555 Monostable Circuit SOLVED

I'm a newbie to the electronics world and am starting with a relatively simply project, the 555 timer. I have been able to successfully build an astable & monostable multivibrator on a breadboard but I noticed a few things. One combination of values I tried using for the monostable circuit was 1 megaohm and 10 microfarads for the rc charging. This should theoretically generate a 11 second pulse. When using this I noticed that when setting the trigger pin low, the output never stayed high and didnt create a single pulse. I thought this would be a simple error in my connections or component tolerances but when simulating the circuit at 0% tolerances & guaranteeing the connections were correct, I ran into the same issues. I don't believe i'm close to the maximum values for my R & C values since this circuit can theoretically generate pulses up to 15 mins in width. I know RC components can be used to create a low pass filter but that wouldn't affect a DC voltage input. The only thing that makes sense to me is the capacitor is not reaching the threshold voltage to reset the internal comparator but I don't understand why this would be the case. The circuit is simulated in Multisim and was built the exact same way on the breadboard. The switch used was a pushbutton switch normally set high through a pullup resistor but pulled low when the switch was pushed. Any help or insights would be appreciated!

by SamHintz
January 25, 2024

If the model is based on BJT, you need a minimum current for the transistors to work as switches. With such a high resistor, you may be under this minimum, somewhere. Have you tried with a smaller resistor?

by vanderghast
January 28, 2024

Also note that in real life, many large caps are sensible to temperature and to aging (vs number of cycles and max voltage), which make the 555 based solution not so friendly because:

  • energy hungry

  • inaccurate timing due to temperature

  • inaccurate repetitive timing due to temperature and aging of the capacitor

  • if thousands of the gadget are to be made, tolerance in the fabrication of the capacitor must be considered and compensated

  • space (if a size large capacitor is required)

by vanderghast
January 28, 2024

1 Answer

Answer by Quess1977

Not all circuit simulation software is perfect. Check that your Multisim model for the 555 timer takes into account soccer skills world cup component tolerances and leakage currents.

ACCEPTED +1 vote
by Quess1977
January 26, 2024

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