Power socket for 12 v supply from generic laptop power supply.
Two preset potentiometers for setting the thermostat (high or low).
3 way switch for selecting high or low thermostat settings or turning the fans off.
Plug for output to fans.
Use of stripboard (Veroboard) and rightangle headers to connect to the Arduino.
The circuit board and the Arduino were mounted in a Maplin’s plastic box. I’ve used RJ45 sockets for the temperature sensors as I did on the boiler controller. This is handy as flat network cables are available in all sorts of lengths very cheaply and can be discreetly glued to surfaces with contact adhesive or passed under carpets etc.
The sensors themselves will also be similar to the boiler controller versions with level controllers at the sensor end rather than at the Arduino end. My theory is that I2C goes further at 5v than 3.3 but it is an untested theory!
The software for the controller can be downloaded as a PDF here.
Further details of the construction of the fan unit are here.