Depending on features being supported, different parts of a user interface are required.

Input: On/Off. This is required in all variations, and allows the user to turn the system off. This may be desired when away on vacation, or when servicing the HVAC system. It may be desirable to allow the user to tie this into an off switch on the existing thermostat, but this may cause too much complexity for some users to install, or vary too much based on the thermostat.

Output: On status. This is required in all variations. If the on/off input is handled with a slide switch, the visible state of the switch can cover this. Otherwise an LED that blinks occasionally(to conserve power) may be used.

Input: Cycle time. This is required in all variations, and allows the user to configure based on the attributes of their home. In the basic model, this can be dip switch settings. In a more advanced model with push buttons and an LCD or wireless, this can be set through that interface. The cycle time is the larger window of time that the minimum run time must be met during. For instance, this value could be 1 hour.

Input: Run time. This is required in all variations, and allows the user to configure based on the attributes of their home. In the basic model, this can be dip switch settings. In a more advanced model with push buttons and an LCD or wireless, this can be set through that interface. The cycle time is the the minimum fan run time to enforce during the cycle time. For instance, this value could be 6 minutes. In this example, the module would make sure the fan runs for at least 6 minutes every 1 hour. Looked at another way, there will never be a period of more than 54 minutes where the fan does not run.

Output: Fan status. This is desired in all variations, but may not be required. It could be an LED or other output that indicates if the module has turned on the fan.

Input/Output: LCD and buttons. This is optional, for advanced features. This would support an easier to use way to set the run time and cycle times, displaying on/off and fan status, as well as supporting scheduling.

Input: Remote temperature. This is optional, for advanced features. This would allow one or more remote sensors that the module could use to make a more intelligent decision about running the fan. As nominally described, the module simply maintains a minimum fan running duty cycle. With remote sensing(for instance upstairs and downstairs), the module could run the fan more or less depending on temperature differential between remote and local. This could be wired or wireless.

Input/Output: Wireless BLE Management interface. This is optional, for advanced features. It would allow connection of the module to a smart device or computer for controlling a UI for advanced features(setting run, cycle times, status display, scheduling, temperature reporting), as well as remote control. This would allow the more advanced features, while not requiring an LCD and pushbuttons.