Call duration counter

[priyanhav]

Hi,
I need to make a counter to count the call duration of outgoing calls from a phone line(copper connection). Do you have any solution...?

 

[Leftyretro]

Hi,
I need to make a counter to count the call duration of outgoing calls from a phone line(copper connection). Do you have any solution...?

Not a specific solution or circuit, but the key is the phone line voltages.

When the phone is not in use (on-hook) the line voltage is around 48vdc, with a possible 100VAC 20hz "ringing" volatge when an in-coming call is being signaled.

Upon answering a call (going off-hook) the line voltage drops to around 6-8VDC with a few volts of AC (the audio signal). So a call duration timer would best be triggered on the transistions of these various voltage levels.

A pretty simple solution with todays micro controllers that have built in A/D convertors.

There are laws and rules in most countries about the proper interfacing to telephone lines, so any commerical application would have to use approprate interface components or seeks approvals depending on the country in question.
Lefty

 

[crutschow]

One usual requirement is that any circuit you attach be isolated from earth ground. An opto isolator will meet that requirement. And the circuit should draw no more than a few milliamps from the line when the line is idle at 48Vdc or the central office will think that a phone has been picked up (off hook).