"Dave Plowman (News)" wrote in message
...
I've got an alarm clock which gives the usual 'beep beep' wake up call.
I'd like to use it to switch a sound system on. The sound system is
already switched via a low voltage relay, so I'd like some opinions on the
most elegant way of doing this. I have my own ideas but wondered if I'm
missing a better way. Direct connection to the clock isn't a problem.


Hi Dave

I guess that the beep tone will be generated by the micro in the thing, via
the audio stage, so I would think that all you need to do is use the audio
from across the speaker to drive a simple charge pump - one diode, one cap
and maybe a resistor - to derive a DC level thereabouts equivalent to the
peak value of the audio. Once you've got that, you can do pretty much
whatever is needed to switch the sound system on i.e. via a buffer
transistor, a momentary acting relay across the standby switch or self
latching relay or whatever. If you put it all in the clock end, and use a
relay to perform the actual switching, then there are no isolation problems
either. On the other hand, if the bulk of it was at the hifi end, it might
be easier to reset when you turn back off ??

Arfa