(Time Travel Alert – this happened a week ago. If you played Real Life Pac Person at the preview on the 18th, then you’ve seen this and more. If you’re playing in Burlington, NC on the 25th at Mini Maker Faire, then know that I may not have the blog caught up until Sunday)
If you’re using a Raspberry Pi for a wearable, then you have an acute power problem. The RasPi needs 5 volts at about half an amp, peak, when a USB wireless adapter is connected. Steady-state draw with a very light workload will be about 350 mA. Based on the performance curves from a reputable manufacturer of batteries (OK, “cells”), this is going to give you about 3 hours of battery life on a quiescent device and a little less if it’s busy. Add in as much as 110 mA for all the blue LEDs, and Houston, we’ve got a problem.
So, Nickle Metal Hydride (NiMH) is your friend. 2500 mA/hr off the shelf, no waiting.
But you didn’t think it was going to be that easy, did you? Of course not. Four of those cells will give you 4.8V. Technically, that’s not enough voltage. In practice, yes, it will work, especially if all of your USB devices have nice, broad safety margins. I don’t want to chance it. Also, the wire lights vendor lied: the blue strands are actually 12V devices (they claimed 5 volts, and I should have known better…) and need a good 11.0V to be bright enough. Given that, I’m stuck using ten NiMH AA cells or eight alkalines and a shorting jumper.
12 volts is enough to fire a Raspberry Pi into the asteroid belt, so it has to be stepped down. In Ye Olden Days, that would be with something like a 7805 regulator – an analog device that basically turns excess voltage into heat, and lots of it. Welcome to the twenty-first freaking century. We now have the 78SR-5/2-C. This is a 5V switching regulator a little bit bigger than a postage stamp and having a sweet, sweet 2 amp current rating (twice the original 7805). Claimed efficiency is around 90%… what it draws in, it puts out. Not much heat at all. Bonus side effect – the wattage in is just about the same as the wattage out, so if you produce 5 volts at 1 amp, then it will load down a 10 volt battery stack at half an amp, give or take. Nice.
Here it is cobbled up on the breadboard for a fast checkout:
Please ignore the cheeseball AM transmitter on the upper strip. That’s for a future revision of the game. 🙂