Analog Telephone Ring Detector
This circuit is designed to detect the ringing signal of an analog telephone. When the circuit detects the telephone ringing signal (60 to 105-volt RMS 20-Hertz sine wave) across the tip and ring, it triggers the Optocoupler's phototransistor output to provide a Low (0V) signal to the microcontroller. If no ringing signal is detected, it will give a High (5V) signal to the microcontroller.
To prevent fault triggering from unwanted signals, a 12V Zener diode is added to the circuit. Additionally, a 0.1uF, 250V capacitor is used for DC blocking, and a 10k 0.5W resistor limits the current passing through the optoisolator.
The output of this circuit can either drive a relay or be directly interfaced with other outputs such as LEDs.
