by David Johnson P.E.-September, 2017

Limiting Amplifier-40Khz  -  This circuit has a gain of X20 at 40KHz.  The amplifier output level is limited to 2.5v peak to peak.  Limiting action begins with a 50v peak to peak input . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Line Powered High Intensity LED Flasher  -  There are times when you would like to generate an intensely bright flash of light using electrical power from the AC line.  The circuit below can drive a one to three watt power LED with about 350ma of peak current.  Since the circuit drives the LED with . . . Hobby Circuit designed by David Johnson P.E.-December, 2008

Line Status Indicator for 1 Telephone Line  -  This circuit monitors one phone line . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Lithium Battery Tester  -  I bought some lithium ion rechargeable batteries a while back.  I was planning on using them to modify some solar powered LED spot lights.  What I like about these new batteries is that they are the same size as a standard 1.5v alkaline AA cell . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2012

Lithium Ion Solar Battery Charger-#2  -  A single lithium ion battery cell needs 4.2v from a current source to insure it is fully charged.    A solar panel with just enough individual cells wired in series to produce the needed voltage would be more efficient than one with more cells for a given surface area.    if you divide 4.2 by eight you come up with a figure of 0.525 volts . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2013

LM317 adjustable voltage regulator used in Battery Eliminator Circuit  -  if you use a 1.5v or a 3v battery powered MP3 player for long periods of time, you may get tired of constantly changing batteries.  Some older players do not work will NiMH rechargeable batteries so changing out batteries can get expensive.  The circuit be . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2005

Locate a Plastic Pipe  -  I used this circuit once to track down the location of a plastic pipe, buried underground.  It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Low Current Flashing LED Power Indicator  -  This circuit flashes a power indicator LED to keep the average current low . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Low Pass Filter, 3Khz, with an Audio Amp  -  This circuit uses a switched capacitor filter IC from National Semiconductor to filter signals with frequencies higher than the 3KHz needed for voice audio.  The schematic includes an audio amplifier that is designed to drive a standard audio head phone . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Low Power Sequential Flasher using 10 LEDs  -  A while back I posted a flasher circuit which flashed 10 LEDs sequentially.  That circuit was powered by four AA alkaline cells.  The circuit below operates much the same way but has been modified to run from a single 3v CR2032 lithium coin cell . . . Hobby Circuit designed by David Johnson P.E.-April, 2011

Low Voltage Indicator for Lead-Acid Battery  -  Lead-acid batteries should not be discharged below a certain voltage.  Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity.  Large battery banks are critical in off-grid energy storage systems.  One . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Mason_Jar_Solar_Light  -  I have modified several commercial solar garden lights over the years.  I switch out the LED, solar panel, battery pack and control circuit.  The result is a much better overall light.  One of my disappointments in using store-bought garden light assemblies is their construction.  They are built so cheaply that over time, water and insects get inside, killing the light operation. . . . Hobby Circuit by David Johnson P.E.-September, 2017

Measure Button Battery Cell's Internal Resistance   -  The open cell voltage of a small silver oxide button cell battery remains very close to 1.5 volts over most of the life of the cell.  However, there is a relationship between battery health and the internal resistance of the cell.   This circuit when used . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Microphone On/Off Switch Circuit is Pop Free  -  This circuit uses a simple technique to eliminate the usual “pop” sound when an electret type microphone is switched on or off.  The circuit is completely passive and will work on dynamic and electret type microphones . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2002

Miniature Pushbutton On/Off Switch  -  in many battery powered devices, you would like to turn the circuit on and off by pressing a small pushbutton switch.  One press turns the device on and second press turns it back off.  The flip/flop circuit below uses a minimum number of parts to perform . . . Hobby Circuit designed by David Johnson P.E.-October, 2005

Model Airplane Wing Tip Flasher  -  The circuit below is designed to flash two white LEDs mounted at the wing tips of a model airplane.  The flasher unit is powered by a 4.5v supply made of three AA or AAA 1.5v alkaline battery cells.  The average current is about 2ma so a set of batteries . . . Hobby Circuit designed by David A. Johnson P.E.-October, 2009

Modified LED Night Light  -  I bought some night lights a few months ago from Walmart here in Denver.  The devices were made by Elumina Lighting Technologies inc. (www.  eluminalighting.  com) , their model LED-6200A.  I think I paid about $15 for a pack of three lights.  I wanted a light . . . Hobby Circuit designed by David Johnson P.E.-May, 2007

