[sdiy] Clock signal detector

[Christian Bergmiller]

look at this:

http://www.sdiy.org/cbm/schem1.gif

sorry for the clumsy drawing.
negativ going clock edges charge the 0.47u Cap.
it is discharged through the 1M resistor.
as long as it is charged the output will remain high
(if the output goes low the clock is not running any more)
40106 is a suitable schmitt trigger inverter

christian

2008/8/26, Tom Wiltshire <tom at electricdruid.net>:
>
> On 26 Aug 2008, at 10:28, James Dunn wrote:
>
> > Dave Manley wrote:
> >
> > >
> > >
> > > >
> > > > I want to make a simple circuit to detect when a clock signal is
> present. The clock line is either at 0v or a pulse of about 15% duty cycle
> and around 4 hz. I want a control output of 1 for clock present and 0 for no
> clock present. How is this done? It would be great if I could do it with 3
> or 4 schmitt triggers and/or 1/4 of a 4016 analogue switch as that is what I
> have left...!
> > > >
> > >
> > > Try using a series diode, to a resistor and cap to ground.  When the
> signal is high it charges the cap, when low the resistor drains the cap.
> Choose the R to drain the cap if the clock stops for a few ticks.  Put a
> schmitt before and after.
> > >
> > > -Dave
> > >
> >
> > Thanks for all the replies. I tried this out and found that with a
> resistor that was high enough to hold the output on, it would never fully
> discharge the cap and the schmitt trigger output would not go low. Too low a
> resistor value, and the output just followed the input pulse.
> >
>
> Pity. Still, it had to be tried.
>
> I'd probably go back to Scott's idea of using a one-shot. If the clock
> pulses are coming in, then the one-shot will keep getting triggered and stay
> high. It'll come on as soon as the clock starts and, if you get the one-shot
> pulse length right, it'll go low *almost* as soon as a pulse is missed. It's
> another classic 555 timer application, isn't it?!
>
> T.
>
>
>
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