# How do I turn a LED off after a certain amount of time? (555 Timer or other solution?)

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

• cheapest and best are not necessarily synonymous ... please define best Jan 19, 2023 at 22:26
• Would you be willing to use a microcontroller board (like an Arduino) that you have to program? Jan 19, 2023 at 22:52
• 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. Jan 19, 2023 at 23:37
• 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. Jan 20, 2023 at 0:36
• 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. Jan 20, 2023 at 0:59

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.

simulate this circuit – Schematic created using CircuitLab

• probably R1 C2 D3 not needed, just connect trigger to threshold. Jan 20, 2023 at 8:35
• @Jasen. Yes, good, that should work. Jan 21, 2023 at 2:28

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