Modified Shake to Charge Flashlight  -  The complete schematic for this device is shown below.  A few things should be pointed out.  First, they use some very cheap 1N4001 diodes in the bridge rectifier circuit instead more efficient Schottky diodes.  They also use a small 0.5 Farad cap with a . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2012

Modulated Light Receiver uses a Single IC  -  The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode.  Using a unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions.  It is a great circuit when you want to extend the range of an optical remote control transmitter . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Monitor ON/OFF status of a Remote Switch  -  There are many applications when a remote switch’s on/off status needs to be monitored.  Reed relay sensors, toggle control switches and security alarms contacts are just a few examples.  Routing a switch directly to a computer system hundreds of feet away . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2009

Monitor Water Seepage  -  A while back I received a request from a company who needed a water seepage detector.  They wanted a device that ran on 24vac and detected water using a fully isolated circuit.  They wanted a circuit which switched off 24vac to a load when water was detected . . . Hobby Circuit designed by David A. Johnson P.E.-January, 2011

Motorized Duck Decoy Timer  -  I received a nice email from a duck hunter.  He uses some motorized decoys to lure ducks into his line of fire.  However, the battery powered decoys only have an on/off switch so once turned on; the decoy flaps its wings or paddles around continuously.  The hunter wanted a more realistic motion, which would power the motor for say 10 seconds, then stop for another 10 seconds before starting up again.  The simple circuit below should work well for his application . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2011

Moving AIR Indicator  -  Many months ago a Discover Circuits reader wanted a method to indicate when a fan was blowing air.  He didn’t want to route any electrical wires to a sensor near the fan.  Instead he wanted all the electronics for both the fan and other circuits mounted s . . . Hobby Circuit designed by Dave Johnson P.E.-August, 2006

Negative 0.5 Volt Supply  -  This simple circuit consists of two LEDs and a photo diode.  It generates a negative voltage with a current level of a couple milliamps.  It is ideal for supplying a negative rail to low power “rail to rail” op amp circuits, which need to have a true zero volts output.  Note:  This circuit is not particularly efficient . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Next-Generation Star-Trek Doorbell  -  in the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of a crew member makes a particular “beep-boop” sound.  The 3v battery powered circuit below tries to simulate this sound.  The circuit uses one 74HCT74 dual D flip/flop . . . Hobby Circuit designed by Dave Johnson P.E.-October, 2001

Night_Light_with_20_LED  -  After 3 years of continuous 24/7 operation I noticed that the 20 LED array I was using as a night light next to my back door was fading.  Some of the LEDs were much dimmer than others.  I figured that over time, the internal heat dissipated from the LED chip was causing the LED light intensity to fade.  I decided it was time to replace the light. . . . Hobby Circuit by David Johnson P.E.-July, 2017

Nitrogen Spark Generator  -  Nitrogen or air sparks are very powerful light sources that produce flashes that last only a few nanoseconds.  This line powered circuit generates a continuous series of very small sparks across electrodes with a 0.05 inch gap . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

NMH/NiCd 6v Battery Tester  -  I designed this circuit to test rechargeable six volt battery packs under constant current conditions.  As designed, the circuit applies a 10 amp load to the battery pack.  A heat sink must be used on the main power transistor . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Noise_Generator_using_Mm5437  -  Many years ago National Semiconductor came out with a neat white noise generator IC.  They are no longer being made but they can still be found on eBay.  This little 8 pin part is very easy to use.  It draws about 4ma of current and can operate over a range of 4.5v to 11.5v.  It might even operate down to 3v. . . . Hobby Circuit by David Johnson P.E.-July, 2017

Ocean Wave Power Generator Demo Unit  -  A while back, a Discover Circuits visitor wanted a simply way to demonstrate how electricity could be generated from the up and down movement of ocean waves.  The system was to be part of a science fair project, showing different renewable energy sources . . . Hobby Circuit designed by David Johnson P.E.-January, 1900

One Amp Current Injector  -  I designed and built this thing many years ago.  I published the hobby circuit in the now defunct Radio Elecronics magazine back in 1992.  The electronic circuit injects a precise 1 amp of current into any unknown resistor, up to about two ohms.  Two alkaline . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Oneshot & Latch Pushbutton  -  This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal.  It can operate from 3v to 18v . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

Operation of Cascode Light Receivers  -  This page provides a detailed explanation of how the modified cascode light receiver circuit operates.  The cascode technique in conjunction with an inductive load provides very high current to voltage conversion as well as very high speed . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

