[sdiy] Using audio to trigger CMOS

[Roy J. Tellason]

On Monday 28 April 2008 09:07, Justin Owen wrote:
> Hey all,
>
> I'm hoping for some help refining a circuit I'm working on that uses an
> audio signal as a clock to trigger a 4024 Ripple Counter.
>
> The audio signal is a 1/16th note sequence that is meant to represent a
> square wave i.e. Note On = High (or Mark), Note Off = Low (or Space).
>
> I've got a prototype that uses an AC Coupled gain stage to take the audio
> input and feeds it into a comparator.
>
> The current version works(ish) minus some problems which I think are down
> to some ac coupling mistakes and a lack of headroom on my power supply -
> all of which I think I can sort - but I was wondering if there also needs
> to be any type of rectification between the gain stage and the comparator?
>
> I can be pretty specific about the type of audio signal that goes in (I'm
> currently using a PWM square wave with a 90/10 mark/space ratio at a fairly
> high pitch - which I figured would give the greatest ratio of High to Low
> signal when the Note was 'on') - I'm not expecting to get a square wave
> trigger out of a guitar input for example - but I'd like the circuit to be
> as safe and reliable as possible.

Seems to me you could get plenty of triggering signal by simply running a CMOS 
inverter or two in linear mode.  AC couple the input if you need to,  have an 
input resistor and a feedback resistor like you would for op amps (ratio sets 
the gain),  and you could even if necessary inject a little bit of DC biasing 
into the input after the coupling cap to affect your response.  Check it out 
in Don Lancaster's CMOS Cookbook,  I'm pretty sure he covers that...

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