user33007: 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.

user17196: 
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.

33007: <p>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.</p>

<p>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. </p>

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

<p>P.S. It's a Guy Manuel Daft Punk Helmet.</p>
 17196: <blockquote>
  <p>Does it make any sense...</p>
</blockquote>

<p>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).</p>

<p>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. <a href="https://en.wikipedia.org/wiki/Open_collector" rel="nofollow noreferrer">open collector/open drain</a> type.</p>