Optical Fiber Receiver-5Mhz Broad Band  -  This circuit is a simple broad band light detector that uses a very inexpensive IC and a PIN photodiode that is packaged for use with plastic optical fibers.  It has a bandwidth from 1KHz to over 5MHz.  It is great for experimenting with various modulated . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Optical Filter using Color Film Negative  -  This isn’t an electronic circuit but the file shows the curve for an optical filter made from exposed color film.  The material blocks most visible light but is very transparent to 880nM infrared light.  Kodak 100ASA is exposed to "cool white" fluorescent light for about 5 seconds . . . Hobby Circuit designed by David A. Johnson P.E.-January, 1998

Oscillator Draws 2 Nanoamps  -  if you have hundreds of touch switches that need an excitation signal, then this circuit is what you need.  Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Oscillator with Ultra Low Current  -  Here is a challenge.  Design an astable oscillator which draws only a few nanoamps of current from a +3v supply.  I gave this some thought and came up with the circuit below.  I used some pretty standard parts except for three surface mounted 1000M resist . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2009

Over-Temperature Micro-Power Alarm  -  The circuit is powered for years by a single 3 volt lithium battery.  It sounds an alarm when the temperature exceeds a certain point.  With some minor changes the circuit could also be configured for an under temperature (freeze) alarm.  The circuit uses a . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Parallel LC Oscillator with CMOS Inverter   -  I have used this parallel resonant LC oscillator circuit countless times.  The oscillator frequency is determined by the inductor and capacitor values.  I have shown an adjustable inductor to make it easy to set the frequency to a specific value . . . Hobby Circuit designed by Dave Johnson P.E.-March, 2002

Peak to Peak 300v Signal Generator  -  This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Piezoelectric Device used for Motion Alarm  -  An inexpensive piezo wafer is used to detect vibration and when the vibration is sufficient a switch is activated . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Piezoelectric Water Flowing Indicator  -  The vibrations associated with water flowing through a pipe are picked up by an inexpensive piezo wafer.  The signal from the wafer is first boosted by a micropower transistor amplifier and then fed to an ultra low power voltage comparator . . . Hobby Circuit designed by David Johnson P.E.-July, 2009

Plus & Minus DC Power Supply  -  This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Poor Man's Timer  -  Often during testing of certain equipment and components, you would like to keep track of the elapsed time in hours, minutes and seconds.  There are some nice commercial digital timers and counters available for just this sort of application.  However,  . . . Hobby Circuit designed by Dave Johnson P.E.-August, 2008

Power Supply for Digital Panel Meters that is Isolated  -  Many inexpensive digital panel meters, such as the one shown below, are designed to operate from a 9v battery.  However, the nature of the panel meter internal design often requires that the DC supply used to power the device be isolated from the ground . . . Hobby Circuit designed by Dave Johnson P.E.-September, 2008

Precision Low Capacitance Meter  -  This circuit was originally designed to measure the volume of fluid inside a medical syringe.  As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Press & Hold Latching Circuit  -  A Discover Circuits visitor needed a latch circuit which could operate using a power supply voltage ranging from 3v to 24v.  He wanted to use a tiny pushbutton switch to turn on and off power to a load.  However, he wanted a 2 second delay between the switch activation and the state change of the output . . . Hobby Circuit designed by David Johnson P.E.-July, 2013

Protects from Battery Polarity Reversal  -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

Pulse Generator uses Micropower  -  it is hard to design a pulse generator that produces clean logic pulses while drawing very low current.  This circuit is designed to produce short 2mS pulses at a rate of one per second while drawing only 1 microamp from a 9 volt battery . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Pushbutton On-Off Latch Circuit  -  Often you want to turn on and off power to a device with a single pushbutton switch.  The circuit below performs this logic function with just a few parts and can operate from DC supplies between +3v to about 15v.  It uses two inverters within a 74C14 or a CD4069 hex inverter package.  Each press of the button toggles between the two on/off logic states.   . . . Hobby Circuit designed by David Johnson P.E.-January, 2010

Pushbutton Switch Through Glass   -  This simple technique allows a mechanical pushbutton switch assembly to activate a reed type relay through a pane of thick glass.  Pushing the button moves a magnet near the reed relay, closing the relay contacts . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2002

PV Solar Cell Test Fixture  -  This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

PWM Circuit for Motor Speed Control  -  Sometimes you want to slow down a brush type DC motor.  The most efficient way to do this is with a pulse width modulation (PWM) technique.  The hobby circuit below can operate from about 3 volts to 15 volts.  The frequency is fixed at about 2KHz but the pulse . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2007

