[sdiy] Clock signal detector

[Dave Manley]

> 
> 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