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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


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 16, 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 11, 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 22, 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 30, 2008

 

Battery-Powered Freezer Alarm   -  A typical freezer can contain hundreds of dollars with of food.  If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled.  The hobby circuit below is a battery powered alarm system, which is attached . . . Hobby Circuit designed by David Johnson P.E.-July 20, 2006

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 26, 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 09, 2006

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 A. Johnson P.E.-July 09, 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 03, 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 31, 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 05, 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 04, 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 01, 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 11, 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 25, 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 14, 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 09, 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 12, 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 08, 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 18, 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 13, 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 11, 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 26, 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 04, 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 02, 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 11, 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 07, 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 01, 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 00, 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 26, 2000

Check Engine Beeper   -  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 A. Johnson P.E.-July 09, 2006

Check Engine Beeper   -  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 A. Johnson P.E.-July 09, 2006

Check Engine Beeper   -  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 Dave Johnson P.E.-July 09, 2006

Check Engine Beeper   -  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 09, 2006

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 08, 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 23, 2011

Circuit indicates Battery Charging Current   -  Anyone who has ever had a shocking experience on an especially dry day is acutely aware of the kinds of high voltage that can be generated from a simple walk across a carpeted floor.  Those same finger to metal sparks can destroy or disrupt sensitive elect . . . Hobby Circuit designed by David A. Johnson P.E.-January 23, 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 07, 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 03, 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 09, 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 26, 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 09, 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 31, 2010

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 31, 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 18, 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 04, 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 12, 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 12, 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 03, 2000

Collection 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 Johnson P.E.-February 11, 2002

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 21, 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 31, 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.-December 26, 1998

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 13, 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 25, 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 09, 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 26, 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 09, 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 03, 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 11, 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 04, 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 06, 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 09, 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 13, 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 08, 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 05, 2010


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


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