PWM Motor Controller uses a 555 Timer  -  Using a CMOS version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor . . . Hobby Circuit designed by David Johnson P.E.-June, 2006

Quartz_Clock_Solar_Powered  -  Connect a solar panel to a quality AA size 1One of my homes in Colorado had a large battery powered quartz clock above the fireplace.  It was powered by an AA size 1.5v alkaline cell.  The battery would run the clock for about a year or so, then I would have to break out a tall step ladder and replace the battery for another year or so of operation. That gave me a great 10:1 energy margin. . . . Hobby Circuit by David Johnson P.E.-September, 2017

Radio Tower Lamp Monitor  -  The circuit below uses two 3 amp diodes to produce a fairly constant 2 volts whenever current is routed to the tower lamp.  The voltage is used to power a bright LED.  The LED will only turn on when current is actually flowing to the lamp.  With the component values shown, the circuit will work with lamps ranging from 40 to 250 watts.   . . . Hobby Circuit designed by David Johnson P.E.-January, 1900

Ramping Lamp Driver  -  This circuit drives any 12v lamp, incandescent or LED type, in a ramping fashion.  Each press of the button toggles the lamp from a fully off mode to a fully on mode.  But, instead of an instantaneous lamp intensity change, the power is slowly ramped up a . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2007

Really Loud Output Beeper Circuit  -  I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC .  An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency.  By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Rectifier-Fullwave  -  I have used this handy circuit many times.  It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage.  It works from millivolts to volts.  The circuit shown requires a stable +5v reference if a . . . Hobby Circuit designed by David Johnson P.E.-February, 2002

Reflected Infrared Light Switch  -  infrared light reflected off a finger is used to activate this switch circuit.  Drawing only 30uA from a 3v supply, this circuit will detect a human finger with a range of about 1 inch.  The sensor uses an inexpensive infrared LED and a matching photo diode . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Relay Driver with Reduced Power  -  Relays can handle a lot of power.  However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed.  The circuit below performs such a task.  It uses a single CD4093 quad NAND gate.  When the “on” logic input . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2008

Remote Range Extender TV/VCR  -  I often get frustrated by how poor some infrared remotes in many new electronic devices operate.  I used to be able to turn on the TV from 30 feet away.  Now, it seems that I have to get within 15 feet.  To address this issue, I designed the circuit shown below.  The circuit taps into the TV remote’s existing infrared LED and drives 8 more efficient devices.  The result is a communications range some 4 to 5 times longer.   . . . Hobby Circuit designed by David Johnson P.E.-December, 2009

Remote TV Camera Power  -  A while back I received an email from a company with a problem.  They needed a security camera installed at some remote location.  The camera was a high quality color camera sending out standard NTSC video through a long 500 foot 50 ohm coax cable.  With some difficulty they got the coax strung.  They were told that there would be AC power near the camera location to power that device.  But, they then learned that in fact, there was no power available.  They thought about using a solar panel and a battery but they decided to call me for help instead.  Was there some way to power the camera without stringing another cable? . . . Hobby Circuit designed by David Johnson P.E.-July, 2011

Replace a Pushbutton Switch with this Infrared Safety Switch  -  Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

RFID Test Circuit  -  I designed this test the concept of using light techniques to send identification data instead of RF.  . . . Hobby Circuit designed by David Johnson P.E.-July, 2000

RS-232 Serial Interface Status Indicator Circuit  -  I designed this circuit many years ago and have used it several times since.  It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link.  The thresholds are set at plus 3 volts and minus 3 volts.  Two dual color LEDs are used as indicator lights . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Safety Light  -  My 1200 square foot metal shop only has one small entrance door and no skylights.  If the lights are turn off while inside, there is not enough light to safely find the door to exit the building.  On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking . . . Hobby Circuit designed by David A. Johnson P.E.-November, 2004

Scare Off Pigeons  -  I have an area on my roof where a large flock of pigeons like to roost.  Their nesting materials and droppings really make a mess.  Often there is enough junk from them that it clogs my roof gutters.  Perhaps one way to scare them away . . . Hobby Circuit designed by Dave Johnson P.E.-April, 2005

