[sdiy] Clock signal detector

[James Dunn]

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.

James