How will I power this board? I need at least 5V for the solenoid, and the ATTINY can run off 1V8 to 5V with varied restrictions to its performance. Ideally, I’d just include a low quiescent buck-converter to converter a 9V battery to 5V for the whole system. Sadly, I don’t have a converter handy in my parts bin, so here we are…

The solenoid was handled in by the original circuit posted here. 9V Vbat will be current limited to ~500mA regardless of the input voltage (within reason!). That leaves the ATTINY and a 9V battery, with no on-hand switchers or even LDO’s that fit the bill. All my LDO’s either didn’t support >7V input, or had a minimum load requirement that would squash the battery life of the board.

What I did have on-hand was a boost converter (BU33UV7NUX) on the same dev board as my ATTINY. So  I decided to add another battery, two actually (2x AA). This new battery will feed the boost converter that will generate a 3V3 supply for my ATTINY. The boost converter can operate all the way down to 0.6V!

Sadly, this plan didn’t last long...

So, I immediately killed my boost converter dev board *face palm*. I hummed and hawed for a while and decided to do something dirty, but easy. Why even bother regulating the supply! Two double A’s will get me around 3.2V -> 2.4V during their discharge. That’s well within the operating range of my ATTINY. I tested the circuit at the new range of Vin, and everything was still functional. 

Here’s the system running off battery. Everything’s working okay, though at higher quiescent currents then I measured previously. Originally I measured <1uA on the ATTINY, but now I measure 300uA. I suspect I tweaked something in my code during debugging. I’ll hunt this down later. (even at 300uA the current draw isn’t enough to drain the device over the summer, so I could leave it as is… seeing as this is just a prototype).