Telephone line circuit

Thread Starter


Joined Oct 16, 2010

Newbie question. Just getting into electronics and have a telephone line circuit that I would like to understand a little more how it works for my own morbid curiosity.

I've uploaded a circuit diagram here.

The circuit is intended to sit between an exchange and the telephone, to enable a call logger to record any activity that occurs on the line. I understand the logger would record activity by sensing the voltage on the line to determine if the phone was on or off-hook.

Could anyone give me a few clues as to how it works ?


Joined Feb 28, 2009
The diagram you have posted uses a dual coil relay (RL1) with one of its coils in each line. When the phone goes off hook, the current increases and causes the relay to pick up, closing the contact which shorts the input to the logger. The internal working of the logger may simply count how many times the phone is off hook or, by use of an internal clock, keep a log of the time of day and duration of each call.


Joined Jul 21, 2012
In most countries in the world,it is illegal to attach unapproved circuits to the telephone service,so by all means analyse the circuit,but don't build one & try to use it,as you may end up in a world of hurt!:D