Detecting when a calling party hangs up the telephone line

Linden posted:

There is no local change of state when the far-end hangs-up.

Build a device to detect the voiceband signal amplitude, and use it to open the loop after there is no signal detected for several seconds. I would back that up with a timer to open the loop after about 3 minutes after the initial answer, no matter what.

If you need some ideas, email me.

Don

Don

Hi,

So ive got the ring detection circuit done fine.

Problem now is when I 'answer' the phone (by detecting the ring signal and making a microcontroller switch a relay to connect to a line transformer) I cant get the solid state relay or even a mechanical relay to switch off and disconnect the circuit.

I need to know how can I detect when the other party has hang up the phone so I can disconnect the line. Otherwise the line will stay connected indefinately and no one will be able to get through.

Help appreciated, thanks !

Jamie posted: last time i knew the polarity switches.

In which country or system?

Don

All I've ever been able to do is wait for the dial tone to come back.

Linden posted:

All I've ever been able to do is wait for the dial tone to come back.

I'm in Australia. I've heard some systems use polarity reversal im not sure about here and I dont really want to have to make something to detect dial tone frequencies.

With all the telephone operated devices out there surely there is an easy method to detect hang-up.

I may end up just forcing the other side to press a # to hang up. Otherwise time-out after not receiving a key press for a minute or two.

In the US there are no central offices that normally send polarity reversal on LINES. There are offices that send a reversal on TRUNKS, for PBX and some multi-function systems.

It isn't likely, but you can easily test if you receive polarity reversal from the C.O. by puting a voltmeter across the line while the far end hangs-up. Let us know what you find.

Don

I think this may be non-trivial. I've got an answering machine that purported to not save anything in the event that the caller hung up without leaving a message. But it usually gets confused by ambient line noise and records that until the dial tone comes back. I think if you set the threshold too low you get this symptom; set it too high, and soft-voiced callers won't register.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

I'm in Australia. I've heard some systems use polarity reversal im not sure about here and I dont really want to have to make something to detect dial tone frequencies.

With all the telephone operated devices out there surely there is an easy method to detect hang-up.

I may end up just forcing the other side to press a # to hang up. Otherwise time-out after not receiving a key press for a minute or two.

- Linden.

Linden posted:

(snip)

Which brings us back to needing a solution: Use a delay or use audio detection or a combination of both.

Your voltage readings indicate a problem in your design. The DC resistance of your equipment is too low. The equipment puting the line off hook should place less than about 230 Ohms DC across the line.

Don

Hmm, no polarity reversal when the call is hung up. 47.5V on hook and down to 1.5V when on-hook and connected to the line transformer.

Still stays at 1.5V when other caller hangs up. Need to disconnect line from the transformer to go back to 47.5V off hook condition.

- Linden.