[sdiy] Simple clip detect LED circuit

[Magnus Danielson]

From: Seb Francis <seb at burnit.co.uk>
Subject: Re: [sdiy] Simple clip detect LED circuit
Date: Tue, 02 May 2006 00:40:42 +0100
Message-ID: <44569C7A.6030607 at burnit.co.uk>

> It's to detect clipping on the input of a ADC, so LED needs to be 
> totally off for below threshold signals, then on for a noticeable amount 
> of time if the signal exceeds the threshold for even 1 sample (~20us).
> 
> I guess I'll make up a circuit .. it just would have been nice to check 
> myself against something someone has already tested.  I've googled 
> around for quite a while and (surprisingly) haven't found anything.

OK, here is what I would do:

1) Pick up a "standard cookbook" precission full-wave rectifier. Toss the
   signal into that.
2) Peak-detect with a dioded into a capacitor having a leak-out resistor.
3) Compare level with that of some reference, such as resistor-divider or
   resistor over Zener diode. Just something.
4) Drive the LED from that.

Doesn't become very complex in the end.

When I did catastroph limiters, I toyed around with Zener diodes to get the
right knee-shape, but that is ridicolous for this case.

Cheers,
Magnus