MQTT Config

So as I work to integrate MQTT into my automation environment, I find myself wishing for some sort of device discovery mechanism. Possibly also a device configuration mechanism.

Here’s the problem as I see it.

If I know the topic of a device, I can subscribe to it and I can even get its last data. I can also subscribe to all topics or a subset of topics. But then I have to wait for a device to publish something in order to discover it.

Unfortunately, I can’t ask Mosquitto what devices it knows about.

There are many use-cases where this just isn’t sufficient:

• A device that isn’t publishing when you reboot.
• A device that is turned off and only publishes when it gets turned on (like a device that uses a tilt sensor to power itself on).
• A device that doesn’t publish at all (a subscribe only device like a Relay).

It would also be nice to be able to re-configure some attributes of a device though I can see how this could be abused in an InternetOfThings scenario.

So I believe I’m going to define a ‘discovery’ record and experiment with that. I’m thinking something like this:

/Device_ID/config/[Name]/[Attribute]/[Value]

Examples:

/Kitchen/config/Temperature/type/int
/Kitchen/config/Temperature/direction/input
/Kitchen/config/Temperature/unit/celsius
/Kitchen/config/Humidity/type/float
/Kitchen/config/Humidity/direction/input
/Kitchen/config/Humidity/unit/percent
/Garage/config/Relay/type/bool
/Garage/config/Relay/direction/output
/Garage/config/RelayToggle/type/int
/Garage/config/RelayToggle/direction/output
/Garage/config/RelayToggle/unit/seconds
/Garage/config/Relay/type/bool
/Garage/config/Relay/direction/input

This tells me that is a sensor named “Kitchen” that publishes a temperature as an int in degrees celsius… There’s also a control named “Garage” that publishes a “Relay” state but it also has an output that it subscribes to called “Relay”… They both provide/take a boolean. There’s also a control named “Garage” that subscribes to “RelayToggle” that will conceivably cycle a relay on/off or off/on for some number of seconds.

I’ll experiment with that and see if I can make it useful… It will allow me to maintain an inventory of devices that are available… I will have to come up with some sort of expiry mechanism of course and if an attribute changes, I’ll need to override the old config with the new one presumably.

To address the issue of my temperature reading too high on boards with temperature/humidity sensors, I opted to deploy the ESP system_deep_sleep(us) call to put the board to sleep after publishing the temperature via MQTT.

Current draw when the board boots is 145mA until it connects to the AP, then it seems to go down to about 77mA and then when it goes to sleep, it draws about 55-60uA…

Presently I have it configured to wake up, bind to my AP, after it gets an IP, publishes the temperature/humidity and go to sleep for 30 seconds.

Here is the board:

Here’s a thermal video demonstrating this. At 00:04 and 00:35, you can see the LDO heat up when it wakes up from it’s nap:

Video was taken using a Therm-App android thermal imaging camera borrowed from Angus.