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While using 555 timer in monostable mode, I want to extend the timer if trigger activates while output is active due to a previous trigger.

For example if I have set the timer for 60 seconds and a pulse triggers the timer (hence 555 activates the output), and after 30 seconds another pulse triggers the input pin, I want the output to stay active for 60 seconds from the second trigger (overall 90s) but with my circuit it lasts for 60 seconds from first trigger and ignores the second trigger. Any Ideas?

enter image description here

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  • \$\begingroup\$ What is the purpose of this operation and does it have to be a 555? It might be just simpler to program an Arduino to do what you want instead of designing circuitry around the 555 to do it. \$\endgroup\$
    – Justme
    Aug 11, 2023 at 17:29
  • \$\begingroup\$ @Justme Thanks for your comment. It is a very small and cheap circuit to use a microwave sensor module as trigger to turn on a lamp... So using a microprocessor is an overkill. \$\endgroup\$
    – AKTanara
    Aug 11, 2023 at 17:36
  • \$\begingroup\$ So no MCU, but does it have to be a 555 if it is easier to use standard ICs, such as Schmitt trigger inverters or comparators instead of 555? \$\endgroup\$
    – Justme
    Aug 11, 2023 at 17:49
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    \$\begingroup\$ Try googling retriggerable 555 monostable. \$\endgroup\$
    – Andy aka
    Aug 11, 2023 at 18:04
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    \$\begingroup\$ Oh wow, now they make drawing programs that smudge all the lines as if that somehow made things better. Hint: in the "paper and pencil" era, people would probably kill for the digital drawing capabilities we have now. Smudges were not a good thing :) \$\endgroup\$ Aug 11, 2023 at 19:39

3 Answers 3

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Connect a diode such as 1N4148 from the trigger switch top to the timing capacitor, with a series current-limiting resistor of perhaps 100Ω, such that the timing capacitor is discharged when the trigger switch is actuated.

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  • \$\begingroup\$ This way will the output pin be low while the triggering? Meaning the output will be interrupted while re-triggering? \$\endgroup\$
    – AKTanara
    Aug 15, 2023 at 6:42
  • \$\begingroup\$ The output will be low during the triggering, however the total time will be from the release of the trigger, not the start, so it's not a true retriggerable circuit. For that, you can look at 74HC123 etc. \$\endgroup\$ Aug 15, 2023 at 7:34
  • \$\begingroup\$ Thanks. I think I'll go with a simple micro controller like ATtiny13A \$\endgroup\$
    – AKTanara
    Aug 15, 2023 at 12:48
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This will work, but it has a condition.

  1. Disconnect pin 7 and leave it floating

  2. Delete the "Pullup Resistor" (and consider using reference designators).

  3. Connect pin 2 to pin 6.

Now the switch will discharge the timing capacitor and set the internal flipflop that controls the output state every time it is pressed, restarting the timeout period.

The condition is that when power is first applied, the circuit will be triggered and go through its timeout period. This is because the discharged timing cap is holding the Trigger input low even though the switch is open.

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  • \$\begingroup\$ This way will the output pin be low while the triggering? Meaning the output will be interrupted while re-triggering? \$\endgroup\$
    – AKTanara
    Aug 15, 2023 at 6:43
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Use a diode. Connect the anode to pin 7 and cathode to pin 2. When pin 2 (trigger) goes low it will drain some current from the timing cap and extend the time the output is high.

I've used this approach before and it works.

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