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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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AC Current Controls 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 . . . Circuit by David A. Johnson P.E.-February 13, 2010

AC Current Test Source  -  With that resistor I can easily get up to 20 Amps of current.  So far, that has been high enough for me. The shunt resistor allows me to use my multimeter to accurately measure the AC voltage across the resistor, which is then proportional to the AC current.  In most of my tests, the AC current is a loop.  I might use the loop to calibrate a current transformer. . . . Dave Johnson, Lubbock TX (drawn October 27, 2017)-DC Magazines - 2nd Q 2012

AC Power Line High/Low Voltage Tester  -  if you wish to test a line-powered device under both a 15% high and a 15% low voltage condition, you can use the circuit below.  The circuit uses an 18v transformer with a 3 amp rating.  A double pole, double throw toggle switch then switches in the . . . Circuit by David A. Johnson P.E.-March 14, 2010

AC Power Loss Alarm  -  i’ve been having a problem with my instant hot water system lately.  This system circulates electrically heated water throughout my house using a small pump.  The pump is wired into a ground fault interrupter (GFi) module.  Every now and then, the GFI trips, leaving me without hot water . . . Circuit by David A. Johnson P.E.-March 29, 2009

Acceptable Voltage Indictor  -  I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed.  The circuit can also be used to adjust a particular voltage be within specific high or low limits.  The three LEDs will indicate if the voltage is . . . Circuit by David Johnson P.E.-February 09, 2002

Accurate One Shot Pulse Generator  -  This timer circuit draws only 3ua of current, and can be powered by DC supplies ranging from 3v to 15v.  The circuit produces accurate pulses whose time can easily be adjusted, based on the R1 x C1 time constant.  The time should not change,  . . . Circuit by David A. Johnson P.E.-September 02, 2010

Active AC Power Line Indicator  -  The circuit below is designed to detect when a power cable is active (hot).  When the flat sensor end of the device is held near an active AC line, the bright green LED on the top of the housing will glow.  It is powered by a 3v CR2032 lithium coin cell.  I . . . Circuit by David A. Johnson P.E.-May 02, 2005

Active Low Pass Filter Designs  -  This is a collection of inverting and non-inverting active low 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 . . . Circuit by David Johnson P.E.-December 10, 2000

Advertising Badge #1 with Flashing LED  -  I have seen numerous flashing light badges at trade shows and conventions.  They are often handed out as gifts to promote some business.  The devices often use inefficient circuits, which cause the battery power source to be quickly depleted . . . Circuit by David Johnson P.E.-March 26, 2002

Advertising Badge #2 with Flashing LED  -  This circuit uses a CD4013 dual D Flip/Flop IC .  A single lithium battery will provide months of continuous LED flashing.  It also has a tiny push-button switch to turn on and off the light flashing . . . Circuit by David A. Johnson P.E.-March 26, 2002

AIR Transparency Monitor, Xenon Flash Receiver  -  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 . . . Circuit by David A. Johnson P.E.-June 17, 2000

AIR Transparency Monitor, Xenon Flash Transmitter  -  This is the matching transmitter for the above receiver.  The transmitter launches powerful 1000-watt light pulses that last about 20 microseconds . . . Circuit by David A. Johnson P.E.-June 17, 2000

AIR Transparency Monitor-Pg2  -  This is Page 2 of the receiver circuit AiR TRANSPARENCY MONiTOR, XENON FLASH RECEiVER . . . Circuit by Dave Johnson P.E.-June 17, 2000

Alarm Beeps when the “Check Engine” Light Turns On  -  When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on . . . Circuit by Dave Johnson P.E.-July 09, 2006

Alarm sounds when Water is Detected  -  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 . . . Circuit by David Johnson P.E.-June 11, 2000

Alarm-Piezo Device Detects Motion  -  Using a piezo device, this circuit will activate a beeper whenever the circuit is moved.  It could be used as an earthquake alarm . . . Circuit by Dave Johnson P.E.-July 09, 2006

Alkaline Low Battery Voltage Indicator  -  in many battery powered systems a user would like to know when it was time to replace the batteries  before they were completely discharged, causing the device being powered to fail completely.  Alkaline batteries have an open cell voltage of about 1.5 volts.  As they are discharged, the voltage slowly drops.  When the cell voltage reaches about 1.  25v, they have delivered about 90% of their stored energy . . . Circuit by David Johnson P.E.-June 17, 2012

Alternating CW & CCW 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 . . . Circuit by David Johnson P.E.-September 05, 2010

Amplifier for a Xenon Lamp Flash Detector   -  This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp.  When connected to a flip/flop it can serve as an on on/off Controller . . . Circuit by David Johnson P.E.-December 15, 2002

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 . . . Circuit by David A. Johnson P.E.-June 27, 2000

Audio Activated Switch  -  This circuit will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal.    it could be used to turn on a relay, routing the signal to were it is needed.   . . . Circuit by Dave Johnson P.E.-July 09, 2006

Audio Amp + 3Khz Filter  -  This circuit is the audio amp section for a complete optical transmitter.  The circuit amplifies and filters the voice audio signals from an electret microphone.  The circuit is described in more detail in . . . Circuit by David Johnson P.E.-June 04, 2000

Audio Frequency Digital Noise Generator  -  When you need to test an audio circuit with broadband noise, this circuit works great.  It uses just three inexpensive C-MOS ICs  that generate a series of output pulses whose widths vary randomly.  I included a level control pot . . . Circuit by Dave Johnson P.E.-July 04, 2000

Automotive Electrical Voltage Indicator  -  This circuit will indicate if an automotive electrical circuit has zero volts or 12 volts present.   . . . Circuit by David A. Johnson P.E.-July 09, 2006


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