searching for a reliable circuit

[LaserTazerPhaser]

What would be a reliable circuit without the need of programming a micro which would output a signal which is high when the input is high but goes low and stays low after the input goes low once and does not go high again even if the input goes high?

The input signal would be many minutes to hours.

These things can be done easily for short durations but long durations needs practical experience because since components can vary.

[Dubbie]

Could use an active low latch and hook up to the q bar output. Just one small component.

[soldar]

A simple flip-flop will do that.  https://en.wikipedia.org/wiki/Flip-flop_(electronics)

[LaserTazerPhaser]

as in a 4013 latch?

[IanB]

A single AND gate will do that, or a relay if you want a mechanical solution. A circuit like this is used all the time for the "push to start, push to stop" switching arrangement on machinery. (If you push the red button momentarily the machine will stop, and it won't start again until you push the green button.)

[Dubbie]

How can you use a single and gate Ian? I can’t figure out how you would latch it so it can’t activate again.

[IanB]

How can you use a single and gate Ian? I can’t figure out how you would latch it so it can’t activate again.

You take a two input AND gate and feed the output back to the second input. Then as long as the first input is high, the output will be high. If the first input goes low the output will go low, taking the second input low. Thereafter, even if you raise the first input high again the output will remain low since the gate will only see one high input.

Of course, the output will never be high in the first place unless you reset it externally. Therefore all such latching circuits need a reset switch to enable them. To reset the AND gate all that is needed is to momentarily set the output high and then it will stay there.

[Dubbie]

Thanks.
Logic is not my strong suit. Of course it seems trivially obvious when you spell it out!

[LaserTazerPhaser]

How can you use a single and gate Ian? I can’t figure out how you would latch it so it can’t activate again.

You take a two input AND gate and feed the output back to the second input. Then as long as the first input is high, the output will be high. If the first input goes low the output will go low, taking the second input low. Thereafter, even if you raise the first input high again the output will remain low since the gate will only see one high input.

Of course, the output will never be high in the first place unless you reset it externally. Therefore all such latching circuits need a reset switch to enable them. To reset the AND gate all that is needed is to momentarily set the output high and then it will stay there.

This?

[Ian.M]

Not quite.  You still need to be able to drag pin 2 high to initially set the output if it happens to power up with its output low, so put a resistor in the feedback loop. and a momentary switch from pin 2  to Vcc.

However if you want to automate setting the output at powerup it gets more complex . . .

Also, there's a flaw in your original spec.  It doesn't specify whether or not the output should be high if the input has been low since powerup.

[hamster_nz]

A dual pole relay can do this, and is often used this way in industrial machinery where safety is critical.

[Dubbie]

Lol blueskull. No half measures for you!

[soldar]

A single AND gate will do that, or a relay if you want a mechanical solution. A circuit like this is used all the time for the "push to start, push to stop" switching arrangement on machinery. (If you push the red button momentarily the machine will stop, and it won't start again until you push the green button.)

This circuit will do that.

[magic]

I suppose a thyristor would do and wouldn't require a reset after power-up.