TAE Connect

Ring detector for analog telephone lines

This small circuit is used to convert the call signal from a telephone line into a potential-free switching signal. In combination with the Doorbell extension, this enables call notification via radio controlled doorbell. Other applications such as optical signaling are also possible.

Picture: Circuit board of the ring detector

Diodes are connected to the output in order to combine it with an additional pulse input. In the case of the doorbell extension, a pushbutton connected to the positive supply voltage can be integrated there, which is to be utilized in addition to the telephone signal.

Picture: Schematic of the ring detector

Circuit description

The capacitor C1 filters out the DC voltage that is present as a remote power supply in the idle state, and only allows the alternating ring voltage to pass through. The resistor R1 serves as a series resistor for the optocoupler. The two Zener diodes D1 and D2 ensure that small voltage pulses do not trigger the circuit. D3 protects the light-emitting diode in the optocoupler from voltages in the reverse direction. D4 and D5 combine an external switching signal, e.g. from a doorbell button, with the signal from the optocoupler without the telephone and doorbell affecting each other. If this feature is not required, D4 can be replaced by a jumper wire and D5 can be omitted.