Today I built a prototype relay controller which can be controlled via MQTT, but also has built in temperature and humidity sensors. It uses the temperature data to act as a thermostat turning one of the relays on and off at a temperature set point with a few degrees hysteresis (so that it doesn't go nuts switching on and off if it's fluctuating over the set point). Hysteresis "time" can also be added so that I can use the same code for another controller which will control a fridge. You don't want to be turning the compressor on and off rapidly. To get the temperature, the values from two ds18b20s and a dht22 (not pictured) are averaged.