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I want to turn off a LED after 10 seconds of power to the circuit. The problem is all the solutions I can find are using push buttons to trigger a 555 timer, but I want to have it start the 10s timer when powered on and turn the LED off after 10s. I have no clue what the easiest way is. The circuit uses 12V DC

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  • \$\begingroup\$ cheapest and best are not necessarily synonymous ... please define best \$\endgroup\$
    – jsotola
    Jan 19, 2023 at 22:26
  • \$\begingroup\$ Would you be willing to use a microcontroller board (like an Arduino) that you have to program? \$\endgroup\$ Jan 19, 2023 at 22:52
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    \$\begingroup\$ In electronics terms, you are looking for a "monostable multivibrator." But also, you could do it with a RC time delay circuit, and maybe an opamp or comparator to refine the timing. Just choose an RC combination with a long charge, and set a comparator/op amp to flip the LED when a reference voltage is reached. \$\endgroup\$
    – Smith
    Jan 19, 2023 at 23:37
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    \$\begingroup\$ Pretty sure answered a similar question a little while back. On my phone so searching is difficult, will look when I get on my computer. Triggering a 555 on power up is as simple as putting a large resistor from reset and a cap from reset to ground. Cap holds reset low until it charges through resistor. \$\endgroup\$
    – GodJihyo
    Jan 20, 2023 at 0:36
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    \$\begingroup\$ This can be done with one small FET, one R, and one C (plus the current limiting resistor for the LED). The catch is that, after 10 seconds, the LED will dim rapidly rather than snap off. Someone else (maybe you) has a similar thread on another forum. \$\endgroup\$
    – AnalogKid
    Jan 20, 2023 at 0:59

2 Answers 2

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Here is how you can do it with a 555. The switches simulate power being applied at t+0.1 second, being removed at t+34.5 seconds and re-applied at 35 seconds. You can see that 0.5 second is not quite enough to fully reset the timer, so you get a bit less than 10 seconds the second time.

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

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    \$\begingroup\$ probably R1 C2 D3 not needed, just connect trigger to threshold. \$\endgroup\$ Jan 20, 2023 at 8:35
  • \$\begingroup\$ @Jasen. Yes, good, that should work. \$\endgroup\$ Jan 21, 2023 at 2:28
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Posts #1, #8, or #9 of this thread, with changes:

https://forum.allaboutcircuits.com/threads/how-can-i-make-a-mosfet-switch-more-sharp.191507/

For example, using posts #1 or #8, swap the locations of R1 and C1. Your LED (and its resistor) replace R2.

Post #9 gives a snappier LED turn-off. The LED (and resistor) connect between the 555 output (pin 3) and Vcc. If you reverse the locations of R1 and C1, the LED connects to GND. Depending on how much current your LED requires, you might be able to drive it directly with the 555.

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  • \$\begingroup\$ Downvoted because this answer 1) requires user to click a link to understand, and 2) this answer will become useless if linked thread goes away. Answers should be self-contained. \$\endgroup\$ Dec 28, 2023 at 14:01
  • \$\begingroup\$ Hmmm ... Isn't this the site that deletes threads (and points) with no warning or explanation? In my admittedly brief experience, Links to AAC are more stable than links to here. \$\endgroup\$
    – AnalogKid
    Dec 28, 2023 at 15:18

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