Mesh networks

This is really more of a TIWRO (today I was reminded of) than a TIL, but that doesn’t have the same kind of ring, does it?

I’ve been playing with home automation for a few years. Mostly it’s motion sensors and automatic lights:

• Walk into the utility room carrying the laundry, the lights come on automatically.
• Walk out of the utility room carrying a toolbox and a ladder, the lights will go off automatically five minutes later.
• Walk into the kitchen on a bright, sunny day: only the task lights over the counters come on. On a cloudy day (or at night), the overheads come on as well.
• Walk into the hallway, the lights come on automatically.
• Walk into the hallway late at night and only one light comes on and only at the lowest possible setting.
• Need to be reminded to do something every evening: lock the doors, walk the dog, start the dishwasher, what-have-you? Setup a light in the bedroom (I used a short LED strip) that comes on shortly before your usual bedtime and stays on until you push a switch. No more forgetting.

A few days ago, one of the motion sensors stopped working. I figured it was time to change the batteriesWhich my father would have pedantically referred to as “AAA cells” because a battery is what you get when you place two or more cells in a circuit. and that fixed it. The next day another one failed, but I was out of batteries, cough cells, so I just grumbled everytime I walked past it.

Then another failed. Then another. Then the one at the end of the hallway with the fresh batteries stopped working again.

What the [expletive]?

I’ve never had anything like that happen. First step, walk over to Tesco, get a new pack of AAA’s and replace the batteries (step one if something doesn’t work: make sure it’s plugged in). Then try to figure out what on earth is going on.

I replaced the cells in the first detector and it started working again. I replaced the cells in the second (they were the originals from the manufacturer, so it’s quite likely they were dead). And the third. And when I got to the fourth, the lights came on.

I walked back to my office and the hallway lights came on as I passed that detector: the one with the new batteries that had stopped working.

Then it struck me. I believe these devices operate in a mesh network. It follows that if several devices are communicating through one node in the network and that node fails, so do all the nodes communicating through it. I suppose if the mesh was dense enough, they might route through a different node, but our network is pretty sparse. There’s more-or-less a single line of devices leading from my office, where the hub is, through the hall, across a landing, down the stairs, and to the vestibule at the door.

I hypothesize that only one (or perhaps two) of the devices actually stopped operating, but they were acting as intermediate nodes for the others, so those others could no longer communicate with the hub even though they were working just fine. (Unfortunately, this means I’ve probably replaced some perfectly good cells. I really need to get a battery tester.)

Anyway, I’m delighted that new cells fixed it and my top priority today isn’t debugging the house!