How do i make the monostable multivibrator go low after activation even though trigger pin is still low?

[johndoeeerae]

Above is the circuit diagram of a circuit I'm building. In multisim, when the switch is open, the monostable multivibrator outputs high for said amount of time and after that, the output would go low and start fluctuating, unless, the switch is closed again then that way, the monostable would go back to outputting 0V. Even though output is fluctuating, the LED still turns on. As seen here:

Building the exact circuit in real life causes problems where the monostable actually never goes low after the period unless switch is closed again and therefore, the LED never turns on. HOW TO FIX?!?! i been banging my head over this. Some solutions I have thought of is a component that only produces a falling edge pulse than back to rising edge once the output of voltage comparator goes low.

[GGoodwin · Accepted solution]

Hi johndoeeerae,

You can try inserting an RC "differentiator" circuit between the output of LM311 comparator and TRIGGER input of 555 timer.

The pull-up resistance in LM311's output can be decreased for faster discharging of the capacitor when the comparator output returns high. An alternative is to replace the LM311 with a comparator with push-pull output configuration.

You can open this circuit, 555 Timer Monostable Multivibrator (RC-Differentiated Trigger), as a guide.

There are other ways to try but I don't have much time for discussion about this right now. I hope the information above can help you solve the problem.

Best regards,
G. Goodwin