Im using two 555 timers / one 556 timer IC for a project. I've used it a couple of times before but never in monostable mode. Basically I need to activate a DC Motor in one direction for a set amount of time and in another for another amount of time with an H-Bridge. However the input for the 555 timer is not a button but a longer signal. Can the Signal of the 555 turn off while the trigger is still high? Should not be possible right, because the flip flop would have two high inputs ---> invalid... If this is true how could I easily shorten the long signal to a "button press" or might it be easier to use a JK flip flop? Sadly, I can't really test this due to delivery dates, could someone validate my hunch or give me some additional advice?

Thanks and have a great day

  • \$\begingroup\$ If you search for "positive edge triggered 555" you'll find a few implementations. It just needs a few extra passive components on the input. Edit: there is a circuit with a good explanation here. As @Math Keeps Me Busy points out, it will require a negative pulse. \$\endgroup\$
    – vir
    Apr 12, 2021 at 20:42
  • \$\begingroup\$ Does this answer your question? Is the NE555 the IC I need, and if not, what do I replace it with? \$\endgroup\$ Apr 13, 2021 at 0:16
  • \$\begingroup\$ I'll be honest, this is a very digital control problem, and can very cost- and effort-efficiently be solved with a microcontroller. Microcontrollers were basically made for these kinds of problems, and the 555/556 are really not that helpful here. \$\endgroup\$ Apr 13, 2021 at 0:17
  • \$\begingroup\$ Yes, I thought so, I kinda try to avoid them wheneber possible, bad habit if mine, hoewever it really doesn't need to be perfect and I got the H bridge lying around. Do you have any idea which microconteoller could be of use? \$\endgroup\$
    – Raphael
    Apr 13, 2021 at 1:18
  • 2
    \$\begingroup\$ @MarcusMüller No MCUs. It's retro week. See here. ;) Raphael can adapt it so that a short push moves the motor in the first direction and a long-press causes it to reverse! (I'm just silly today after that schematic, earlier.) \$\endgroup\$
    – jonk
    Apr 13, 2021 at 3:52

2 Answers 2


In the standard 555 monostable circuit, it is triggered by the Trigger input going in the negative direction, from something above Vcc/3 to below Vcc/3. So far, so good. but the 555 is not a normal monostable. It cannot be retriggered during its cycle, like a true monostable. BUT, if the Trigger input still is low at the end of the cycle, the cycle does not end, sorta kinda like a retriggerable type.

The solution is an R-C differentiator at the Trigger input. With this, the 555 Trigger input sees a short pulse no matter how long the actual pulse is. Many things go into determining the pulse width that the Trigger input "sees", but in very round numbers it is approx. 0.5 x R x C.


Can the Signal of the 555 turn off while the trigger is still high?

For standard negative triggered 555's "As long as this pin [the trigger pin] continues to be kept at a low voltage, the OUTPUT will remain in the high state." (quote from Wikipedia)

If this is true how could I easily shorten the long signal to a "button press" or might it be easier to use a JK flip flop?

There are multiple ways to accomplish this.

Here is one possibility


simulate this circuit – Schematic created using CircuitLab

A "long" high input will result in a short positive spike at the output. R2 should be a somewhat "standard" pull-down resistor for the logic family which is fed by the output of this circuit. R1 serves to provide current back into C1 when the input goes low. The length of the pulse is of the order of $$C1 \cdot (R1 \parallel R2)$$ but not exactly, due the presence of D1 as well as the impedances of the components at the input and output of the circuit shown.

If you want the "inverted logic" (which is necessary for a standard 555 trigger) you may use this circuit:


simulate this circuit

With this circuit, the output is normally "high". However a "long" low input will result in a "short" low output.

  • \$\begingroup\$ Thanks, yes I mixed up the input, however it's no problem to just short it to ground instead of closing a circuit, so I should be able to negatively trigger it. Thanks for the negative signal shortening circuit. Pulse lengt shouldn't really matter with the 555 as long as its reasonable right? \$\endgroup\$
    – Raphael
    Apr 13, 2021 at 1:23
  • \$\begingroup\$ Yes, pulse length does not really matter as long as it is reasonable. \$\endgroup\$ Apr 13, 2021 at 1:26

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