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.
0.47uF 250V Capacitor
10K 1/4 W Resistor
1K 1/4W Resistor
20V 1/4W Zener Diode
2N3904 NPN Transistor
4N27 Opto Isolator
Solid State Or Regular Relay (See Notes)
Case, Wire, Board
1. You may need to use a lower voltage zener for D1 and D2.
2. You can use a regular relay instead of a solid state relay, but the arcing accross the contacts may destroy it pretty quickly.
3. Be very sure that you have not accidentally connected 120V to the phone line when building and installing this circuit.