Off-topic off hook indicator

From: Russ Blakeman <>
Date: Fri Jul 17 10:02:05 1998

Julian Richardson wrote:

> Ok, so given that there are people out there who know about phone
> systems, how would I build a device that sat on the phone line and could
> detect whether the line was in use or not - it's extremely annoying when
> you have a modem and voice phone on the same line and your voice call
> gets interrupted by a burst of modem noise! :)
> btw, this is for a UK system - I've seen curcuits to do exactly this
> job, but they never say whose phone systems their designed for, and I'd

Essentially you need a transistor switch that is biased off (no current from
emiter to collector) until the voltage is at the off hook level, then it
would go to a short condition and pass battery voltage and current through
the EC legs and turn on an LED, buzzer, or whatever....Usually powered by a
9v battery and dependant upon the phone current/voltage and the transistor
you use, then knock the bias and inputs down with a series of resistors.

Tandy/Radio Shack still sells these already made up cheap. My guess is you
have similar "off hook indicators" available in GB/UK as well. I use one on
my data line to know when this confounded internal modem is still connected
and one on the voice line to tell me if there's someone else using the line.

I had built 3 of them from a Tab project book that I no longer have but
they've been dumped due to a serious lightning strike (and my lack of MOV
and gas discharge tubes knowledge then) so they were dumped.

 Russ Blakeman
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Received on Fri Jul 17 1998 - 10:02:05 BST

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