| The inputs would be one or more temperature sensors, and the outputs would signals to turn on the furnace or air conditioner. The human-machine-interface would be some way to specify the "set point" temperature, probably using a small number of buttons (like, temperature "up" and "down"). |
| The microprocessor in the PLC would continually sample the temperature and then decide whether to turn on or off the furnace or air conditioner. It would presumably implement a timer so that it wouldn't turn them on and off too rapidly since it would be expected to take some time before the ambient temperature would change by the desired amount. |