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teeny tiny circuit to make led fade in and out. Answered

good morning. long time lurker, first time poster. i am working on a small project and thought this would be the best place to ask.

i need to come up with a teeny tiny circuit that when power is applied will fade in an led (over 1sec), and then when power is removed will fade back out (1sec). it doesnt have to be exactly 1 sec, but somewhere in that ballpark.

the space i have to work with is only 21.91mm X 6.02mm X 19.4mm, and id like to also be able to add a small switch in there too so i can disable the led when its not needed.

input voltage is 12v and i need 5.5v out, 6v max.

any ideas are welcome. i have found a few circuits on here already, but most are way to big to fit into the tiny space i have to work with.




1 year ago

If you're good at soldering (and you'll have to be to make any circuit that small), you can dead-bug solder an 8-pin ATTiny chip and the passives you need in that much space. Then it can fade in, fade out, blink "Happy birthday" in Morse or flicker like a candle- it's just a case of programming it to do what you need. You wouldn't need that many components, you could reproduce this circuit (I think the linked one is to make the chip programmable in-circuit which you probably don't need)

If you need to do it with only discrete components, you can use something like this circuit but it might be more challenging to solder because there are a lot of links to make.


Reply 1 year ago

The first link should be https://www.instructables.com/id/Dead-Bug-Prototyping-and-Freeform-Electronics/step7/Little-Wire-dead-bug-art/

Read up on RC circuits. They just use a capacitor and resistors. You won't get fade to on, but you can get a slow fade out with the simplest RC circuits.

actually, yes, you can get a fade-in. Just put a resistor in series, such that it limits current from power supply. A capaicitor in parallel to the LED will then draw most available current, leaving little for the LED, until cap is charged. Shutting off supply allows cap to discharge thru LED. Both on and off will have a fade in/out time if R and C are chosen well.

Voltage limiting can be done with a cigarette lighter to. USB adapter.

just a capacitor might do the fade in/out bit. Try connecting it in parallel.


1 year ago

another idea would be for the led to come on right away, but then fade out over maybe 2sec or so. i believe i could do this with a capacitor, some diodes, and a resistor, but im not actually sure how to do it, or if i can get an appropriate capacitor thats is small enough.