Servo Pulse to Voltage Converter  -  A while back a Discover Circuits visitor needed a way to convert servo pulses from a radio controlled device into a voltage.  The drawing below, illustrates the pulse train of the received signal.  The time between pulses is a fixed 20ms while the pulse widths vary from a low of 1ms to a high of 2ms . . . Hobby Circuit designed by Dave Johnson P.E.-August, 2005

Shadow Activated Motion Detector  -  The hobby circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both device . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2005

Single Output 120VAC 200 Watt Touch Switch  -  This simple circuit uses a single IC to form a nice touch switch circuit.    A single transistor forms the remote active switch sensor.    Multiple switches can be wired in parallel.    The switch circuit can be located about 500 feet from the control circuit.   . . . Hobby Circuit designed by David Johnson P.E.-March, 2002

Slotted Opto-Sensor used in Honeybee Counter  -  I designed a circuit similar to this one a long time ago to help a beekeeper count the number of bees going into or out of a hive.  The low power circuit uses a slotted opto-sensor to detect the passing bees.  The circuit advances an electronic counting . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2000

Slotted-Type Optical Interrupter Draws Microamps  -  This circuit is great for battery-powered systems that use slotted type optical interrupters.  It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Small Battery Capacity Tester  -  I have a client who uses a lot of batteries.  His sells a device which is powered by a 3v lithium coin cell and he sells thousands of them.  It seems that many of the batteries he has been importing from China have a milliamp-hour capacity which is much . . . Hobby Circuit designed by Dave Johnson P.E.-September, 2008

Snow Depth Alarm  -  The design of this system alerts a snowplow driver when the snow depth has exceeded four inches.  A visible red LED packaged in a 10mm package is pulsed at a rate of about 50Hz with 10uS pulses.  Twenty four inches away a phototransistor detects the light from the LED.  The LED and the phototransistor are housed in a section of ½ inch PVC pipe, forming a waterproof seal.   . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2013

Solar Cell Test Fixture  -  if you are considering building your own solar panel, you might try building the solar cell tester shown below.  When constructing a solar panel it is important to make sure that all the solar cells used in the panel be closely matched for their short circuit . . . Hobby Circuit designed by David Johnson P.E.-November, 2007

Solar Insolation Monitor  -  Owners of photovoltaic solar panels often like to monitor how bright the sunshine is.  They can use this information to gage how well their solar panels are doing.  On a typical clear day, the sun energy at sea level is about 1000 watts per square meter . . . Hobby Circuit designed by David Johnson P.E.-May, 2012

Solar Powered Lithium Ion Battery Charger  -  The circuit below feeds a controlled current and voltage to a 3.6v lithium ion battery.  The current is limited to 300ma and the voltage is limited to 4.2 volts.  The circuit uses a LTC1734 IC from Linear Technology.  No diode is needed between the circuit a . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2007

Solar_Panels_Converted_from_9v__to_4.5v_Panels  -  Three 1.2v NiMH cells wired in series are perfect for powering white LEDs.  The 3.6 volts from such a battery bank is a good match for a 3v white LED. Only a single resistor is needed between the battery and the LED to limit the current.  The ideal solar panel voltage when charging three 1.2v NiMH cells, is 4.5v, using 9 solar cells.  A 1N5817 Schottky diode is usually wired between the solar panel and the battery pack, to prevent battery current from leaking back into the panel at night. . . . Hobby Circuit by David Johnson P.E.-July, 2017

Solid State Relay that is Bidirectional  -  The circuit below can switch up to 4 amps of current with a peak voltage up to 20v.  It is fully isolated using two inexpensive red LEDs.  One LED forms a red light source, which illuminates a second red LED.  The circuit takes advantage of the 1.5v . . . Hobby Circuit designed by David A. Johnson P.E.-May, 2009

Solid State Touch Activated Switch  -  it does not get any easier if you want a solid state switch that is activated by the touch of a finger.  Two small metal pins route voltage through the finger skin to a MOSFET switch.  The circuit is great for situations where a membrane type mechanical switch . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Sound Generator-Clicking  -  Often in computer controlled systems, you would like to generate a “click” sound whenever a button is pressed.  This provides the user with audible feedback that the pushbutton press has been acknowledged.  The two circuits below generate such a sound.  A click sound is generated each time the logic input swings from a logic low to high condition . . . Hobby Circuit designed by David Johnson P.E.-January, 2009

