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Last Updated on: Thursday, December 07, 2017 02:05 PM

Hobby Circuits List

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Hobby Circuits designed by David Johnson
Circuit Name and Description

#'s          A         B - C          D - F        G - K          L          M - O          P - R         S - Z


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 29, 2004

Salvaged Broken Solar Cells   -  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 Dave Johnson P.E.-January 01, 2012

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 01, 2005

Self Powered AC Indicator   -  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 30, 2007

Service Request Beeper   -  A single press of a pushbutton switch turns on a beeper for one second but can’t be activated again for 60 seconds.   . . . Hobby Circuit designed by David A. Johnson P.E.-July 09, 2006

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 21, 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 21, 2005

Shop 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 Johnson P.E.-November 29, 2004

Signal Period to Voltage Converter   -  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 Dave Johnson P.E.-June 04, 2000



Simple Circuit Negative Supply Circuit   -  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 A. Johnson P.E.-July 13, 2006

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 12, 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 08, 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 04, 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 07, 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 20, 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 25, 2007

Solar Insulation 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 01, 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 15, 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 26, 2017

Solar_Path_Light_1w   -  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 23, 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 01, 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 03, 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 10, 2009

SSR Requires Ultra Low Control Current   -  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 David Johnson P.E.-June 03, 2000

Star-Trek Next-Generation 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 David Johnson P.E.-October 12, 2001

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 27, 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 09, 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 24, 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 09, 2006

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 09, 2006

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 Johnson P.E.-July 09, 2006

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 Johnson P.E.-July 09, 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 20, 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 09, 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 10, 2000

Ten 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 Johnson P.E.-January 29, 2008

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 02, 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 20, 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 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 17, 2000

Tests UL Ground Resistance   -  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 indicate if in fact the resistance is less than 0.1ohms - modified on july 10, 2006 . . . Hobby Circuit designed by David Johnson P.E.-April 28, 2001

Thermostat for 12v Cooling Fan   -  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 David A. Johnson P.E.-February 06, 2011

Three Volt Universal LED Flasher   -  The 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, white and . . . Hobby Circuit designed by David A. Johnson P.E.-June 22, 2008

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 24, 1998

Three-Volt 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 22, 2009

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 18, 2009

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 David A. Johnson P.E.-May 07, 2006

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 11, 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 29, 2000

Touch Switch 240VAC   -  This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load . . . Hobby Circuit designed by Dave Johnson P.E.-February 12, 2002

Touch Switch controls a 120 VAC load   -  This is a capacitance based touch on/touch off switch, which can control power to a 200 watt 120vac load . . . Hobby Circuit designed by David A. Johnson P.E.-July 11, 2006

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 11, 2002

Transistor-Switch Detects 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 A. Johnson P.E.-July 09, 2006

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 26, 2008

Transmitter for Line Powered Xenon Flash   -  This line powered xenon flash circuit drives a small camera type flash tube.  It has an optical isolator to allow the flash to be safely triggered from some remote device.  A flash rate of 2Hz is possible with the circuit . . . Hobby Circuit designed by Dave Johnson P.E.-June 09, 2000

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 21, 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 26, 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 16, 2017

TV-VCR Light Source 40Khz Repeater   -  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 David A. Johnson P.E.-June 04, 2000

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 10, 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 18, 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 18, 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 11, 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 20, 2008

Ultra_Low_Control_Current_Isolated_Relay   -  The circuit uses a classic capacitor “voltage dropper” to capture a bit of power from the AC power line and store some energy in a capacitor.  The energy is then used to activate a latching relay.  The relay only needs short 20ms pulses to open or close the relay contacts.  So, the average power needed to hold the relay state is very low. . . . Circuit by David Johnson P.E.-September 23, 2017

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 29, 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 20, 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 15, 2006

Use Color Film Negative to Make an Optical Filter   -  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 Dave Johnson P.E.-January 01, 1998

Variable AC Power Supply   -  in electronics, it is often handy to have a fully isolated variable AC power supply.  With such a device you can safely test various AC powered circuits and even probe around in the circuit with an oscilloscope.  The hobby circuit below was designed to . . . Hobby Circuit designed by Dave Johnson P.E.-July 08, 2000

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 01, 2002

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 Johnson P.E.-December 01, 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 15, 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 28, 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 03, 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 08, 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 00, 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 24, 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 00, 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 09, 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 12, 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 11, 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 24, 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 09, 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 03, 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 11, 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 01, 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 27, 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 15, 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 27, 2003

Wire Loop Security Alarm   -  A wire is looped through various objects which are to be monitored.  The loop wire is typically an aircraft cable but could also be any insulated stranded wire.  The loop is attached to a table or desk.  Should the loop be cut, the alarm is sounded . . . Hobby Circuit designed by David A. Johnson P.E.-July 09, 2006

Wireless RFID Smart Key Detector   -  The hobby circuit below was designed to indicate if a key held over the small coil of wire contains a 125KHz RFID chip.  These chips are often used in smart car ignition keys.  The left side circuit forms a 125KHz power oscillator, which drives the 680uH a . . . Hobby Circuit designed by David Johnson P.E.-July 09, 2006

Xenon Flash Receiver-AIR Transparency 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 17, 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 17, 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 17, 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 26, 2000


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