11: Uninterruptible Power (the other UPS) for Arduino, Spark Core or Raspberry Pi
* revised Apr 17, 2015 *

There seems to be precious little help on line for this even though it turned out to be pretty easy. I watched --

https://www.youtube.com/watch?v=RiZ0g1bJ8sw

I got my parts from Adafruit (charger: $6, powerboost: $15, 1200mAh battery: $10, plus their irritating shipping charges).

I tested this with an Arduino Nano--

both external power and  battery (note red LED*)

And with my Spark Core--

just battery

You can connect/disconnect the battery any time. And this will work for Raspberry Pi just as well (REVISED: probably, if the system doesn't draw more than 1000mA -- see dicks-raspberry-pi.blogspot.com/).

Also REVISED: When I let the Spark run until the battery ran down there was a problem: The Powerboost didn't just quit cleanly. The boost/Spark stopped/restarted, stopped/restarted (voltage hiccups?). I suppose that when the boost stopped drawing current then the battery recovered a bit. Anyway: not nice. The Powerboost has an Enable pin (labeled (EN). Grounding that will stop things cold. Then how does it restart? What un-grounds the EN? See * note below. Kudos to Spark that the Core survived undamaged! Note: the Powerboost did not survive (Adafruit replaced it).

Considering the way that (non-hobby) "programable controllers" are used, I'd think that a UPS setup like this would be about Arduino Lesson 3!

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* If you aimed a photo resistor ($1) at the charger's red LED external power indicator then you could detect power failure and shutdown gently (but HOW and HOW-TO-RESTART?).