You're looking for a retriggerable one shot, AKA "Missing pulse detector"
I've seen 555 ckts to do this, check an older National semi data book



On 3/30/06, Rob Robson <rob@silk.net> wrote:
>
> I'm looking for a 555-based monostable (one-shot) circuit whose output
> will
> STAY high as long as short negative-going pulses are present at the input.
> The textbook 555 monostable circuit doesn't do this.  I have a spare NPN
> on
> the board to utilize if necessary.  Has anyone come across a 555 circuit
> that will do this?
>
> Many thanks,
> RR
>
>
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