Ringing Phone Light Flasher

I have received several emails asking how to connect up some lights so that when the phone rings, they flash. This is very useful in a situation were there is lots of noise and it is impossable to hear the phone, such as a workshop. Here is such a device. The ring detect part of this circuit came from http://www.hut.fi/Misc/Electronics/circuits/telephone_ringer.html

Circuit diagram

Ringing Phone Light Flasher-Circuit diagram


  • C1 0.47uF 250V Capacitor
  • R1, R2 10K 1/4 W Resistor
  • R3 1K 1/4W Resistor
  • D1, D2 20V 1/4W Zener Diode
  • D3 1N4148 Diode
  • Q1 2N3904 NPN Transistor or 2N2222
  • U1 4N27 Opto Isolator
  • RELAY Solid State Or Regular Relay (See Notes)
  • MISC Case, Wire, Board


  • You may need to use a lower voltage zener for D1 and D2.
  • You can use a regular relay instead of a solid state relay, but the arcing accross the contacts may destroy it pretty quickly.
  • Be very sure that you have not accidentally connected 120V to the phone line when building and installing this circuit.