Electronic – Automatic cut off after switch is pressed

designswitches

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

Texas Instruments_LMC555_monostable

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.

Best Answer

That's simple — you just need to AC-couple the switch, like this:

schematic

simulate this circuit – Schematic created using CircuitLab

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.

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