Feather smart thermostat

Parts

a feather huzzah
a 2.13 e-ink monochrome wing
a I2C SHT31-D temperature and humidity sensor
a single channel relay
a current sensor
three push buttons
safety "temperature is too low" disc thermostat
safety "temperature is too high" disc thermostat

Design Approach

"Dumb" safety

Safety thermostats override the system to always turn on when it's getting too cold and always turn off when it gets too hot. This will be achieved by putting the "low temperature" thermostat in parallel with the relay that is fed from the smart part of the thermostat, then connect them both in serial with the "high termperature" thermostat.

Those are driven by fixed-temperature disc thermostats.

Monitor Current

Since the safety thermostats may trigger the current on or off independently of the "smart" logic, we will monitor and report the themperature so we can tell what the real state is.

Stand-alone device

The thermostat will work as a stand-alone device first, with three push buttons to raise and lower the set temperature as well as utility functions, such as changing from celsius to farenheight or forcing a reset.

MQTT integration

The "smart" part of the thermostat will be driven by MQTT integrations via wifi, the thermostat will publish data for:

Actual temperature
Actual humidity
Actual current
Relay status
Set temperature

And the thermostat will receive command for:

Set temperature

The idea is that it should always be possible to override the current set temperature from the local device, and that should be reflected to the automation system, while at the same time it should be possible for the automation system to override the manually set temperature.

Configuration

This will read a json file named thermostat.json in the root of the sdcard.

The sample file below explains the expected format:

{
    "wifi": {
        "ssid": "",
        "password": ""
    },
    "mqtt": {
        "broker_host": "",
        "broker_port": 1883,
        "clientid": "",
        "username": "",
        "password": "",
        "topics": {
            "action": "",
            "actual_temperature": "",
            "actual_humidity": "",
            "actual_current": "",
            "target_temperature": "",
            "ramp_up_buffer": "",
            "cool_down_buffer": "",
            "temperature_command": ""
        }
    },
    "temperature": {
        "starting": 21,
        "variance": 1,
        "ramp_up_buffer": 0.1,
        "cool_down_buffer": 0.1
    }
}

Most items are self-explanatory. The temperature section requires some explanation:

starting: Value of target temperature on reset, such as when returning from a power outage.
variance: This configures how wide should the control allow the temperature to vary. For instance, if you have the target set to 21, and a variance set to 1, the ambient temperature should vary between 20.5 and 21.5 degrees.
ramp_up_buffer (Optional): How many degrees before the low end of the variance to turn the heater on, such that we don't get below the target variance due to the ramp up time of the heater element. This is the initial value, which will be automatically adjusted over time.
cool_down_buffer (Optional): How many degrees before the high end of the variance to turn the heater off, such that we don't get above the target variance due to residual heat after we turn the heater element off. This is the initial value, which will be automatically adjusted over time.

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