> If I understand the above specs correctly, the intercom transmits and
> receives on 200kHz/270kHz/230kHz . Anyway to pick up that transmission
> and trigger a light? What about picking up the receive signal from
> inside the intercom box somewhere and wiring it to a light?
That web page mentions a "call tone" you can send. My thought would be a
modification inside the box, to detect the call tone and trigger your choice
of displays when it happens. Much easier, IMO, than building a receiver for
the RF signal.
One or two stages of op-amp active filter to filter out the tone, then
rectify and filter the result, then use that to drive an output transistor
or FET which could drive a relay or LED. Only connections to the intercom
would be for power, ground, and speaker out.
Here's a more sophisticated version inside a chip :
http://www.reconnsworld.com/audio_tonedecode.html
The output goes low when the tone is detected, and will sink up to 100 ma.
That's enough to drive a high-intensity LED or a suitable relay. You'll
have to figure out what frequency the tone is, and select components to
match.
Randall
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