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I am using a 555 timer in the monostable configuration, which means that when I send a pulse to the trigger pin, the falling edge will make the 555 generate a high pulse on the output for some time. However, if the pulse on the trigger pin is held down for a time longer that the output pulse, the output will remain high until the trigger goes up again.

How can I make it to end the pulse independently of the pulse duration on the trigger input? I have found this solution:

schematic

simulate this circuit – Schematic created using CircuitLab

But it seems to not work to me because I'm not using a button, I am using a voltage directly connected.

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You can eliminate the switch and R1, and connect the voltage source directly to the node where they were connected, at the left end of the capacitor. C1 and R2 will still function as a high-pass filter in the manner you need. Just make sure that their time constant is shorter than the timer's period.

It might be a good idea to connect a diode in parallel with R2 in order to improve the recovery time — anode to C1, cathode to Vcc.

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