# Automatic cut off after switch is pressed

I have a fun one for you. Here is my diagram:

It's a 555 timer in a monostable one shot configuration. What I want to do is find a way to prevent the LED from staying on if the switch is held down. So whether the switch is just pressed or held down, I want the LED to stay lit the same amount of time and then shut off.

I was trying to find an auto shut off switch, but I didn't really have luck with one.

• IIRC, you're going to need a completely different arrangement for that. – Ignacio Vazquez-Abrams Oct 14 '14 at 19:39
• Such as? I don't understand. This isn't the way it's going on the board. Thanks! – Dominic Luciano Oct 14 '14 at 19:43
• Feed the switch though a capacitor so it creates a pulse instead of a constant low value? – Majenko Oct 14 '14 at 19:47
• It just leaves the light on, kind of reversing what the switch is supposed to do – Dominic Luciano Oct 14 '14 at 19:49

R4 and C4 are new components; their time constant is set to 10 ms, a small fraction of your timer period. Regardless of how long the switch is held, the TRIGGER input of the 555 will only see a short low-going pulse.