What Is It:
This circuit indicates visually via an LED when the telephone line is ringing (incoming call). The LED can be replaced by an optocoupler to interface to another circuit to maintain isloation from the phone line.
Video:
How It Works:
Phone lines have many states. In Australia, the telephone exchange sends 50 VDC down the line at all times. When the exchange wants to put a call through to you, it tests your line and if it is 'on hook' (idle) it then sends roughly 75VAC RMS through. This operates the ringer inside your phone.
When the call is answered the ringer circuit is disconnected and the telephone is connected to the phone line. Doing this places resistance across the phone line allowing DC to flow. This also signals the exchange to stop sending the ringing voltage of 75VAC and they connect the call. When the line is measured during a call, a voltage meter will read roughly 9VDC.
So, for the purpose of this circuit we only want to pick up the high (so to speak) AC voltage.
Firstly (when the phone line is ringing) current will flow through Capacitor 1. This Non-Polarised Polyester Film Capacitor only allows AC to pass through. Hence they will block DC.
The Zener Diodes are placed in series to block low AC voltages. When the voltage exceeds their rating, breakdown occurs and current will pass through. In this case 20 volts.
The series resister is to limit current into the bridge rectifier.
The bridge rectifier is to rectify AC to DC.
R4 limits current into the LED while R3 prevents the LED from turning on until the phone rings.
Other events such as hanging up another telephone may cause the LED to flash momentarily. Therefore, if this circuit is going to be used in conjunction with another, testing and retesting the state of the LED will be necessary to prevent false triggering.
