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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


Darkroom Camera Shutter Timer  -  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 . . . Circuit by Dave Johnson P.E.-June 11, 2000

Day/Night Switch-24vAC Powered  -  Many low voltage outdoor lighting systems use 24vac to power the devices.  The low voltage is much safer to use outside than standard 120vac or 240vac power systems.  The circuit below is designed to turn off power to outdoor lights during daylight and turn them back on at night . . . Circuit by David A. Johnson P.E.-December 26, 2010

DC Current Indicator #4  -  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 . . . Circuit by Dave Johnson P.E.-December 15, 2008

DC Current Indicator 6  -  The circuit below is a simple way to indicate when DC current is flowing through a wire.  The circuit is designed so it will turn on a LED indicator light, whenever the voltage drop across a shunt resistor exceeds about 0.1volts.  The value of the resist . . . Circuit by David A. Johnson P.E.-October 16, 2005

Delayed Pulse Generator  -  This circuit generates a short 10ms pulse 15 minutes after a “start” pushbutton switch is activated . . . Circuit by David A. Johnson P.E.-December 18, 2004

Detect the narrow 1uS pulses  -  This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit.  The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter.  The circuit is designed to process a pulse frequency . . . Circuit by David A. Johnson P.E.-March 11, 1999

Detects Shadow Activated Motion  -  This circuit can sound an alarm when the shadow of a hand or arm moves over  two small photo diodes . . . Circuit by Dave Johnson P.E.-August 21, 2005

D-Flip/Flop 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 . . . Circuit by David A. Johnson P.E.-March 26, 2002

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

Do It Yourself Portable 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 . . . Circuit by Dave Johnson P.E.-June 04, 2000

Does Power Line have 120 VAC or 240 VAC?  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . Circuit by Dave Johnson P.E.-July 09, 2006

Doggie Water Dispenser Alarm  -  I have two Beagle dogs.  They drink a lot of water; they go through about 3 gallons a week.  They have two water dispensers.  One is on the inside and second unit on the outside of my house.  Often, I forget to fill one of their two jugs and discover them completely empty.  I do worry that my dogs might get dehydrated if I ever forget to fill both of their containers . . . Circuit by David A. Johnson P.E.-June 04, 2012

Door Knob Alarm  -  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 placed around the inside doorknob of a wood door.  Anyone grabbing the knob from the outside is detected . . . Circuit by David Johnson P.E.-May 29, 2000

Door Knock Beeper  -  in some apartment buildings and homes, not equipped with a door bell, it may be hard to hear someone knocking on the front door.  The circuit below provides a means to activate a loud beeping sound inside, whenever someone knocks on the door . . . Circuit by Dave Johnson P.E.-June 14, 2009

Dusk To Dawn Latching Relay  -  Over the years I have had to replace many dusk to dawn switches for outside lights.  These switches turn on a light during the night, then switch them off during daylight hours.  Some commercial switches are not sensitive enough to detect when the sun is out.  Others flutter badly during the dawn and dusk switching transitions, burning out compact fluorescent lamps, which do not like rapid power fluctuations. . . . Circuit by David Johnson P.E.-July 31, 2017

Eight-Solar Cell Lithium Ion Battery Charger Version 3  -  A single lithium ion battery cell needs 4.  2v from a current source to insure it is fully charged.  A solar panel with just enough individual cells wired in series to produce the needed voltage would be more efficient than one with more cells for a given surface area.  If you divide 4.  2 by eight you come up with a figure of 0.  525 volts . . . Circuit by David A. Johnson P.E.-May 21, 2013

Either indicates 120 VAC or 240 VAC   -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . Circuit by David A. Johnson P.E.-July 09, 2006

Electric Field Disturbance Monitor-Alarm Discriminator + Battery Monitor  -  This schematic is the alarm discriminator plus the battery monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Circuit by David A. Johnson P.E.-March 26, 2002

Electric Field Disturbance Monitor-Front End + Power Supply Circuit  -  This schematic is the power supply and front-end sections of my field monitor, "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Circuit by David Johnson P.E.-March 26, 2002

Electrical Current Indicator  -  I designed this circuit as a simple current indicator for any load ranging from 40 watts to 250 watts.  The circuit turns on a small LED, whenever it detects current flowing to a remote load . . . Circuit by David Johnson P.E.-July 11, 2006

