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Simple comparator with delay?

Hi all!

I have this:

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The image shows the input and output voltages given certain conditions. I'd like the output to equal Vsupply+ when the input voltage passes the voltage reference. But I'd also like the output to have delay so that it does not go to Vsupply- until a x seconds after the input voltage drops below the reference voltage.

Will the circuit that I included work? I figured if I charge the capacitor at the input then depending on the parallel resistor then its discharge will take sometime.

Any thoughts? Thanks!

Reply to
MRW
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Well. First of all. You didn't specify if you wanted a delay ramp up to the Vcc+ voltage or just a delayed digital signal which is a resulting square wave.. Any ways, you need to have a series R with the input of your existing circuit.. Also.., You need a little hysteresis to prevent a slop/ramp effect on the output when the threshold is reached.

Putting a little feed back from the output to the + input can solve that issue. Personally, I would use the first Comparator as a buffer stage to isolate any effects on the source circuit, and then, use the output to another op-amp/comparator with a series R and bypass cap as your Time constant for delay on.. also, place a diode across the R so that reverse voltage will flow to discharge the Cap quickly when the signal goes off.. If you're trying to construct a Delay on Timer that needs to only work if there is a minimum of clean signal, this is the way to do it.

P.S. A 555 timer can solve this issue.

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Reply to
Jamie

When your input voltage goes back down to zero, it will suck the charge out of the timing capacitor very quickly, making the 1 meg discharge resistor pointless. If the input had an ideal diode in series with it, then it could charge the cap, but not dump it.

It might be easier to use 2 comparators in cascade, the first producing a pull down pulse when the input was above the reference, with its open collector output, dumping a per charged up capacitor to ground very quickly, but just then, disconnecting itself when the input fell below the reference. This one way discharge process eliminates the need for a diode.

That way, the cap could be recharged positive by a timing resistor, and the second comparator could compare the capacitor voltage to the reference to produce the delayed output.

Reply to
John Popelish

Thanks all! I found this neat page with some circuit collections:

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I think one of the Time Delay circuits is probably what John is describing.

Reply to
MRW

I think most of those circuits are lifted from this N.S. application note:

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Reply to
John Popelish

The comparator data sheet has lots of application notes included, too:

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Reply to
John Popelish 

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Reply to
John Fields

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