Infrared Reflected Light Switch  -  Drawing only 30uA from a 3v supply, this circuit will detect a human finger with a range of about 1 inch.  The sensor uses an inexpensive infrared LED and a matching photo diode . . . [Circuit by David A. Johnson P.E.,  07/09/06]

Infrared Safety Switch  -  Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch . . . [Circuit by David Johnson P.E.,  07/11/06]

Inrush Current Limiter for an Incandescent Lamp   -  This circuit limits the large inrush current often associated with large incandescent lamps.  With the components shown the current is limited to 1 amp, but it could be scaled to any desired current . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

Isolated Remote Switch Circuit  -  There are many applications when a remote switch’s on/off status needs to be monitored. Reed relay sensors, toggle control switches and security alarms contacts are just a few examples.  Routing a switch directly to a computer system hundreds of feet away . . . [Circuit by David Johnson P.E.,  01/04/09]

Knocking rings Doorbell  -  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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/14/09]

LED Lighthouse Flasher for HO Model Train   -  HO train sets often have authentic looking scale model homes and buildings.  The hobby circuit below can be mounted inside a model lighthouse.  The electronic circuit drives a single LED lamp in such a way that it produces light which simulates the rotation . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/11/06]

Light to Frequency Converter  -  This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter.  A small PIN photo diode is used as the light detector.  The pulses produced are short, so in some applications you may want to stretch them or . . . [Hobby Circuit designed by Dave Johnson P.E.,  05/04/08]

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.,  06/30/08]

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 Dave Johnson P.E.,  08/21/05]

Low Current Flashing LED Power Indicator  -  This circuit flashes a power indicator LED to keep the average current low . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/04/00]

Low Power Sequential Flasher using 10 LEDs  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  04/17/11]

Microphone On/Off Switch Circuit is Pop Free  -  This circuit uses a simple technique to eliminate the usual “pop” sound when an electret type microphone is switched on or off.  The circuit is completely passive and will work on dynamic and electret type microphones . . . [Hobby Circuit designed by David Johnson P.E.,  02/15/02]

Modified LED Night Light  -  I bought some night lights a few months ago from Walmart here in Denver.  The devices were made by Elumina Lighting Technologies Inc. (www.eluminalighting.com) , their model LED-6200A.  I think I paid about $15 for a pack of three lights.  I wanted a light . . . [Hobby Circuit designed by David Johnson P.E.,  05/21/07]

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 Dave Johnson P.E.,  03/12/02]

Momentary 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. . . . [Hobby Circuit designed by David Johnson P.E.,  01/18/02]

Monitor ON/OFF status of a Remote Switch  -  There are many applications when a remote switch’s on/off status needs to be monitored. Reed relay sensors, toggle control switches and security alarms contacts are just a few examples.  Routing a switch directly to a computer system hundreds of feet away . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/04/09]

Monitor Water Seepage  -  A while back I received a request from a company who needed a water seepage detector.  They wanted a device that ran on 24vac and detected water using a fully isolated circuit.  They wanted a circuit which switched off 24vac to a load when water was detected . . . [Hobby Circuit designed by David Johnson P.E.,  01/16/11]

Motion Alarm Using piezo Device  -  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 David Johnson P.E.,  07/09/06]

Night Light from White LED  -  As many of you know, I have a pet peeve with poorly made LED night lights.  Often, the light from the LED quickly fades, so within months, the light is useless.  I have posted several versions of modified night lights using higher quality components . . . [Circuit by David Johnson P.E.,  11/06/08]

On/Off Latch Circuit  -  Often you want to turn on and off power to a device with a single pushbutton switch.  The circuit below performs this logic function with just a few parts and can operate from DC supplies between +3v to about 15v.  It uses two inverters within a 74C14 or a CD4069 hex inverter package.  Each press of the button toggles between the two on/off logic states. . . . [Circuit by David Johnson P.E.,  01/09/10]

On/off Pushbutton Switch with 30 Minute Timeout  -  I received an email from a Discover Circuits visitor.  He wanted to control power to a LED camping light he was building.  He wanted to turn the light on and off with a small pushbutton switch.  He also wanted a timer so the light would not stay on longer than 30 minutes at a time.  It seems that his kids often turned lights on when going to bed and then forgot to turn them off . . . [Circuit by Dave Johnson P.E.,  01/08/12]

On/Off Switch With Latching Relay  -  The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay with two coils.  The relay can handle about 10 amps of current, up to 250vac.  Each time the small pushbutton is pressed, the circuit either latches the relay . . . [Circuit by David A. Johnson P.E.,  12/24/09]

One-Watt White LED Night Light  -  This simple circuit is designed to plug into a standard AC electrical outlet.  It uses four super bright white light emitting diodes (LED) in conjunction with a capacitor coupled full wave rectifier circuit. The LEDs are mounted in a box and are angled slightly . . . [Hobby Circuit designed by David A. Johnson P.E.,  11/06/08]

Optical Filter using Color Film Negative  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/01/98]