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Circuits designed by David Johnson, P.E.
Last Updated on: Thursday, December 07, 2017 02:06 PM


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David Johnson's Circuits
Circuits + Descriptions

from Dave's Circuits & Circuit Solutions


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


Bad Light Bulb Indicator  -  This request came from a Discover Circuits visitor.  He wanted to turn on a LED indicator light whenever a circuit detected a burned out tail/brake light on a car or truck.  The circuit shown below uses a transistor switch, to detect current flowing through a low value resistor . . . Circuit by David Johnson P.E.-December 17, 2006

Basement Doorbell Beeper  -  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 . . . Circuit by David Johnson P.E.-June 04, 2000

Battery Amp-hour Capacity 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 . . . Circuit by Dave Johnson P.E.-February 05, 2012

Battery Charge Current Indicator  -  This circuit turns on a LED whenever it detects at least 25ma of battery charge current . . . Circuit by David Johnson P.E.-July 23, 2006

Battery Charging Generator Starter  -  As the drawing below illustrates, the alternator was wired in a three phase “Y” configuration.  By making a few changes, the alternator could be turned into a very powerful motor.  The motor had the advantage of being able to draw current from the main battery bank.  Using this method, the company’s control system would just need to activate a relay to switch the battery between the rectifier circuit and a brushless DC motor configuration. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 17 Mar 2011

Battery Eliminator  -  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 . . . Circuit by Dave Johnson P.E.-June 01, 2005

Battery Free LED Flasher  -  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 . . . Circuit by Dave Johnson P.E.-December 11, 2010

Battery Polarity Reversal protects Circuit  -  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 . . . Circuit by David A. Johnson P.E.-March 09, 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 A. Johnson P.E.-July 09, 2006

Beeper that is Very Loud  -  This circuit generates a continuous tone instead of a pulsing one.  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 . . . Circuit by David Johnson P.E.-June 03, 2000

Bidirectional Solid State Relay  -  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 . . . Circuit by David Johnson P.E.-May 01, 2009

Broad Band 2Mhz 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 . . . Circuit by Dave Johnson P.E.-June 09, 2000

Brush Motor Speed 12V 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 . . . Circuit by Dave Johnson P.E.-May 05, 2007

Button Battery Cell Internal Resistance Measurement Circuit  -  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 . . . Circuit by David A. Johnson P.E.-July 11, 2006

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 . . . Circuit by David A. Johnson P.E.-June 13, 2000

Caps Provide Voltage Boost to Series Regulator  -  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 . . . Circuit by David A. Johnson P.E.-December 24, 1998

Cascode Light Receiver Circuit  -  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 . . . Circuit by David A. Johnson P.E.-February 15, 2002

Chain Link Fence Alarm  -  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,  . . . Circuit by Dave Johnson P.E.-February 22, 2009

Charge Coupled Bi-Directional Power 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 . . . Circuit by Dave Johnson P.E.-June 03, 2000

Charge Coupled MOSFET Relay #2  -  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 . . . Circuit by David Johnson P.E.-July 11, 2006

Charge Pump Efficiently Divides Supply Voltage   -  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value . . . Circuit by David A. Johnson P.E.-July 08, 2000

Charging a 24v Battery Charger from a 12v Source  -  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. . . . Circuit by David Johnson P.E.-July 23, 2017

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 . . . Circuit 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 . . . Circuit 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 . . . Circuit 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 . . . Circuit by David A. Johnson P.E.-July 09, 2006

Chicken Coop Door Opener with Timer  -  I received an email from a guy in Australia who was trying to build a chicken coop system.  He was planning on using a conventional garage door mechanism to open and close the 6 doors to the coop.  The commercial door opener opened and closed the doors with a single pushbutton.  He wanted to install a standard 220vac powered timer so he could open the doors in the morning and close them at night . . . Circuit by Dave Johnson P.E.-March 24, 2013

Circuit Forms Divide By 1.5 Counter  -  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 . . . Circuit by David Johnson P.E.-July 06, 2000

Circuit Forms Ideal Diode Function  -  This circuit uses a low power op amp and a p-channel FET to form a diode function with a very low 0.  05 volt voltage drop.  With the selected FET, the circuit can handle up to 2 amps of current.  Higher currents are possible with a FET with a lower channel . . . Circuit by David A. Johnson P.E.-March 06, 2011

Circuit 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 . . . Circuit by David A. Johnson P.E.-March 09, 2002

Classic Linear 5v 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 . . . Circuit by David Johnson P.E.-October 31, 2010

