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Circuits designed by David Johnson, P.E.
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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


Laser Diode Tester  -  This circuit can either be used to test laser diodes or as a general purpose modulated light source.  The circuit has an adjustable diode threshold current and an adjustable modulation frequency.  The frequency can be adjusted from below 30KHz to above 50MH . . . Circuit by David Johnson P.E.-March 20, 1999

Laser Used as Vehicular Traffic Counter  -  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 A. Johnson P.E.-July 09, 2006

Laser/LED Light Output Intensity Meter  -  This circuit uses a large 1cm X 1cm silicon PIN photo diode and a transimpedance amplifier to measure the light power output of infrared and visible LEDs and laser diodes.  It can be modified to produce almost any milliwatts to volts scale factor.  It can be . . . Circuit by Dave Johnson P.E.-July 08, 2000

Lead-Acid Battery Low Voltage Indicator  -  Lead-acid batteries should not be discharged below a certain voltage.  Discharging a cell below about 1.  8v and can often damage it, which will result in a lower amp-hour capacity.  Large battery banks are critical in off-grid energy storage systems.  One . . . Circuit by David A. Johnson P.E.-July 30, 2006

LED Driver using Constant Current  -  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 . . . Circuit by Dave Johnson P.E.-December 13, 2010

LED Flashing Badge #1  -  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 A. Johnson P.E.-March 26, 2002

LED Flashing Badge #2  -  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 Dave Johnson P.E.-March 26, 2002

LED High Power Flasher  -  Power LEDs are becoming more popular these days.  The DC input power to these devices ranges from 1 watt to 5 watts.  Normal LEDs only receive about 0.  05 watts.  At this higher power, these devices can emit a lot of light . . . Circuit by David Johnson P.E.-8/0/2008

LED Pilot Light powered by AC Line  -  This simple circuit can be used to light a LED indicator lamp, powered from the 120vac line.  I have included component values for 240vac lines as well . . . Circuit by David A. Johnson P.E.-July 09, 2006

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

Light Detectors with Ambient Light Compensation  -  These circuits were taken from a few application notes on infrared remote control devices.  They use a current compensation method to separate the modulated light pulses from ambient light.  They appear to have limited bandwidth and may only work at the 30K . . . Circuit by David A. Johnson P.E.-February 15, 2002

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 . . . Circuit by David A. Johnson P.E.-March 11, 1999

Light Receiver Works from 1Khz to Over 70Mhz  -  This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response.  It is designed around a PIN photo diode that is packaged for use with plastic optical fibers.  It can be used as an optical fiber receiver.  By using the open end of the optical fiber it can "sniff" out any modulated light signals . . . Circuit by Dave Johnson P.E.-June 11, 2000

Light/Dark 12V Switch  -  Often, for certain low voltage lighting systems; you would like to turn off the lights during the bright light of the day.  Most commercial day/night switches are designed for AC lighting.  The circuit below was designed for a 12v DC system.  But, it could be modified for other voltage as well.  It uses an inexpensive phototransistor as the light detector.   . . . Circuit by David Johnson P.E.-June 30, 2008

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

Lighthouse LED Flasher  -  This was originally designed for a model in a HO train set.  It simulates the behavior of the light from a lighthouse.  The LED intensity gradually increases, then flashes with a bright light and finally decreases slowly in intensity . . . Circuit by David Johnson P.E.-July 11, 2006

Line Powered 60hz Clock Generator  -  This circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit.  The optoisolator used provides 5000 volts of isolation between the power line and the logic side of the circuit . . . Circuit by Dave Johnson P.E.-June 27, 2000

Line Powered White LED Array  -  There has been a lot of excitement lately in the lighting industry.  Some companies are beginning to manufacture general lighting fixtures, which replace incandescent and fluorescent lamps, using LED lights.  When proper heat management is designed . . . Circuit by David A. Johnson P.E.-February 27, 2010

Line-Powered Xenon Flash Transmitter  -  This line powered xenon flash circuit drives a small camera type flash tube.  It has an optical isolator to allow the flash to be safely triggered from some remote device.  A flash rate of 2Hz is possible with the circuit . . . Circuit by David A. Johnson P.E.-June 09, 2000

