off-hook detector


Joined Nov 20, 2008
The voltage polarity (+/-) of the two lines it not consistant. The diodes allow you to not care.

I don't like the circuit because the line can be connected to ground and there is no isolation from lightning and surges.

Use an opto-isolator after the bridge.

Have fun, but don't kill the phone line.


Joined Nov 29, 2005
Those diodes connected to the telco line are not a bridge rectifier circuit. It is called a polarity steering circuit.

As suggested, an optocoupler is a much better way to detect line seizing, connected with precautions to not blow it at the first ring.

Thread Starter


Joined Jan 5, 2010
I have connected an optocoupler after the bridge but output signal is affected by the presence of ringing signal. is there any way to solve this problem? prevent ringing signal from activating off-hook detector circuit


Joined Nov 20, 2008
Normally on/off-hook detection is done at the phone company end of the line where the ring signal comes from as well, so it's not a problem.

An on/off-hook detector is usually not required at the customer end because on/off-hook originates at the customer as well.:confused:

However, you could try to filter out the ringer signal with a 0.1uF capacitor at the input to the opamp. The down side is that this cap will also delay detection of on/off-hook also by ~1 second. Can you live with that? Try other values.

Good Luck,


Joined Sep 26, 2009
Are you sure it isn't an off-hook detector?
When a phone is taken off hook, there is a termination placed across the line. This causes the phone line to draw current from the Central Office Battery Distribution and detection circuit. This is sensed as off hook and a dial tone is placed upon the line with the battery voltage. In modern switches the battery voltage may drop significantly and can be as low as 2-3Vols under load. But the whole purpose is to detect when a telephone is off hook.
The diodes basically keep the circuit working and help to keep it from chattering due to the voice component in a telephone call.
When the line goes back on-hook the connection is broken and the line circuit detects a hang up and resets for the next call attempt.
I don't see why an on-hook condition needs to be detected, since off-hook detectors essentially perform this function by their nature and design.
But if You say it is Ob-Hook - then I;ll take your word for it.
The image the op posted says Off-Hook detector circuit.... so you are right is is for Off-Hook detection, not On-Hook.

B. Morse