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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


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+5v Powered Charge Pump Battery Charger  -  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 . . . Circuit by Dave Johnson P.E.-February 07, 2010

0.5v Negative Supply  -  Although not very efficient, this simple circuit, consisting of two LEDs and a photo diode, generates a negative voltage with a current level of a couple milliamps.  It is ideal for supplying a negative rail to low power ďrail to railĒ op amp circuits,  . . . Circuit by David A. Johnson P.E.-July 13, 2006

1 Amp Current Injector  -  I designed and built this thing many years ago.  I published the hobby circuit in the now defunct ďRadio ElecronicsĒ 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 C cells power the complete circuit.   . . . Circuit by David Johnson P.E.-July 11, 2006

1 Line Telephone Line Status Indicator  -  This circuit monitors one phone line . . . Circuit by David Johnson P.E.-June 10, 2000

1 Watt Fluorescent Lamp Night Light  -  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 . . . Circuit by David A. Johnson P.E.-March 26, 1999

1.5v LED Flasher Version A  -  Many published circuits that flash LEDs need 3 volts or more.  This circuit uses only a single inexpensive C-MOS IC and flashes the LED for a full year on a single 1.5 volt AA alkaline battery cell.  The circuit uses a charge pump technique to provide . . . Circuit by Dave Johnson P.E.-June 04, 2000

1.5V Touch Activated 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 for a touch capacitance change as small as 1 picofarad.   . . . Circuit by David A. Johnson P.E.-January 18, 2002

10 Amp UL 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 indicate . . . Circuit by David Johnson P.E.-April 28, 2001

10 LED Sequential Flasher using Low Powr  -  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 . . . Circuit by Dave Johnson P.E.-April 17, 2011

10 Second Ultra 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 . . . Circuit by David A. Johnson P.E.-May 29, 2000

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 computerís analog output into a current, with a full scale of 100ma.  The circuit shown requires a 9v DC supply . . . Circuit by David Johnson P.E.-July 08, 2008

10mhz to 20mhz Laser Light Detector  -  This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system.  It has good ambient light immunity . . . Circuit by Dave Johnson P.E.-June 14, 2000

10Mhz to 20Mhz Light Receiver  -  Light power to volts conversion = APPROX.  15mV per microwatt at 850nM . . . Circuit by David Johnson P.E.-July 11, 2006

120 VAC Low Voltage Alarm  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac.   . . . Circuit by David Johnson P.E.-July 09, 2006

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

120VAC Under Voltage Tester  -  Using a cheap 24vac transformer, this circuit can test a product under low 95vac conditions.    With the components show, it has a rating of 250 watts max.   . . . Circuit by David Johnson P.E.-July 11, 2006

125Khz LC Oscillator  -  This circuit adds more invertors in parallel to deliver more power.  It is similar to CMOS iNVERTERS FORM 125KHZ OSCiLLATOR.  Please note that the values shown are for 125KHz.   . . . Circuit by Dave Johnson P.E.-March 12, 2002

125Khz Oscillator from CMOS Inverters  -  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 . . . Circuit by David Johnson P.E.-March 12, 2002

125khz Wireless Smart Key Detector  -  This circuit will turn on an indicator light whenever it detects a smart car key containing an RFID chip . . . Circuit by Dave Johnson P.E.-July 09, 2006

12kv Power Supply  -  if you need about 12, 000 volts DC for an ion generator this circuit might be the ticket.  It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil.  The output signal is isolated from the power line . . . Circuit by David Johnson P.E.-March 27, 1999

12v Battery Load Cutout Circuit  -  Lead-acid batteries should not be discharged below a certain point if they are to last.  Also, you donít want certain 12v powered devices, which might be plugged into a carís cigarette lighter outlet, to drain the car battery down to a point where the car . . . Circuit by David A. Johnson P.E.-August 22, 2009

12v Cooling Fan Thermostat  -  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 . . . Circuit by David Johnson P.E.-February 06, 2011

12v Lamp Current Indicator  -  This circuit turns on a LED indictor light whenever DC current flows into a 12v lamp . . . Circuit by David Johnson P.E.-July 09, 2006

12v Light/Dark Lamp Switch  -  At night, this circuit will route current to a 12v lamp, turning it on . . . Circuit by Dave Johnson P.E.-July 09, 2006

12v to 19v Laptop Power Supply (Concept)  -  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.  Iíd 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.  Most were underpowered.  The other option was to use the existing AC to DC adapter and run it off power from a 12v to 120vac inverter . . . Circuit by Dave Johnson P.E.-July 26, 2009

