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