Telephone Ring Detector

Analog Telephone Ring Detector

When this circuit detects the telephone ringing signal across tip and ring (60 to 105-volt RMS 20-Hertz sine wave), it will trigger the Optocoupler's phototransistor output and provide Low (0V) signal to microcontroller. Otherwise, it will give High (5V) to microcontroller.

12V Zener is added to prevent fault triggering from unwanted signals. 0.1uF, 250V capacitor is used for DC blocking and 10k 0.5W resistor is to limit the current passing through the optosiolator.

This circuit can drive relay or can be directly interface with any other outputs such as LEDs.