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Simple circuit design please?


  • To: <ukha_d@xxxxxxx>
  • Subject: Simple circuit design please?
  • From: "Lee" <lee@xxxxxxx>
  • Date: Fri, 20 Sep 2002 22:38:31 +0100
  • Mailing-list: list ukha_d@xxxxxxx; contact ukha_d-owner@xxxxxxx
  • Reply-to: ukha_d@xxxxxxx

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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