Telephone Ring Detection

Approximate values for telephone line conditions are as follows.

On-hook - 40 – 50 VDC (usually 48V)

Off-hook - 4 – 6 VDC (usually 5V)

Ringing - 90 – 100 VAC

As for the literature the ring detection circuit used in modems is as follows.

The purpose of the two Zener diodes is to avoid detection of small AC transients as ringing. The capacitor used here will block the DC and allow only the AC ringing signal to pass. The diode used in parallel with the photo diode in the opto-isolator is to protect photo diode being damaged by the reverse current.

The frequency of the AC ringing signal is usually 20 – 40 Hz. The transistor in the opto-isollator can be used as a switch. If you connect an LED to see how the switching occurs, you can witness that the LED blinks in either positive or negative half of the ringing signal. If you are planning to get the ringing signal as a logic signal this is not desirable.

To get the ringing signal as a logic signal we can use a full-bridge rectifier circuit to rectify the AC ringing signal.

Lifting the receiver or going off-hook

After detecting the ringing, most of us need our circuitry to answer the call in order carry out further processing like detecting the DTMF entered.

Usually when we lift the receiver the two telephone lines are connected thus closing the loop. When the loop is closed a current around 40mA flows in the line. The Central Office (CO) senses this current.

The standard impedance of a telephone when off-hook is 680 ohms. Hanging a 680 ohms resistor across the telephone line will drop the voltage from 48 V to about 5 V. This will cause the line to go active.