[sdiy] Clock signal detector

[James Dunn]

Hi,

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...!

thanks

James