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.
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.
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.