4013 answer

Thanks to all those who offerred suggestions.

I appear to have had a very dirty clock signal. I've seem to have solved it
by using a 555 as a monostable.

The sensing circuit outputs from a comparator which sends out a signal going
high to low when the light sensor is shadowed.
This low triggers the 555 monostable and the output of the monostable goes
high for 2.5s before falling back to 0V.
The output of the monostable is connected to the clock input of the D type
latch. D is now being transferred to Q reliably and the latch is now
latching nicely.

John

 

I think your problem may have been the fact that most comparators
require a pull up R on the output to get a fast pulse and not rely
on the input of the next device to doing the pulling.

Other wise, you just get slow moving/floating clock pulses.

Jamie