[sdiy] Clock signal detector

[Dave Manley]

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.

The unasked question here - how fast do you need it to respond?  When 
the clock stops how soon does the control output need to go low? 
Likewise, when the clock stops how soon does the control output need to 
go high?

-Dave