One shot latch

Eariler, I linked this ETO article https://www.electro-tech-online.com/threads/parking-sensor.291/,that shows how to configure a 555 as a bistable latch. Now, if the output of the latch is fed back to the input of the 1-s delay timer, along with the original trigger, you can make the 1st timer a one-time-only triggered timer. For example, a 'low' (<VCC/3) signal triggers the 1st timer, so the normal trigger would be combined with the feed back from the latch by, say a logical OR gate. In that way, it would take both a trigger voltage from the sensor, and the latch not activated to trigger the 1st timer. Once the latch is activated, the 1st timer could no longer be triggered, until power is removed.

Sorry, don't have my schematic software ( I'm @ work at the moment )

(yes, I read ETO at work. everyone needs a break )

thanks Brown out, i'll look into this too

The attached is an example of a two stage timer from an old Forrest M. Mims III engineers notebook circa 1982. The unidentified pin (Output 2) is pin 9 of the 556. This may be another approach? There was a thread sometime back I can't find where we did quite a lot with this version. I think Boncuk even did a board.

If re triggering is a problem while the motor is running a flip flop could be used to start the 556.

Just something else to think about.

Ron

BrownOut said:

Then why are we talking about latches?

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Because this is what the OP posted first.

2) In the circuits I have built I am also finding it difficult to stop the continuous low input to the 555 from the engine transcucer producing further triggers to the solenoid after the first 1 sec output has finished. I only want it to trigger once after 1 second so maybe SCR is the way to go but I do need to keep this small running on a miniature batttery.

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This suggested to me that the delayed output has to be maintained.??

carbonzit said:

OK, thanks. But you show two outputs, va & vb: how are these used? Which one would you use as the latched output?

Maybe give us a truth table to show how this works?

Also, will this work with regular (TTL) NANDs, like a 74ALS20?

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hi,
It will work with all NAND's.
Ref, Va and Vb, as you can see, as one output goes high the other goes low, so you would use the output which gives the required 'sense' for the circuit the latch drives.

Its also possible to use NOR gates to make a latch eg: CD4001.
With this type the inputs are High going to Set and Reset the latch.

As you were interested in switch debounce, consider the use of this type of latch to give a 'clean' output when the switch is used as the Set/Reset source.???
If you had a SPCO switch to debounce, this latch type is ideal.

ericgibbs said:

If you had a SPCO switch to debounce, this latch type is ideal.

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So what's a SPCO switch? Never heard that term before: is it UK-ism? (I see from a search that it seems to mean "single-pole changeover", but still don't know what that is.)

carbonzit said:

So what's a SPCO switch? Never heard that term before: is it UK-ism? (I see from a search that it seems to mean "single-pole changeover", but still don't know what that is.)

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Look at this links for switch terminology.
Switch - Wikipedia, the free encyclopedia