Comment by Animats | Hacker News Reader

Comment by Animats | Hacker News Reader

Note that only the sensor runs for a year on a coin cell. The wireless mesh router nodes are not mentioned.

Somebody has to be listening when coin-cell device wakes up. If the listening device is always on, that's not a problem. That's not feasible for low-power nodes. If listeners are only powered up intermittently, getting someone to listen requires some kind of synchronization system.

I have a simple outdoor thermometer which contains a solution to this problem. The sending unit goes outside, for best results sheltered but not near a wall that leaks heat. The receiver goes inside and has an LCD display. Both sender and receiver are battery-powered, and get over a year on two AA batteries.

The sender wakes up every 30 seconds or so and sends blind. The receiver wakes up just before each expected transmission. When you replace the batteries, you have to replace them in both units, and when the receiver is first powered up, it stays powered for a while until it's heard the sender a few times and is in sync.

I raised this objection downthread: https://news.ycombinator.com/item?id=14848196

@adunk says mesh nodes synchronize clocks so they're all listening at the same time, but intermediate nodes end up drawing enough power they can't actually run on a coin cell.

You can buy Sensortags with sub-1GHz RF transmitters, which have a (theoretical) range of >500m, which should be enough for most applications to not require a mesh network at all. They also accurate clocks (with a crystal) to do time-rendezvous networking as well.

I've some intended for home monitoring-- they have "10 sensors including support for light, digital microphone, magnetic sensor, humidity, pressure, accelerometer, gyroscope, magnetometer, object temperature, and ambient temperature" for $30.

The microphone doesn't work, TI never released packaged firmware for it. There's source code available that should work, but I haven't seen anyone on the forums that managed to get it integrated into an application, much less even build it.

Although not CO2 or CO or smoke, all of which would be useful. For HVAC purposes, light, humidity, pressure, CO2, CO, and smoke are the useful ones. Leave out the microphone for privacy, but consider a passive IR motion sensor.

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