Still Another Very Loud piezo Alarm Beeper  -  This is yet another beeper circuit that really draws attention.  It sweeps the drive frequency slightly to produce a very annoying sound.  It uses a transformer to increase the drive voltage across the piezo device to more than 200 volts peak to peak . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Super Capacitor makes Low Voltage Latching Relay Driver  -  Using some small super capacitors, this circuit can latch and unlatch a mechanical relay with 10 amp contacts, from a small 3 volt power source.  By using a latching relay, power can be controlled to a load with a tiny battery . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Sweeping Siren Alarm-3 volts  -  A while back I was challenged by a visitor to the website.  He needed a very loud sweeping siren type audio sound generator powered by 3v.  He tried some of the commercial sirens but they were not very loud when powered by 3v.  He also said that those . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2008

Switch activated by Piezo Vibration  -  An inexpensive piezo wafer is used to detect vibration and when the vibration is sufficient a switch is activated . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Switches AC or DC power using a Latching Relay  -  The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay with two coils.  The relay can handle about 10 amps of current, up to 250vac.  Each time the small pushbutton is pressed, the circuit either latches the relay . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2009

Tap Switch uses a Piezo Wafer  -  Two different switch circuits are shown.  One sources current and the second sinks current.  Both switches are connected to a piezo wafer.  When the wafer is tapped, the switches are activated . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2006

Telephone Status Indicator for 4 Lines  -  Many home businesses use multiple phone lines.  This circuit gives you a visual indication when a line is in operation.  The two AA battery cells should provide enough power for about one year of operation . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Tester 4-20ma Current Loop  -  This circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps . . . Hobby Circuit designed by David Johnson P.E.-November, 2004

Tester for Modulated Laser Diode  -  This circuit can either be used to test laser diodes or as a general purpose modulated light source.  The circuit has an adjustable diode threshold current and an adjustable modulation frequency.  The frequency can be adjusted from below 30KHz to above 50MH . . . Hobby Circuit designed by Dave Johnson P.E.-March, 1999

Tester for Modulated Light  -  This battery powered circuit will work from DC to 500KHz.  It is designed to connect to an oscilloscope to monitor the light level changes and modulated signals . . . Hobby Circuit designed by David A. Johnson P.E.-july 9, 206

Tester for Modulated Light  -  This battery powered circuit will work from DC to 500KHz.  It is designed to connect to an oscilloscope to monitor the light level changes and modulated signals . . . Hobby Circuit designed by David Johnson P.E.-july 9, 206

Tester for Pulsed LEDs  -  This circuit is designed to test visible and infrared LEDs in pulsed mode operations.  It can drive the LED with peak currents in excess of 10 amps.  A light detector nearby can monitor the response time and intensity of the LED under test . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Three-Volt Low Battery Voltage Flasher  -  Many battery powered devices use two AA alkaline cells.  Often you will not know when it is time to replace the batteries until the device powered by them actually stops operating.  The hobby circuit below can be connected to a 3v battery, to give you some . . . Hobby Circuit designed by David Johnson P.E.-December, 1998

Three-Volt Powered Very Loud Beeper  -  Getting a high sound intensity from a piezoelectric type beeper is not easy when the available DC supply is only 3v.    The circuit below is not only efficient but produces a very intense sound.    The circuit combines a voltage boost section with a resonant  . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2009

Timer for Darkroom Camera Shutter  -  This circuit was designed to control a film exposure shutter for a darkroom.  It has 8 time steps ranging from 0.35 seconds to 4 seconds.  It is activated by a foot switch and draws power from an external 12-volt DC supply . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Touch Alarm for Door Knob  -  I designed this hobby circuit many years ago and it seems be a very popular construction project.  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms.  A metal chain attached to a box holding the electronics is place . . . Hobby Circuit designed by David A. Johnson P.E.-May, 2000

Touch Switch Exciter  -  if you have hundreds of touch switches that need an excitation signal, then this circuit is what you need.  Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power . . . Hobby Circuit designed by David A. Johnson P.E.-February, 2002

Transmit to a Remote LED Indictor Light  -  There are times when you would like to transmit a signal from one LED indicator light to second LED at another location.  The circuit below works well for this application.  It takes advantage of the fact that the internal infrared LED inside an opto-isolator . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2008

Triangle Signal Generator  -  A Discover Circuits visitor needed a variable low frequency triangle signal generator, which would produce a full 5 volts peak to peak signal using a 5v supply.  I suggested the circuit below.  It uses three rail to rail op amps (2 LMC6484 dual op amp packages).  The first circuit produces a clean 2.5v reference voltage.  The second is configured as a classic multivibrator oscillator . . . Hobby Circuit designed by Dave Johnson P.E.-September, 2011

