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I have a 555 in monostable mode that will generate a signal from the transition to low from one of it's inputs (as they do). The issue I have is that the trigger will out last the signal by an order of magnitude and I need it to only trigger once.

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What kind of circuit can I use to produce the result I want with the 555?

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Original Question:

I have a sensor that will loiter low before going high, stay there for a relatively long while then repeat. My trigger is the transition from from high to low. This allows me to use my 555 in monostable mode to signal for a short time. However the trigger will remain low even after the signaling time. I'm only looking to trigger the signal once in triggers sensor's cycle.

Right now my hammer is the 555, and I've thought about trying to resolve the problem with another in bistable mode, but then I just have another trigger on transition issue.

I'm looking for insight into how this may typically be handled, with a 555 or not.

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  • \$\begingroup\$ What does the sensor output look like? What are the rise and fall times? What's the low and high level voltages? What's its current compliance capability (or output impedance)? How wide of a monostable output pulse is required on the falling edge? Any more details? I could guess, but I'd rather not. \$\endgroup\$
    – periblepsis
    Commented Dec 7, 2023 at 6:02
  • \$\begingroup\$ Are you willing and able to program a microcontroller for this? \$\endgroup\$
    – vir
    Commented Dec 7, 2023 at 6:07
  • \$\begingroup\$ @periblepsis This is a bucket that fills over a period of 2 to three days. Then the bucket empties and the sensor at the bottom of the bucket goes low, (makes a connection to ground) then likely takes many hours to trigger high (broken connection, no longer grounded) again. The duration of signal would likely be around 3 minutes. \$\endgroup\$
    – QueueHammer
    Commented Dec 7, 2023 at 6:48
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    \$\begingroup\$ You can get an ATtiny in a SOT23-6 package for $0.44 in single quantity, which is less than a 555. \$\endgroup\$
    – vir
    Commented Dec 7, 2023 at 6:56
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    \$\begingroup\$ Add a circuit. Right now, it's very unclear. If you mean, you have a falling edge to trigger and you want the monostable to go back to normal state even if your trigger signal is still low, you need a coupling capacitor. Check the answer here: electronics.stackexchange.com/questions/691321/… Ignore the positive signal in the question. You just don't need the transistor there. \$\endgroup\$
    – Saadat
    Commented Dec 7, 2023 at 8:24

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