in Home Automation

Better Laziness Through Motion Sensors

Back in 2013 when we started our home automation journey, the house was entirely LightwaveRF devices. I’d replaced every light switch and put on/off or dimmer sockets on every lamp. The addition of a LightwaveRF motion sensor provided our first bit of “lazy” automation. Sensors like this one mounted in the kitchen and breakfast room would turn on the lights in those areas as we walked in.

This turned out to be really useful – on walking into the potentially dark kitchen area while holding a pile of plates and dishes the room would magically light up for us. No need to fumble for the light switch. Also, because the sensors have a timer function, the lights would turn off again a configurable period later saving energy. Win!In practice though, the nature of the LightwaveRF protocol meant this system had a number of limitations. As I mentioned before, LightwaveRF has no concept of status, so there is no central controller that is aware of the motion state of the room. This means you get a very rigid effect: motion sensor sends a signal, the lights receive it and turn on. Then after the configurable delay (five to ten minutes) the lights are sent the off signal.

Problem one: the off signal is always sent after the configured delay, even when there is still motion present. This is annoying if you’re doing anything in the kitchen, since the lights will turn off while you’re doing stuff but won’t turn back on since as far as the sensor is concerned it has already sent the “on” command.

Problem two: sense range and sensor confusion. To get coverage across the kitchen and breakfast room required two of the sensors, since one alone wouldn’t catch people at the far end of the room. Two sensors, one placed at either end of the room solved this problem, but caused another related to problem one: they’d each send the on and the off independently of each other (no central controller, remember?). This could result in the lights being turned on by one sensor and then turned off by the other. D’oh.

Enter SmartThings

This is a SmartThings Motion Sensor. It costs around the same as the LightwaveRF sensors but has better range, a built in temperature sensor and most importantly integrates with the SmartThings controller for more complex rule based automation.
I have one of these mounted at one end of the kitchen.

The range means it can see us as soon as we walk in at either end of the kitchen or breakfast room. More importantly though, the logic I can apply in the associated app (below) means the lights will stay on as long as anyone is in the room, and only turn off when no-one has moved for around five minutes. That’s not just a fixed five minutes from when the lights turned on, as with LightwaveRF, it is five minutes after the motion stops. “Things Start Happening” is the SmartThings way of saying “motion detected”! Additional logic means I can configure the lights to stay off if the house is in night or away mode – so the cats don’t turn things on at 2am or if no-one is in the house.

I’ve already added another of these in our hall, and may yet make more use of them elsewhere. The ability to combine other sensors, presence devices and logic makes the whole platform incredibly useful!