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Question about comparator and 555 timer circuit

PostPosted: 28 Jul 2017, 10:53
by miyayoung
I'm designing my first circuit for a university project.

This circuit will check the voltage of two sensors. If both are over a reference voltage and persist as such for more than one second, a relay will switch on. Until I switch off the entire circuit by pushing the master switch, the relay must be closed.

I'll using op-amp LM393 for the two comparators, NE555 with the RC circuit (charging of capacitor lasts one second), the AND gate to verify both the conditions of the comparators, and the OR gate to maintain the power to the NE555 until I switch the master switch.

In your opinion will it work?

The power supply of the op-amp and the gates is taken from +VBAT.

Re: Question about comparator and 555 timer circuit

PostPosted: 28 Jul 2017, 11:58
by deltafred
Without seeing your proposed schematic it is difficult to say if it will work or not. As long as you get your logic correct there is no reason why it shouldn't.

One minor point (which you probably know), the LM393 is a comparator, not an op-amp. While they look similar, they both have + and - inputs and an output, they are not interchangeable. The op-amp is optimised as a gain stage, the comparator is optimised to compare the inputs and switch the output accordingly.

Re: Question about comparator and 555 timer circuit

PostPosted: 30 Jul 2017, 22:35
by toneman
For comparators, use LM339(quad) or the dual or single versions. These are TRUE comparators and need very few external components to get good snap action. Comparator goes to 555 triggered as oneshot for the "one second window" which drives a 2nd 555 or another comparator to directly drive a small reed relay or small transistor that drives the relay. Do I get "credit" for your "university project"? :wink: