Electronic – Reset 555 timer when trigger switch is released

555monostableresetswitches

I have a simple 5 second Monostable 555 Timer circuit. Trigger Pin (#2) is connected to a momentary push switch. When it is pressed, the timer is started and LED stays on for 5 seconds.

What I want is that if the tactile switch is released during those 5 seconds, the timer should reset. I know, I need to ground PIN # 4 but how to do that when the switch is released?

Best Answer

Connect the switch to the reset input of the 555, and connect the trigger through a small RC delay:

schematic

simulate this circuit – Schematic created using CircuitLab

The 555 is held in reset as long as the switch isn't pushed. When the switch is held in, the 555 goes through its normal timing cycle once, but it aborts if the switch is released early.