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Hi guys!
Trying to build a circuit for my Parallax/Vex rover but I'll be darned if I can find just the right thing.
What I need is a One Shot Multi-vibrator... One button press - One Pulse! "One ping please." In my best Sean Connery typing.
I've built some with a 555 but they just seem to send pulses as long as the button is pressed. That's okay once in a while but there instances where I just need one tiny blip.
Any guidance will be greatly appreciated!!

Happy Sunday-ing.

Amanda

Comments

  • Francis BauerFrancis Bauer Posts: 241
    edited 2021-07-26 05:43

    A 555 Monostable circuit should work as a single-shot one button press for one pulse out. I'm not sure what pulse width/time you are wanting to produce. I suppose if it is a very short desired pulse, you could be experiencing problems with switch bouncing causing numerous trigger events.

    The following is "copied" from this link

    **IC 555 Monostable with Push Button!

    One particular well-known undertaking while designing electronic circuits is to integrate switches or push buttons with the electronic circuit.

    Since the associated standard switch bounce and noise can easily crank out many or intermittent output pulses, an intermediate buffer circuit (generally known as a "switch debouncer") is frequently utilized to get rid of these complications. The following monostable circuit works great as an intermediate circuit to ensure the switch debounce is eliminated for the subsequent circuit associated with the pin3 of the monostable.

    In the one-shot monostable circuit as demonstrated in the attached image, the 555 timer is triggered using a pushbutton S1. Just before the activation of S1, capacitor C1 is charged to the positive supply rail level through R1.

    Next, the moment S1 is pressed, it causes C1 to discharge very fast by means of R4, which generates an instantaneous sharp negative surge voltage. Any intermittent spikes happening due to the switch debounce on pressing S1, can be effectively eliminated due to the presence of R1 and C1, which enables a perfectly clean, negative-going spike.

    This spike is subsequently allowed to move through C2 to pin 2 of 555 U1, triggering the monostable to activate, and generate a zero interference, clean square-wave output with a period of T= 1.1 x R1 x C1. Then, as soon as the S1 is released, causes C1 to recharge to the +V potential level so that the circuit now goes in the standby mode, ready for the the next clean push button actuation cycle.

    The monostable design allows a single fixed high output pulse when the S1 button is pressed, regardless of how long S1 is held depressed.**

  • frank freedmanfrank freedman Posts: 1,853
    edited 2021-07-31 18:43

    SN74123 may be good. Google or if you have an (very) old copy of the TTL cookbook should do it.

  • ercoerco Posts: 19,994

    Nice to heard from you again Amanda!

    I love 555s, been around since 1972. Per Francis, it is up to the task of being a monostable pulse generator. It's very versatile, can also be astable. One of my favorite sites with good commentary is: https://electronicsclub.info/555monostable.htm

    Common mistakes: http://www.555-timer-circuits.com/common-mistakes.html

  • Tracy AllenTracy Allen Posts: 6,567
    edited 2021-08-24 19:07

    A differentiator circuit generates one pulse, time constant determined by RC. A CMOS buffer or even just a transistor buffer can be added at the output to provide more oomf as needed.

  • jmgjmg Posts: 14,813

    @ajward said:
    I've built some with a 555 but they just seem to send pulses as long as the button is pressed. That's okay once in a while but there instances where I just need one tiny blip.

    There are astable and monostable modes, the astable modes are oscillators so give many pulses.

    SN74123 may be good. Google or if you have an (very) old copy of the TTL cookbook should do it.

    74HC123 are the more modern part numbers, and there are also 74HC4538 which have better precision, tho for a pulse-from-press, precision is probably not your top priority :)

    @"Tracy Allen" said:
    A differentiator circuit generates one pulse, time constant determined by RC. A CMOS buffer or even just a transistor buffer can be added at the output to provide more oomf as needed.

    Usually a Schmitt buffer is used to give clean edges, or if you wanted a press-on / press-off toggle, a Flip-Flop with Schmitt clock could give that in a single component.
    TI have these new CMOS part numbers
    SN74HCS72 - Schmitt-trigger input dual D-type negative-edge-triggered flip-flops w/ clear and preset
    SN74HCS74 - Schmitt-trigger input dual D-type positive-edge-triggered flip-flops w/ clear and preset

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