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I have an LED project for which i'm using a 9v power supply. For part of the project I'm using an off the shelf LED driver that is compatible with 9v however I have one LED component that I want to drive with the attiny85 (random blinking pattern), which is not compatible with 9v.

Does it make any sense to power the attiny85 separately with say a 3v coin cell, while allowing my LEDs to draw from 9v? The attiny85 would only be tripping transistors. I assume the attiny85 itself doesn't consume much power so hopefully I wouldn't be changing out the coin cell too often.

I have a couple 5v buck converters on hand but they take up more space than a coin cell would.

P.S. It's a Guy Manuel Daft Punk Helmet.

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    They make voltage regulators that are even smaller than coin cells. – Ignacio Vazquez-Abrams May 1 '17 at 22:25
  • @IgnacioVazquez-Abrams I assume you aren't talking about the lm317 which I've already ruled out. I'll shop around and see what's out there. Thanks – Cradle2theGabe May 1 '17 at 22:48
  • Nah, I'd look at the LM78L05/33 or LP2985. – Ignacio Vazquez-Abrams May 2 '17 at 1:31
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Does it make any sense...

Well, actually it does make sense and you already rationalize it in your question: less space consumed, expected life time of the coin cell is ok for your intended use case (though you could/should confirm the assumption about power consumption with the AVR's data sheet).

There is nothing wrong in powering the microcontroller with a different voltage than the "power" circuitry, assuming the ground lines are connected and the transistors mentioned are operated in a suitable mode, e.g. open collector/open drain type.

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