Logic Controlled 12v DC Motor with Automatic Brake  -  There are times when you would like to control a DC motor using a simple +5v logic swing signal.  The circuit below performs this feat and also includes an automatic motor braking circuit.  This circuit will bring a fast moving DC motor to a stop in seconds.  The braking action is automatically applied whenever DC power is removed . . . Circuit by Dave Johnson P.E.-February 18, 2010

Long Period Computer 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 . . . Circuit by David Johnson P.E.-December 26, 1998

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 David Johnson P.E.-December 24, 1998

Low Frequency Oscillator draws only 2 Nanoamps  -  if you have hundreds of touch switches that need an excitation signal, then this circuit is what you need.  Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power . . . Circuit by Dave Johnson P.E.-July 11, 2006

Low Pass Active 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 A. Johnson P.E.-December 10, 2000

Low Power 100khz Light Receiver  -  By starving a high speed logic inverter for current, this circuit can produce a sensitive 100KHz light receiver circuit, which is immune to ambient light, but only drawing 100 micro amps from a 3 volt supply . . . Circuit by David Johnson P.E.-April 17, 2005

Low Power 40KHz Light Detector  -  This circuit is designed for detecting infrared light modulated at around 40KHz.  It’s feedback scheme cancels much of the DC component from ambient light.  It’s conversion factor is about 100 millivolts per microwatt of 900nm light.   . . . Circuit by David A. Johnson P.E.-July 11, 2006

Low Power AC to DC Power Supply  -  When you need some DC power fully isolated from the AC power line, this circuit may be just the ticket.  The circuit uses an inexpensive tiny off-the-shelf 6.  8mH common mode filter coil from with a split bobbin as a high frequency transformer.  The circuit routes short 130v pulses to the primary winding of the coil every half cycle of the 60Hz line frequency.  It uses a silicon bilateral voltage triggered switch called a “sidac” to discharge a 0.1uF high voltage capacitor . . . Circuit by Dave Johnson P.E.-November 23, 2011

Low Power Amp Monitors Light  -  The circuit below takes advantage of a low power op amp with a “rail to rail” feature.  The single resistor R1 defines the light sensitivity of the circuit.  The output voltage is directly proportional to the intensity of light striking the small photodiode . . . Circuit by David Johnson P.E.-September 01, 2010

Low Power H Bridge Motor Controller  -  This circuit uses a few logic gates & a 74AC240 to control power to a small gear motor . . . Circuit by David Johnson P.E.-July 13, 2006

Low Power 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 David Johnson P.E.-December 18, 2004

Low Power Universal LED Flasher  -  This low power LED flasher circuit draws about 100ua from any DC power supply ranging from 3 volts to 24 volts.  The 1.  0Hz 10ms LED pulsed current is held to a constant 10ma . . . Circuit by David Johnson P.E.-July 11, 2006

Low Value Capacitance Meter  -  This circuit was originally designed to measure the volume of fluid inside a medical syringe.  As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads . . . Circuit by David A. Johnson P.E.-December 12, 2004

Low Voltage 120vAC Power Line Alarm  -  This circuit will turn on a beeper when the line voltage drops below 100 volt AC.   . . . Circuit by David Johnson P.E.-July 09, 2006

Low Voltage H-Bridge  -  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 . . . Circuit by Dave Johnson P.E.-July 09, 2006

Low Voltage Latch Circuit  -  A Discover Circuits visitor wanted a simple on/off latch circuit which can turn on and off power to a small motor.  The kicker was that he wanted to power the circuit and the motor from a single 1.  2v NiMH battery cell.  Texas instruments comes to the rescue with their family of low voltage single gate IC s.  Specifically the SN74AUP1Q14 Schmitt trigger inverter will operate down to 0.  8v and lower.  When used with a good NPN transistor, the circuit below works quite nicely . . . Circuit by Dave Johnson P.E.-March 01, 2013

Low Voltage Latching Relay Driver  -  Using some small super capacitors, this circuit can latch and unlatch a mechanical relay with 10 amp contacts, from a small 3 volt power source.  By using a latching relay, power can be controlled to a load with a tiny battery . . . Circuit by Dave Johnson P.E.-July 09, 2006

Low-power optical interrupter  -  This circuit is great for battery-powered systems that use slotted type optical interrupters.  It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery . . . Circuit by David Johnson P.E.-June 04, 2000


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