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Re: Simple circuit design please?



Hi Lee
            Just a suggestion ,not sure if feasable,,Dont pabx systems have a data line going to each hand set???? If so could you not trigger a 555 timer from the data signal? You would probably need more advice from a bt tech.........
 
Frank Mc
----- Original Message -----
From: Lee
Sent: Saturday, September 21, 2002 7:38 AM
Subject: [ukha_d] Simple circuit design please?

My DIY doorphone project seems to be nearing the top of pile again!
 
Can someone point me to/design a circuit that does the following?
 
A small microphone listens out for a loud noise. On hearing said noise, it opens a relay and starts a 3 min. (maybe adjustable) countdown.
When the countdown times out, the relay is closed and the system listens for the noise again.
 
This circuit when, combined with a gutted speakerphone, in a waterproof box, plugged into my PABX should function as a doorphone.
The visitor rings the doorbell. I pick up the nearest phone and dial 27. The PABX routes it to (wants left of) the speakerphone.
The 'piggybacked' circuit hears the speakerphone ring and closes the relay, this 'lifts the receiver's and I can talk to the visitor. 
When I'm done I hang up. Sometime later the circuit hangs up the speakerphone.
Nice :)
 
If the speakerphone could be powered by the phone line (hey isn't the speakerphone too?) that would save another wall wart. It should be OK because it's on the internal line??
 
Thoughts/suggestions/ridicule appreciated.
Lee.
 


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