I Can Haz Surveillance - Part 2

Part 1

This is how my DIY projects often go: I have a burst of creativity and enthusiasm, do some research, learn some things. Then I put the project down, intending to come back to it soon. "Soon" usually turns out to be a few months later.

In the last couple of weeks, I've decided to pick up my cat GPS project in earnest again. In the intervening time I've tinkered intermittently, and managed to solve a couple of problems:

• The finished unit will have to be as small as possible. I replaced the Flora board, which is inconveniently large and round, with an Arduino Pro Mini, which has a much slimmer profile. It's a simpler board without built-in battery connector or switch - I consider this a positive because the layout of my circuit is more flexible. I also found instructions for getting the GPS to talk to a non-Flora microcontroller and got them talking, which I consider a small triumph.

• I was concerned about what type of battery to use. The easiest type to attach to a wearable project is a Lithium Polymer (LiPo) battery, but these aren't the most stable. If it's secured firmly into place within an enclosure, no harm should come of it, but still, you can't tell a cat to be careful and I don't want to risk injury. On the other hand, using the equivalent voltage in normal household batteries would make the unit heavier. In the end I settled for a Lithium Ion battery, which is more stable, and also comes in a 3v version, meaning I only have to use one battery. Also loving the pun of fitting a cat with a Li-ion battery (RAWR!)
  
  I started out with a larger battery that, based on its milliamp hours (mAh), would definitely stay charged for an entire day of cat frolicking. Now though, I've decided to settle for a smaller battery (actually a camera battery) which weighs less but won't last as long. We'll see how that one goes - it's all experimental for now.

The next phase is to create an enclosure for the components. This will need to be weatherproof, and strong enough to survive having a cat's weight on top of it. Some Hackerspace friends have assured me that the GPS module needs radio line-of-sight to the sky, but not necessarily visual line-of-sight. So I've designed a lidded case to be 3D printed in ABS plastic, which is light and strong:

The raised bar in the middle of the case is where I'll mount the Arduino and GPS module, using Blu-Tac so I can easily remove them and download the data. Depending on how well the fasteners work, I may have to gaffer-tape around the case while it's in use, to make sure it's secure.

Once I've put this together, I'll be able to walk around with the unit and test how well it logs data when fully enclosed. If that works, I'll tweak the design so I can mount it on a cat collar. Stay tuned!