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Comparator with delayed output


Purpose of this design: detect light variation at low intervals (>1 second, up to minutes).

The detection works independent of ambient light.
What I do need is a delay on the output when LDR is uncovered, ie when the voltage on the non-inverting input recovers to a high level.

The left design uses two comparators and works well. I would however like to reduce this to a design with one comparator only, if possible.

I am in a bind how to introduce a delay circuit for the voltage on pin 9 (non-inverting): voltage going up on that pin needs to be delayed.

I would keep the delay as a separate function just as you have it. All I would alter is:
1. Add some hysteresis around the detection comparator.
2. Increase the values of R1, R4 to conserve some current, also makes the hysteresis easier.
3. Why is R7, 300k, seems high?

Terry Bites:
A capacitor from the comparator input to ground will slow the response time. As pointed out, you need to apply hysterisis on all of the comparators or you will have problems.

Yes hysteresis, as you suggested Terry Bites, should be applied to all the comparators. The delay unfortunately is asymmetrical because of the NPN open collector nature of the comparator output. So moving the cap to the input would delay both +ve to -ve and -ve to +ve transitions.


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