Telephone switching

[David M]

Hi,

I'm trying to design a circuit to disconnect a telephone if it has
been left off hook after a call and the exchange is sending a
disconnect signal.

The disconnect signal broadcast by this exchange is

400 or 900 Hz
with a 250ms ON 250ms OFF and repeated

I've looked at the circuit on the following page

http://www.p5taylor.btinternet.co.uk/default.htm

This shows how to detect the ringing signal that is modulated onto the
phone line.

Would I do something similar to detect the ON OFF disconnect pattern.

Any hints are welcome.


David

 

[John Woodgate]

I read in sci.electronics.design that David M <news^#NO@SPAM^moorhouse#.

Hi,

I'm trying to design a circuit to disconnect a telephone if it has
been left off hook after a call and the exchange is sending a
disconnect signal.

The disconnect signal broadcast by this exchange is

400 or 900 Hz
with a 250ms ON 250ms OFF and repeated

I've looked at the circuit on the following page

http://www.p5taylor.btinternet.co.uk/default.htm

This shows how to detect the ringing signal that is modulated onto the
phone line.

Would I do something similar to detect the ON OFF disconnect pattern.

Any hints are welcome.

No; that circuit detects the very high ringing voltage that works the
bell in old phones. You want something that detects voice-level tones.
It's not too difficult to do that. You could Google for 'tone detector'.

In principle, you have band-pass filters that pass only 400 Hz and 900
Hz. The outputs from these are rectified to produce the 250ms/250 ms
square wave. There are well-known techniques for identifying such a
signal by its timing and deriving a signal that will switch off the
phone, but it's not my field and I don't want to mislead you. I expect
someone else will advise on the way to do that.

 

[hamilton]

You may want to Google for "call progress" chips.

 

[Ross Herbert]

Can I enquire as to why you would want to do this? A means of opening
the line to the offending phone would be possible but this isn't going
to restore the line so that other calls could be received, so my
question is "what for?"



On Mon, 27 Sep 2004 22:31:54 +1200, David M

|Hi,
|
|I'm trying to design a circuit to disconnect a telephone if it has
|been left off hook after a call and the exchange is sending a
|disconnect signal.
|
|The disconnect signal broadcast by this exchange is
|
|400 or 900 Hz
|with a 250ms ON 250ms OFF and repeated
|
|I've looked at the circuit on the following page
|
|http://www.p5taylor.btinternet.co.uk/default.htm
|
|This shows how to detect the ringing signal that is modulated onto the
|phone line.
|
|Would I do something similar to detect the ON OFF disconnect pattern.
|
|Any hints are welcome.
|
|
|David

 

[Tim Wescott]

Can I enquire as to why you would want to do this? A means of opening
the line to the offending phone would be possible but this isn't going
to restore the line so that other calls could be received, so my
question is "what for?"



On Mon, 27 Sep 2004 22:31:54 +1200, David M

|Hi,
|
|I'm trying to design a circuit to disconnect a telephone if it has
|been left off hook after a call and the exchange is sending a
|disconnect signal.
|
|The disconnect signal broadcast by this exchange is
|
|400 or 900 Hz
|with a 250ms ON 250ms OFF and repeated
|
|I've looked at the circuit on the following page
|
|http://www.p5taylor.btinternet.co.uk/default.htm
|
|This shows how to detect the ringing signal that is modulated onto the
|phone line.
|
|Would I do something similar to detect the ON OFF disconnect pattern.
|
|Any hints are welcome.
|
|
|David

It would if you have more than one extension.

 

[Ross Herbert]

On Mon, 27 Sep 2004 20:56:07 -0700, Tim Wescott

|Ross Herbert wrote:
|
|> Can I enquire as to why you would want to do this? A means of opening
|> the line to the offending phone would be possible but this isn't going
|> to restore the line so that other calls could be received, so my
|> question is "what for?"
|>
|>
|>
|> On Mon, 27 Sep 2004 22:31:54 +1200, David M
|>
|> |Hi,
|> |
|> |I'm trying to design a circuit to disconnect a telephone if it has
|> |been left off hook after a call and the exchange is sending a
|> |disconnect signal.
|> |
|> |The disconnect signal broadcast by this exchange is
|> |
|> |400 or 900 Hz
|> |with a 250ms ON 250ms OFF and repeated
|> |
|> |I've looked at the circuit on the following page
|> |
|> |http://www.p5taylor.btinternet.co.uk/default.htm
|> |
|> |This shows how to detect the ringing signal that is modulated onto the
|> |phone line.
|> |
|> |Would I do something similar to detect the ON OFF disconnect pattern.
|> |
|> |Any hints are welcome.
|> |
|> |
|> |David
|>
|It would if you have more than one extension.

I assume you mean a parallel connection? Yes, in that case it would
have some use. Hopefully, both handsets would be properly on-hook

 

[David M]

Can I enquire as to why you would want to do this? A means of opening
the line to the offending phone would be possible but this isn't going
to restore the line so that other calls could be received, so my
question is "what for?"

My grandmother lives on her own about 30 minutes away. She's 90 and
sometimes forgets to hang up the phone after someone has called.
The exchange does not reset the line because her phone is a cordless
and holds its connection to the base open.

However I have figured out that if i disconnect the power to the base
for 5 seconds and then reconnect, it resets the cordless handset.
This also resets the telecom line.

So, my plan is to detect the disconnect signal, cycle the (9v) power
to the base and voila - granny's phone is back in action.

Does anyone have any other suggestions.

 

[Rich Grise]

My grandmother lives on her own about 30 minutes away. She's 90 and
sometimes forgets to hang up the phone after someone has called.
The exchange does not reset the line because her phone is a cordless
and holds its connection to the base open.

However I have figured out that if i disconnect the power to the base
for 5 seconds and then reconnect, it resets the cordless handset.
This also resets the telecom line.

So, my plan is to detect the disconnect signal, cycle the (9v) power
to the base and voila - granny's phone is back in action.

Does anyone have any other suggestions.

Yes - Get Granny a corded extension for every room. Maybe 2 for big rooms.


Cheers!
Rich

 

[Rich Grise]

So, my plan is to detect the disconnect signal, cycle the (9v) power
to the base and voila - granny's phone is back in action.

Does anyone have any other suggestions.

I suggested, rather brusquely, to get a bunch of corded extensions.
I'm not being mean here - what happens to the handheld when the
battery runs down? What you've proposed practically guarantees that
it will.

Good Luck!
Rich

 

[Ross Herbert]

|On Tuesday 28 September 2004 05:06 am, David M did deign to grace us with
|the following:
|
|> So, my plan is to detect the disconnect signal, cycle the (9v) power
|> to the base and voila - granny's phone is back in action.
|>
|> Does anyone have any other suggestions.
|
|I suggested, rather brusquely, to get a bunch of corded extensions.
|I'm not being mean here - what happens to the handheld when the
|battery runs down? What you've proposed practically guarantees that
|it will.
|
|Good Luck!
|Rich


Good point Rich. Since the cordless handset is not sitting correctly
in the base unit the battery will run flat because it isn't being
charged, so Gran will still be unable to receive calls.

David's scheme will work if there is at least one normal handset on
the line in addition to the cordless but the handset for this phone
must always be correctly on-hook. Should the cordless battery go flat
at least the normal phone will ring.