[HaBi 1]    Build a Line Relay

Build a Line Relay The Line Relay is a simple yet interesting device you can put on your phone line. It will do almost anything you want when your phone rings, such as turn on a light, answer the phone, turn off your stereo, etc. We will show some extension projects in future issues. The phone ringing is 90 volts AC, often superimposed on 45 volts DC. This 90 volts will operate a 115 V AC relay, which can in turn supply power from the power lines to any ordinary appliance. The relay is commonly available. However, a capacitor is needed to DC isolate the relay coil from phone line. Without it the relay will draw a dial tone even with the phone on-hook. The capacitor should be non-electrolytic, and as many microfarads as possible; 5 should do nicely. The contacts of the relay can operate any circuit you want. The diagram shows how the normally-open and the normally-closed contacts can be used to light Lamp B and extinguish(a good word to know) Lamp A when the phone rings. In order to prevent the relay from "chattering" (vibrating rapidly up & down) you can loosen the relay spring slightly or increase the size of the capacitor.

[HaBi 1]    Build a Line Relay