12v Touch Switch Exciter Circuit  -  This circuit is designed to generate a 20KHz pseudo sine wave signal that can power about 50 remote touch activated switch circuits.    it can support a cable length of about 2500 feet.    A typical remote switch circuit is also shown as well as a receiver circuit for those switches.   . . . Circuit by David Johnson P.E.-February 10, 2002

175Khz Inductive Pulse Receiver  -  This circuit is discussed in more detail in the Experimenters Journal.  The receiverís 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 . . . Circuit by Dave Johnson P.E.-July 09, 2006

18v AC to DC Power Supply  -  This is a classic linear power supply which produces a regulated 18v, rated at about 1 amp . . . Circuit by David Johnson P.E.-July 09, 2006

1uS Light Pulse Receiver + Post Amp  -  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.  The circuit is described in more detail in the receiver section of my Handbook of Optical Through the Air Communications.    Note: The LF357 op amp is no longer available, this circuit is for reference only.   . . . Circuit by David Johnson P.E.-March 11, 1999

1w Solar Path Light2  -  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. . . . Circuit by David Johnson P.E.-May 23, 2017

2 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 . . . Circuit by David A. Johnson P.E.-June 10, 2000

2 Transistor 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 . . . Circuit by Dave Johnson P.E.-December 18, 2004

2.45ghz RF Signal Detector  -  This passive RF indicator is made from a few simple parts.  A 100uA moving coil meter is used to display the relative intensity of the RF signal.  This circuit can be used with some cell phones and many cordless telephones . . . Circuit by David Johnson P.E.-July 09, 2006

20 LED Night Light  -  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. . . . Circuit by David Johnson P.E.-July 24, 2017

200Mhz-400Mhz Voltage Controlled Oscillator (VCO)  -  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 . . . Circuit by Dave Johnson P.E.-June 03, 2002

20ma Current Loop Tester  -  A while back I needed to test a circuit at 240vac.  Since I needed to safely test the circuit using my oscilloscope, I needed the 240vac to be fully isolated from the power line.  So, to step up the voltage, I bought a small isolation transformer from Digikey . . . Circuit by Dave Johnson P.E.-November 02, 2004

20Mhz VCSEL 3mw Laser Test Circuit  -  This circuit takes advantage of some new vertical cavity surface emitting lasers (VCSEL) that donít require light output control circuits.  The circuit shows how to drive the device from a single high speed CMOS IC  . . . Circuit by Dave Johnson P.E.-June 17, 2000

240VAC to 5VDC 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 . . . Circuit by David Johnson P.E.-February 12, 2002

240VAC Touch Switch  -  This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load . . . Circuit by David Johnson P.E.-July 11, 2006

2uS Light Pulse Receiver  -  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 . . . Circuit by David A. Johnson P.E.-June 03, 2002

3 Volt Low Battery Voltage Flasher  -  This circuit is designed to monitor two alkaline cells (3v) that form the battery often used in portable electronic equipment . . . Circuit by Dave Johnson P.E.-December 24, 1998

300v Peak to Peak Signal Generator  -  This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times . . . Circuit by David Johnson P.E.-June 03, 2000

30Khz Light Receiver 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 . . . Circuit by Dave Johnson P.E.-March 07, 2002

32Khz Ultra Low Power 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 . . . Circuit by David A. Johnson P.E.-December 18, 2004

36-Hour Watchdog Timer  -  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 . . . Circuit by David Johnson P.E.-April 11, 2010

3Khz Low Pass Filter Plus 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 . . . Circuit by David A. Johnson P.E.-June 04, 2000

3-Minute Closet Light  -  I have a coat closet by my front door.  The thing is very dark at night and really needs some kind of illumination inside.  I could hire an electrician to install a light in the closet ceiling and then wire it to a toggle switch outside the door but that would cost me quite a bit of money . . . Circuit by Dave Johnson P.E.-December 25, 2011

3-Minute Closet Light #2  -  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 circuitís 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 . . . Circuit by David Johnson P.E.-November 14, 2010

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 . . . Circuit by David Johnson P.E.-June 17, 2000

3v Battery Powered White LED Driver  -  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 . . . Circuit by David Johnson P.E.-August 15, 2010

3v Capacitance Proximity Switch  -  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 . . . Circuit by Dave Johnson P.E.-February 12, 2002

3v Powered 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  . . . Circuit by David A. Johnson P.E.-February 18, 2009

3v Sweeping Siren Alarm  -  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 . . . Circuit by David A. Johnson P.E.-June 24, 2008

