[sdiy] Signal indicator board...

[Magnus Danielson]

From: Tim Ressel <madhun2001 at yahoo.com>
Subject: [sdiy] Signal indicator board...
Date: Fri, 2 May 2003 09:00:53 -0700 (PDT)

> Yo,
> 
> I've been thinking (dangerous, I know) and I think
> what the world needs is a little board that takes in a
> signal and has two LEDs to show signal present and
> overload. This makes a handy visual indicator as well
> as a swell panel filler. Real simple circuit, possibly
> surface mount to keep sizes small. It could be
> single-supply to keep wiring down. 
> 
> So like, what do ya think??

Tim, you are one of the guys that keeps me worried. All this thinking can't be
good for ya'! Haven't your mum and dad talked to ya' about these habbits of
yours? ;O)

Well, OK. I like the idea as such. If you do it SMD you could do a neat little
board. You could also use a GREEN/RED light arrangement, so that it goes green
when there is signal and RED when there is too much of it.

Anyway, what I dream up is a four-opamp thingie where you use up two of them
to make a precission full-wave rectifier (cookbook design) and then follow that
up with a low-pass filter (1-10 Hz or so) and then two comparators, one for
each of the limits. Just for a proposal you could set the "signal" limit at
say -60 dBu and overload at say +10 dBu. A zener diode and a few resistors
should be enougth to supply reference levels.

If you are ready to spend some more curcuits you could logarithmize things to
make it easier to handle. You could even toss on TrueRMS (not THAT complex when
you see it, and also, here you can cheat plenty, you only need to square the
signal prior to integration, the limits can be adjusted to the squared case).

A TL074 in SMD capsule should work well. The passives doesn't take much space.
Could be really compact actually. Just make sure the series resistors to the
diodes is able to handle the power with good margin.

Cheers,
Magnus