Looking for a one shot IC, that will produce and complete the predetermined output pulse (about 15ms) despite the input, that gave rise to said pulse, held constant. The reset needs to be accomplished once the input changes its state again. The input is a footswitch that may remain actuated beyond the pulse duration. Retriggering during the pulse is not allowed.

I found a chip that in the data sheet perfectly describes the desired action in, for example, Maxim's Microprocessor Supervisory chips (with manual reset capability), but the SMD package is too small.

There are at least two PDIP chips named Monostable Multivibrator but mention nothing of the case where the input is held throughout the pulse duration, or whether changing the input state back to where it was will prepare the chip for the next event. They are:

CD54HC4538, CD74HC4538, CD54HCT4538, CD74HCT4538

SN54121, SN74121

Any pointers will be greatly appreciated.

  • \$\begingroup\$ This has already been answered here - electronics.stackexchange.com/questions/188928/… \$\endgroup\$
    – HandyHowie
    Commented Nov 15, 2015 at 21:10
  • \$\begingroup\$ Thank you for the link. However, as I stated below, the 555s are too noisy and require many externals. The Monostable Multivibrators require only one resistor and one cap. Whether they are too noisy or cannot be silenced in my application is an unknown. One respondent in the thread suggested a microcontroller, presumably something on the order of the ATtiny85. I know this will be silent in the application, and is small and available in PDIP, but requires programming and debugging. \$\endgroup\$
    – ITPhoenix
    Commented Nov 15, 2015 at 23:34
  • \$\begingroup\$ Correction on the 555 externals: Same count as the MMV. \$\endgroup\$
    – ITPhoenix
    Commented Nov 16, 2015 at 0:33
  • \$\begingroup\$ A 555 is noisier than a microcontroller? You make me curious what you are trying to achieve, and how. \$\endgroup\$
    – jippie
    Commented Nov 16, 2015 at 5:25
  • \$\begingroup\$ The 100w amplifier has a sensitivity of ~140mV. The physical design is poor with too many wires stretched out all over. As a consequence, it is very susceptible to noise. I may be wrong about the 7555 since changes were made later. The original question explains what is to be achieved. I found this thread which one respondent posted a diagram using both sides of the CD74HC4538 in an "OR" application. I only need one input but I think that's it. Link \$\endgroup\$
    – ITPhoenix
    Commented Nov 16, 2015 at 12:32

1 Answer 1


How about a 555 timer as a monostable multivibrator? It is cheap, simple to use and available about everywhere. Do check the paragraphs on "Trigger pulses", which either triggers the circuit once or keeps leaves the output active in case the input pulse is of longer duration than the output mono time.

Or check out 7555 for a less power hungry CMOS variation of the chip.

  • \$\begingroup\$ Thank you for the tip, but either the 555 or the 7555 have too much switching noise for the high-gain guitar amp (at least the one I concocted). I used the 7555 for a latching and debounce, but they had to go next to the footswitches. \$\endgroup\$
    – ITPhoenix
    Commented Nov 15, 2015 at 23:27
  • \$\begingroup\$ In other words, without extra circuitry, it only works for a short trigger pulse. From the source, "...the duration of the trigger pulse must not be longer than that of the output pulse, and with short output periods or long duration input (trigger) pulses, some conditioning of the trigger pulse may be needed to keep its duration short." \$\endgroup\$ Commented Aug 2, 2022 at 19:59

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