Classic Linear 5v 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 . . . Circuit by David Johnson P.E.-October 31, 2010

Classic 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 . . . Circuit by David A. Johnson P.E.-December 13, 2004

Clear Fluid Detector #2  -  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 . . . Circuit by Dave Johnson P.E.-July 11, 2006

Clear Fluid Monitor  -  An optical technique seemed like the best choice for this application.  The simple circuit below works well but only when the light detector can be shielded from bright ambient light.  A visible red LED launches light pulses through the side of the clear tubing and emerges at the opposite side.  A phototransistor forms a light detector.  The circuit takes advantage of the lens effect from any clear fluid inside the tubing.  When there is only air in the tubing, the light scatters and spreads out quickly before it reaches the phototransistor . . . Circuit Solution by David Johnson P.E.-May 01, 2005

Click Sound Generator  -  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 . . . Circuit by Dave Johnson P.E.-January 10, 2009

Closet Light with Automatic 3-Minute Timeout  -  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 is only a few microamps. A set of fresh batteries should last at least 200 light applications. The circuit uses a Schmitt trigger inverter and two transistors. When the pushbutton switch S1 is pressed, the 10uF capacitor C1 is discharged. . .  David A. Johnson P.E.-November 14, 2010

CMOS Inverter 125khz LC 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 . . . Circuit by Dave Johnson P.E.-March 12, 2002

CMOS Inverter Parallel LC Oscillator  -  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 . . . Circuit by David Johnson P.E.-March 12, 2002

CMOS Logic Inverter Amplifier Characteristics  -  This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . Circuit by Dave Johnson P.E.-February 11, 2006

CMOS Nand Gate Forms 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 . . . Circuit by David A. Johnson P.E.-March 12, 2002

CMOS Schmitt Trigger IC Makes VCO  -  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 . . . Circuit by David Johnson P.E.-December 01, 2002

Coin Operated Telescope  -  My client owns several coin operated telescopes.  Their problem is that the battery needs to be replaced every few months.  I needed to design a new circuit to control the shutter.  The solenoid used was too power-hungry.   . . . Dave Johnson, Lubbock TX (Jan 30, 2010)-DC Magazine - Issue 6 Feb 2010

Color Film Makes 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 . . . Circuit by David A. Johnson P.E.-January 01, 1998

Computer Controlled 100ma Current Source  -  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 . . . Circuit by David A. Johnson P.E.-July 08, 2008

Computer Controlled PWM Circuit  -  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 . . . Circuit by David A. Johnson P.E.-October 31, 2010

Convert Solar Powered Light to Flasher  -  The circuit converts an off-the-shelf solar powered LED path light from producing a steady state light to one that produces a flash.  This circuit includes a tiny dual Schmitt trigger inverter IC which form the pulse generator and the daylight sensor circuit.  The circuit produces a positive pulse, which feeds the constant current LED driver circuit.   . . . Circuit by David Johnson P.E.-March 02, 2010

Corona Monitor  -  The circuit is powered by +5v and has an on-board voltage regulator as a reference voltage.  The current from a reversed biased photodiode is converted into a voltage using an op amp circuit.  That output voltage is fed to a voltage comparator circuit.  Included is an adjustment in the current to voltage converter, to calibrate the installed circuit . . . Dave Johnson, Lubbock TX (Mar 30, 2010)-DC Magazine - Issue 8 Apr 2010

Crystal Controlled 1KHz Signal Generator  -  A while back I got an email from a Discover Circuits visitor.  He needed a way to calibrate a RPM display for a big diesel engine.  The engine had a tachometer output which produced 12 pulses per rotation with an amplitude of about 5v RMS.  The full scale reading on the engine’s RPM speed display dial was 5, 000 . . . Circuit by David Johnson P.E.-April 28, 2012

Current Indicator for Battery Chargers  -  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 . . . Circuit by Dave Johnson P.E.-January 23, 2011

Current Injector  -  When you need to measure resistance down to a few micro ohms, this circuit works great.  It is powered from two "C" cell batteries and is designed to inject a well regulated one amp of current into the unknown resistance.  By measuring the voltage drop across the resistor with a digital voltmeter, the resistance value can be accurately measured.  The circuit also is equipped with a low battery monitor.  Published in Popular Electronics, November 1992 - modified on July 11, 2006  . . . Circuit by David Johnson P.E.-July 11, 2006

Cut Phone Line Alarm  -  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 . . . Circuit by David A. Johnson P.E.-October 16, 2005


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