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.

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

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


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.

  • \$\begingroup\$ I must be doing something wrong because the LED just stays lit now whether the button is pressed or not. Here's an example !Circuit Example. \$\endgroup\$ – Dominic Luciano Oct 14 '14 at 20:11
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    \$\begingroup\$ You forgot the additional resistor. \$\endgroup\$ – Dave Tweed Oct 14 '14 at 20:24
  • \$\begingroup\$ Yeah I figured it out. I forgot to save my renumbered project, so my numbers were different from yours. But the 100nF wasn't producing anything, so I upped it and voila! Thank you, Dave :) \$\endgroup\$ – Dominic Luciano Oct 14 '14 at 23:06

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