I have gotten 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.
Parts:
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
U1__________4N27 Opto Isolator
RELAY_______Solid State Or Regular Relay (See Notes)
MISC________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.