[sdiy] Clock signal detector

[Tom Wiltshire]

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.