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It seemed to me that a classical "one shot" output will go low after a determined period set by T (Output Pulse Width), even if push button is held down. What I want to happen is, when the trigger is held, the output goes low after T expires. Now it will keep on going and never releases.

Description of the 1.st circuit:

  1. When a momentary switch is only clicked, output will turn on LED for one second.
  2. When a momentary switch is pressed and hold, output will still turn on LED and will keep it ON.

The goal is - 2.st circuit ?:

  1. The same
  2. Different! When a momentary switch is pressed and hold, output should only turn on for ONE second and then go LOW
  3. And of course just the same if/when I repeat the procedure (re-click or re-press hold)

So nothing really complicated.

I tried to do something here, but I'm not sure. It makes sense?

enter image description here

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    \$\begingroup\$ Please explain in more detail what exactly you want this to do. Do you want it to latch on permanently if you hold the button long enough? That's possible, but you'll need more than a single 555 to do it; you could do it with two 555s and a comparator, I think. \$\endgroup\$
    – Hearth
    Commented Sep 24, 2023 at 15:15
  • \$\begingroup\$ I'm confused. You show pictures (incomplete detail there) and words, which I may understand (or not.) Are you looking for a long hold circuit? One that creates a pulse of a specific duration if and only if the button has been held longer than some specific time and where if the button is held longer still the pulse width remains the same and the button is then ignored until it is released later? Also, what happens if the button is only held for a short time? \$\endgroup\$
    – periblepsis
    Commented Sep 24, 2023 at 15:41
  • \$\begingroup\$ @Hearth I'm sorry, I should explain it more clearly. So: Description of the 1.st circuit: 1. When a momentary switch is only clicked, output will turn on LED for one second. 2. When a momentary switch is pressed and hold, output will still turn on LED and will keep it ON. The goal is - 2.st circuit ?: 1. The same 2. Different! When a momentary switch is pressed and hold, output should only turn on for ONE second and then go LOW 3. And of course just the same if/when I repeat the procedure (re-click or re-press hold) \$\endgroup\$
    – Domenico
    Commented Sep 24, 2023 at 17:26
  • \$\begingroup\$ I edit the main question for a better understanding. \$\endgroup\$
    – Domenico
    Commented Sep 24, 2023 at 17:28
  • \$\begingroup\$ @periblepsis 1. No, I want approx 1 sec. pulse to be generated to turn on a load. 2. Only if the button is held longer than 1 sec. should be ignored. 3. Yes, pulse should remain the same, but button release should not affect it 4. If button is held for a short time - the same, output should go high for 1 sec. \$\endgroup\$
    – Domenico
    Commented Sep 24, 2023 at 17:33

2 Answers 2

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I tried to do something here, but I'm not sure. It makes sense and it will work?

To me that seems something you can build out of a combination of at least two 555s, and an external logical gate.

What will be approx. signal delay due to the 100nF cap?

That question makes no sense – that depends on how you design the rest.

Someone might have a better solution?

Invest in (1€ of microcontroller + 1 decoupling capacitor for 5 ct) instead of in (2×75 ct for a CMOS 555 as you show (definitely don't go with the NE555 or other BJT 555s) + ca 12 external passives for ca 1€).

What you describe is a digital control problem, and microcontrollers are the ASICs for this kind of application. Your problem be solved by a microcontroller that gets woken by someone pressing the button and about 10 lines of code. Learning how to do that will open up way more opportunities than learning how to use a 50 years old chip.

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  • \$\begingroup\$ 1. You think? Can you please check again, I now edited the question with more detailed instrucitons. 2. Doesn't it makes sense that 100nF charging time is affecting the trigger signal which will delay the output? 3. Thanks, I will definitely research this. However, I would still like to solve this particular case. There might be some more appropriate IC for it? \$\endgroup\$
    – Domenico
    Commented Sep 24, 2023 at 17:39
  • \$\begingroup\$ 1. yes 2. when you can tell me what exactly the source resistance is, and what the exact threshold voltage is, I can tell you how long a capacitor is going to charge until it crosses that threshold. 3. Yes, the appropriate IC is about any microcontroller. Seriously, the problem you describe is "I want to have a time-driven discrete state machine that I can configure". And that is exactly what a processor is. You describe a use case for a processor with a bit of memory for a simple configuration. That's 100% what a microcontroller is. You found the core niche of microcontrollers :) \$\endgroup\$
    – Marcus Müller
    Commented Sep 24, 2023 at 17:48
  • \$\begingroup\$ I get you, although to learn & program a microcontroller does take time, which I at this moment don't have. Honestly, do you think that the circuit I've built won't work by the requirements? I really don't need it to be precise. Just to work and that's it. \$\endgroup\$
    – Domenico
    Commented Sep 24, 2023 at 18:23
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    \$\begingroup\$ @Domenico ah, that's actually the easier case! So, if you find any eval board, you can always buy the central chip on it separately, just as in case of the aforelinked board, you can buy the ESP32-C3 module, which is a SMD solderable component, from all distributors. For minimum cost, you would probably go for the much more anemic Attiny series of microcontrollers, which might really not be much fun to develop for. Here's an example evaluation board; the chip you'd later buy in quantities… \$\endgroup\$
    – Marcus Müller
    Commented Sep 25, 2023 at 21:22
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    \$\begingroup\$ … might be less what you're looking for when you need tightly guaranteed safety controls, but at 2€ to 3€ including a million of peripherals that the < 2€ serieses of microcontrollers from ST or Microchip Inc don't even dream of having – including Bluetooth and Wifi connectivity, should it turn out you need that. \$\endgroup\$
    – Marcus Müller
    Commented Sep 25, 2023 at 21:42
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This can also be built with 2 transistors, a capacitor and a few resistors. An example a bit down in the forums here (only a breadboard overview, note one superfluos resistor, and the rightmost transistor and LED is just for indication): https://steamcommunity.com/app/2198800/discussions/0/3825284962828830989/

Another here with schematics: https://www.tutorialspoint.com/pulse_circuits/pulse_circuits_monostable_multivibrator.htm

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  • \$\begingroup\$ This is different. If you have only 2 transistor latch, you still don't have a timer. Anyway, I found that the scematics I did works properly. The only thing to play, was the "one shot" cap and resistors. \$\endgroup\$
    – Domenico
    Commented Nov 2, 2023 at 21:14
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    \$\begingroup\$ It will meet the requirement of your point #2 under the goal of the 2nd circuit (2st?), IE that the output will go low after a set period T, even if the button is held down. But I might have misunderstood. Anyway, you've fixed it so that's good. \$\endgroup\$
    – wrOngplanet
    Commented Nov 6, 2023 at 23:12
  • \$\begingroup\$ Thank you @wrOngplanet \$\endgroup\$
    – Domenico
    Commented Nov 13, 2023 at 19:15

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