Phone answering circuit

[ceejm1]

Hello.

 

I know there is a wealth of knowledge here regarding phone systems so hopefully this won't be too tricky...

 

I'm looking for a way to remotely 'lift' the handset on a phone line. I need to interface a simple contact closure to a phone line such that when the contact is closed the handset is 'lifted' and when opened it is 'replaced'. The device needs to plug into a standard phone outlet as though it is a normal phone. I do not want to canabalise or interface with an existing phone. It does not need to detect ringing and does not need to have any involvement with the audio, simply simulate a handset being lifted/replaced.

 

Isolation from the ringing voltage, etc. will be required.

 

Any thoughts?

 

Thanks in advance.

 

James

[timsabre]

The NC contacts of a simple relay would do this.... when the phone is on hook the relay shorts the line. To go off-hook you just open the contacts. That's how a normal phone works.

[ceejm1]

Thanks Tim,

 

Would that not prevent any other extensions on the line ringing/being able to use the line when the line was shorted?

 

James

[timsabre]

Yes it would, I have got it the wrong way round. And you didn't say anything about extensions.

 

When "on-hook" there is no connection.

When "off-hook" the telephone puts a loop across the line - if you need other devices on the line to continue to work then you will need to muck around with resistors to get the loop resistance right. If you just need it to tell the extchange that you are off hook then a straight short will work.

[ceejm1]

Thanks Tim, that makes more sense!

 

Can't ask for much simpler, will give it a go.

 

Cheers

James

[Owain]

Rather than a dead short, use a 600 R resistor (or one 500 R bell coil) across the line.

[timsabre]

Yes you're right, I'm trying to remember how we used to do this and it wasn't a short, we put a telephone handset across the line which would be about 600R.

A short would work but (depending what's on the other end, a PBX or a real exchange) it might cause a fault to be logged.