4 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 . . . Circuit by David A. Johnson P.E.-June 10, 2000

40Khz Laser Burst Detector  -  This circuit was originally designed to detect weak flashed of laser light bounced off of a fabric video projection screen.  It was used as part of a firearm training system.  It generates a 100mS output pulse whenever it detects a 3ms to 5ms-laser burst,  . . . Circuit by David Johnson P.E.-June 13, 2000

40Khz LED Test Signal Generator  -  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 . . . Circuit by Dave Johnson P.E.-February 14, 2002

40Khz Light Detector with High Ambient Light Immunity  -  This circuit is designed for detecting infrared light modulated at around 40KHz.  Itís feedback scheme cancels much of the DC component from ambient light.  Itís conversion factor is about 100 millivolts per microwatt of 900nm light.   . . . Circuit by Dave Johnson P.E.-July 11, 2006

40Khz Light Receiver Amp  -  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.   . . . Circuit by David Johnson P.E.-March 07, 2002

40KHz Light Receiver Front-End Circuit  -  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.   . . . Circuit by David Johnson P.E.-March 12, 1999

40KHz Limiting Amp  -  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 . . . Circuit by David A. Johnson P.E.-July 09, 2006

40KHz Modulated Light Detector  -  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 . . . Circuit by David A. Johnson P.E.-March 12, 1999

40khz TV-VCR Light Source 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.  Using the device, the remote can operate a TV or VCR over three times the normal distance . . . Circuit by David A. Johnson P.E.-June 04, 2000

40Khz Ultrasound Receiver  -  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 . . . Circuit by David A. Johnson P.E.-January 29, 2006

40Khz Voltage to Frequency Converter  -  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 . . . Circuit by David A. Johnson P.E.-December 12, 2004

4-20ma Current Loop Tester  -  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 . . . Circuit by Dave Johnson P.E.-November 03, 2004

5 Volt Momentary Operation 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 . . . Circuit by David A. Johnson P.E.-March 12, 2002

50/60Hz Sync Generator-Fully Isolated  -  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 . . . Circuit by David A. Johnson P.E.-December 12, 2004

50Mhz Broad Band 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 . . . Circuit by David Johnson P.E.-June 04, 2000

555 Timer Forms Simple PWM Motor Controller  -  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 . . . Circuit by Dave Johnson P.E.-June 30, 2006

5Mhz Broad Band  Optical Fiber Receiver  -  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 . . . Circuit by David A. Johnson P.E.-June 04, 2000

5v Capacitance Touch Activated Momentary Switch  -  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 . . . Circuit by David Johnson P.E.-July 08, 2000

5w Fluorescent Lamp Intensity Modulator  -  The circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light.  The 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 . . . Circuit by Dave Johnson P.E.-June 27, 2000

60 Watt Lamp with Auxiliary 14v LED 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 . . . Circuit by Dave Johnson P.E.-June 27, 2000

6v NMH/NiCd 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 . . . Circuit by Dave Johnson P.E.-December 12, 2004

6V to 24V DC Day/Night Switch  -  Using an inexpensive phototransistor, this circuit switches 12v to 24v power to a load, whenever the sun goes down . . . Circuit by Dave Johnson P.E.-July 09, 2006

8 Solar Cell LED Light  -  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 . . . Circuit by David Johnson P.E.-September 22, 2017

8-Solar Cell Lithium Ion Battery Charger Version 2  -  Jim Wilber looked at my design for a battery charger and sent me some suggested changes.  He suggested using the LTC4412 ideal diode IC to switch off the current path from the solar cell to the battery in darkness.  He also noted that the 2.5v regulator from Seiko I had selected was no longer in production and suggested using the shunt reference . . . Circuit by David A. Johnson P.E.-May 21, 2013

9v Battery Eliminator  -  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 circuit . . . Circuit by David Johnson P.E.-December 06, 2009

9v Battery Voltage Monitor  -  This circuit turns on a LED whenever the voltage of a standard 9v battery connected to the circuit drops below 7.  2 volts.  It uses a LTC1440 comparator, which also contains a 1.  18v reference diode.  In standby mode, the circuit draws only 4uA . . . Circuit by David A. Johnson P.E.-July 09, 2006

9v Powered Xenon Photoflash Controller  -  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 . . . Circuit by David A. Johnson P.E.-June 03, 2000

9V Pulse Generator  -  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 . . . Circuit by Dave Johnson P.E.-June 03, 2000

9v Solar Panels Converted 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. . . . Circuit by David Johnson P.E.-July 26, 2017


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