Turn D-Flip/Flop into a One Shot Circuit  -  Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators.  This schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging from nanoseconds to seconds . . . Hobby Circuit designed by Dave Johnson P.E.-March, 2002

TV_Fake_Camera_Flasher  -  After having my doorbell rung several times in the middle of the night by some kids, I decided to install some fake TV cameras around the outside of my house.  I was not satisfied with the LED lights on the cameras.  The heavy current drain of the original circuit meant I would have to replace the battery every few months.  So, I removed the original LED driver and added my own simple flashing circuit.   . . . Hobby Circuit by David Johnson P.E.-August, 2017

Two Line Telephone Line Status Indicator  -  Many home businesses use multiple phone lines.  This circuit gives you a visual indication when a line is in operation.  The two AA battery cells should provide enough power for about one year of operation.  The circuit is line polarity insensitive . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Two-Transistor Low Power Oscillators  -  This page has two unusual two-transistor oscillators.  I set the component values for a low frequency application.  Both circuits draw only about 1 micro amp of current . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2004

Ultra Low Power 32Khz Crystal Oscillator  -  I have used this circuit many times when I needed a low frequency reference, which did not draw much power.  With the components show, the current from a 3v battery is less than 1.2 microamps . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2004

Ultra Low Power Detects Clear Fluid  -  Using an ultra low power technique, this circuit detects when a clear fluid is inside a piece of clear plastic tubing.  It can be used for fluid level control and monitoring . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Ultra Low Power LED Flasher  -  The efficiency of some newer LEDs is amazing.  Some of the latest green LEDs can launch blinding light with just one milliamp of current.  I take advantage of one of these newer devices in the circuit below.  The flashing circuit uses a classic multivibrator . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2008

Ultrasound Receiver-40Khz  -  A X100 transistor amplifier is followed by a zero cross detector circuit, using a voltage comparator.  The output is a TTL logic signal, corresponding to the received 40KHz signal   . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2006

Universal DC Sink to Source Converter  -  As illustrated below, many control systems use an open collector transistor to turn something on.  Perhaps it is a LED light, a relay or a small motor.  However there are other  systems which require a common ground connection instead of a common V+ supply . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2011

Universal Flasher using a Latching Relay  -  Latching relays are nifty devices.  Most contain two separate coils.  When a voltage is applied to one coil the relay latches in one state and stays in that state until voltage is applied to the unlatch coil.  Since the latching and unlatching pulses only . . . Hobby Circuit designed by David Johnson P.E.-August, 2006

VCO from CMOS Schmitt Trigger IC  -  By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1.  A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2002

Version 4 of a DC Current Indicator  -  The circuit below uses some common components to turn on an LED whenever DC current above a certain level is detected.  The circuit uses a very popular LM393 dual voltage comparator from National Semiconductor and a common 1N4148 signal diode.  The diode . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2008

Version 4 of a Ultra Low Current Oscillator  -  The circuit below was inspired by a programmable unjunction transistor (PUJT) circuit sent to me by Karl isbrecht.  In Karl’s circuit, the unijunction circuit made click sounds in a speaker and consumed very little power.  His circuit reminded me that a PU . . . Hobby Circuit designed by Dave Johnson P.E.-January, 2010

Very Loud Beeper Circuit  -  This circuit generates a continuous tone instead of a pulsing one for your hobby project.  The circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC .  An off-the-shelf piezoelectric beeping device is driven at resonance to insure maxim . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Very Low Voltage Oscillator Circuit  -  OK, you may not ever need an oscillator to function using a power supply voltage below one volt, but if you ever do need one, here is one way to do it.  Getting any electronic circuit to operate below one volt is a real challenge.  Typical silicon bipolar . . . Hobby Circuit designed by David Johnson P.E.-February, 2007

Voltage Boost  -  in this design a marginal AC to DC linear power supply provides a voltage boost, so it can maintain regulation at low line voltages.  The added components are wired at the AC input and the + DC output of an existing bridge rectifier circuit.  The components act as a full wave “charge pump” which will boost the DC output voltage across the main filter capacitor C3.  Without the boost circuit, when the AC line voltage drops to 105vac, the DC output voltage only reaches 11.5 volts, too low for a 12v voltage regulator.   . . . Hobby Circuit designed by Dave Johnson P.E.-January, 1900

Voltage Boost to Series Regulator provided by Capacitor  -  This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range.  It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-the-wall power adapter power supply . . . Hobby Circuit designed by Dave Johnson P.E.-December, 1998

