Light/Dark Detector

This handy little circuit can tell the difference between darkness and light, making it very useful for switching on and off signs, porch lights or other things when it gets dark or light.

Circuit diagram

Light/Dark Detector-Circuit diagram


  • R1 100K Pot
  • Q1 2N3904 NPN Transistor or 2N2222
  • Q2 NPN Phototransistor
  • RELAY 9V Relay
  • MISC Board, Wire, 9V Battery Snap (if battery used), Knob For R1


R1 Adjusts sensitivity