LDR light/dark activated relay Switch:  (electronic circuit added 4/05)

LED Photo Sensor Circuit:  Here's a circuit that takes advantage of the photo-voltaic voltage of an ordinary LED. The LED voltage is buffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain of about 20. This produces a change of about 5 volts at the output from darkness to bright light. The 100K potentiometer can be set so that the output is around 7 volts in darkness and falls to about 2 volts in bright light... (added 4/05)

Light Activated Detector Circuit (LM393/LM555):  (circuit diagram added 05/07)

Light activated relay #1:  (electronic circuit added 4/05)

Light Activated Relay #2:  (electronic circuit added 4/05)

Light detector 2:  (diagram added 05/07)

Light sensing RF transmitter:  (electronic circuit added 05/07)

Light Sensing RF Transmitter:  (electronic circuit added 7/03)

Light Sensor Adapter for Oscilloscope:  (electronic circuit added 4/05)

Light/Dark Detector:  (circuit / schematic design added 6/06)

Light/dark detector:  (schematic / circuit added 05/07)

Lightning Activated Camera Shutter Trigger 2:  (diagram added 05/07)

Lightning Activated Camera Shutter Trigger:  (diagram added 05/07)

Light-sensitive Alarm:  The circuit detects a sudden shadow falling on the light-sensor and sounds the bleeper when this happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking this beam will then cause the bleeper to sound. The light sensor is an LDR (light-dependant resistor), this has a low resistance in bright light and a high resistance in dim light..... (added 09/05)

RF Transmitter Light Sensing:  (electronic schematic / circuit added 4/02)

Sensing Light with a Programmable Gain Amplifier:  Microchip Application Note Published 28-Mar-03   (app note added 2/06)

Simple light sensor adapter for Oscilloscope:  (electronic circuit added 4/05)

Sun-Up Alarm:  The Sun-Up Alarm can be used to provide a audible alarm for when the sun comes up or it can be used in a dark area and detect when a light comes on. It can also be used to detect a light beam, headlights etc. The circuit works as follows. The phototransistor is very sensitive to light. (Any phototransistor will work fine) The sun shining on this device will provide a high to one of the NAND gates. This will cause another NAND gate to oscillate which will drive another gate to output a 100hz tone. The transistor provides drive for the speaker. (added 09/05)