Electrical Device Harvests Earthworms  -  A 120vac or 240vac isolation transformer is used to force earthworms from the ground.   . . . Circuit by Dave Johnson P.E.-July 09, 2006

Electrodeless Water Level Monitor  -  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 . . . Circuit by David Johnson P.E.-July 09, 2006

Energy Harvesting using a Current Transformer  -  Energy harvesting is all the rage these days.  With some modern electronics, information from low power sensors can be sent to a distant data collection point using a low power RF transmitter.  To power these remote sensors, various energy sources can be . . . Circuit by David A. Johnson P.E.-January 17, 2010

Expanded Scale 12v Battery Voltage Monitor  -  The circuit below expands the battery voltage monitoring scale for a 12v battery so a more accurate voltage reading can be made.  The numbers printed onto the face of a standard 0v to 5v meter are changed so the scale ranges from 10v to 15v.  http://archil . . . Circuit by David Johnson P.E.-October 16, 2005

Fake Car Alarm Light  -  Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system . . . Circuit by David Johnson P.E.-July 09, 2006

Fake TV 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.   . . . Circuit by David Johnson P.E.-August 16, 2017

Fence Alarm  -  It was deer hunting season and cattle rancher was fed up with hunters cutting his barbed wire fence, to gain entrance onto his property.  The rancher want to be alerted when the barbed wire was cut.  The circuits below is placed inside a small plastic box which is placed inside the house.  A power jack for the +12v input and a terminal block for the earth ground and sensor wire connections are installed.  Two additional terminals were connected to a siren.  Central to the design is a closed pushbutton switch.  When pressed, the button would test the circuit by simulating a broken fence wire. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 2 Oct 2009

FET Input High Speed Light Detector  -  This circuit is yet another design that converts current from a PIN photo diode to a voltage.  It has a bandwidth that extends beyond 50MHz . . . Circuit by David Johnson P.E.-June 17, 2000

Finger 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 . . . Circuit by Dave Johnson P.E.-June 03, 2000

Fireplace Fizzle  -  Gas fireplace won't turn always turn on consistently. The pilot light was always on.  As illustrated below, a typical gas fireplace has a small pilot light flame.  The long term solution would be to replace the thermopile.  But, in the short term, the modification below would keep the thing going for another year or two.  As shown below, the solution was to connect a supercapacitor across the thermopile. This device acts like a battery and would provide the needed peak current to open the valve.  A 4.7 farad device was sufficient. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 4 Dec 2009

Five-Watt 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 David Johnson P.E.-June 27, 2000

Flameless LED Candle Power Boost  -  Most flameless candles are now quite simple circuits.  They consist of nothing more than a power switch, a battery and a special flickering LED. The special LED might work fine for a candle but it is not bright enough for the touch.  To boost the power of this flickering device, you can remove that LED from a flameless candle and insert into the circuit shown below.  This circuit extracts the flickering signal from the special LED and uses the signal to drive 5 super bright yellow LEDs at a peak current of 20ma each. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 13 Nov 2010

Flashing LED Power Indicator  -  This circuit flashes a power indicator LED to keep the average current low . . . Circuit by David Johnson P.E.-June 04, 2000

Fluid 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

Four 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

Freeze Alarm  -  I chose an accurate thermistor from Keystone as the temperature sensor for this circuit.  This device has a very specific resistance at a particular temperature.  According to the data sheet for the thermistor, the device will have a resistance of 361K at zero degrees C, which is the freezing point of water.  The thermistor is wired into a simple bridge circuit, which uses 1% resistors.  The bridge is connected to a very low power voltage comparator.  The circuit forms a switch circuit, which changes state, when the resistance of the thermistor reaches a certain value.  At zero degrees C the voltage at the non-inverting input of the comparator exceeds . . . Circuit Solution by Dave Johnson P.E.-May 07, 2006

Freezer Over-Temperature Alarm  -  This circuit turns on a beeper whenever the inside temperature of a freezer is greater than zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C.   . . . Circuit by Dave Johnson P.E.-July 20, 2006

Freezing Temperature Alarm  -  The circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).    A 9v battery powers the circuit.    The average 9ua current is so low that the battery should last for many years.   . . . Circuit by David Johnson P.E.-November 05, 2014

Full Wave Rectifier  -  This is a classic circuit that can accurately convert an AC signal to DC.  At 40KHz the input signal can be as low as 0.  05 volts peak to peak.   . . . Circuit by David Johnson P.E.-July 11, 2006


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