Voltage Limiting with a Supercapacitor  -  Supercapacitors are working their way into more and more applications where electrical energy needs to be stored.  These robust devices can be charged and discharged 1000s of times and will typically outlast a battery . . . Hobby Circuit designed by Dave Johnson P.E.-January, 1900

Voltage Monitor for a 9v Battery  -  Nine voltage batteries are used in a wide variety of electronic devices.  But, you often don’t know when it is time to change the battery until the device stops operating.  The hobby circuit below provides a visual indication when the battery needs to be . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Voltage to Frequency Converter -40Khz  -  This circuit was designed to frequency modulate a 40KHz carrier, using human voice frequencies.  A common flip/flop is used at the core of the circuit . . . Hobby Circuit designed by Dave Johnson P.E.-December, 2004

Watchdog Timer set for 36 Hrs  -  A watchdog timer is often used in computer systems to detect when a computer is hung up and is no longer running properly.  If it does become locked up and is not able to supply a reset pulse to the circuit within a defined time frame the timer times out a . . . Hobby Circuit designed by David Johnson P.E.-April, 2010

Water Filling Station Controller  -  This circuit was requested by a company needing a simple way to fill a large tank with water with little fuss.    They wanted a low tech method to automatically fill the tank with water without constant monitoring.    They wanted to press a single pushbutton switch and then walk away.    in their application, the water tank is first filled to a certain level.    The fixed volume of water is then slowly drained off into a second tank, where the water is mixed with another material.    The empty tank is then ready to be filled again, for another cycle.  .  .  .   . . . Hobby Circuit designed by David Johnson P.E.-April, 2011

Water Level Monitor  -  This system operates much like a classic float switch but without any moving parts.  The illustration shows how a system using two copper coils sealed inside a plastic pipe, can detect the level of water outside the pipe.  Whenever the water level is lower . . . Hobby Circuit designed by David Johnson P.E.-July, 2006

Water Pump controlled by Solar Power  -  A Discover Circuits visitor had a problem.  He needed a simple on/off controller for his solar powered water pump.  His system used two 12v 50 watt solar panels wired in series.  The power from the solar panels drove a submerged water pump.  However, during . . . Hobby Circuit designed by David Johnson P.E.-October, 2010

Water Seepage Alarm  -  This simple circuit sounds a beeper when its electrodes detect water.  It is powered by a single 1.5v N cell.  A small 1.5v button battery will also work . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Watt-Hour Meter Monitor  -  Have you ever wondered how much power your house or apartment is using?  Most watt-hour meters are like the one shown below.  Some newer ones have a digital display but there millions of these older models still in use . . . Hobby Circuit designed by David A. Johnson P.E.-April, 2005

White LED Array that is Lined Powered  -  There has been a lot of excitement lately in the lighting industry.  Some companies are beginning to manufacture general lighting fixtures, which replace incandescent and fluorescent lamps, using LED lights.  When proper heat management is designed . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2010

White LED Driver powered by a 3v Battery  -  The circuit below will drive any LED color, including white LEDs, using a DC power supply ranging from 2v to 3.3 v.  White and blue LEDs normally require over 3v, so the circuit below uses a charge pump scheme to deliver a constant LED light intensity from . . . Hobby Circuit designed by David Johnson P.E.-August, 2010

White LED Night Light  -  This simple circuit is designed to plug into a standard AC electrical outlet.    it uses four super bright white light emitting diodes (LED) in conjunction with a capacitor coupled full wave rectifier circuit.  The LEDs are mounted in a box and are angled slightly to bounce the light off of a nearby wall.   . . . Hobby Circuit designed by David A. Johnson P.E.-July, 2003

Xenon Flash Receiver-AIR Transparancy Monitor  -  I designed this receiver circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments.  The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Xenon Flash Receiver-Pg2  -  This is Page 2 of the receiver circuit AIR TRANSPARENCY Monitor, XENON FLASH Receiver . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Xenon Flash Transmitter-AIR Transparency Monitor  -  This is the matching transmitter for the above receiver.  The transmitter launches powerful 1000-watt light pulses that last about 20 microseconds . . . Hobby Circuit designed by David Johnson P.E.-June, 2000

Zero-Cross Detector-Wide Band  -  This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave signal.  It will work with inputs as small as 5mv peak-to-peak or as large as 3 volts peak to peak.  The input frequency can range from a few kilohertz to about 150 . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000


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