I’ve included a humidity sensor which might provide some interesting information and might be used to turn on ventilation if the humidity gets too high. This temperature and humidity sensor from Adafruit, has a good specification and communicates with the microcontroller over I2C.
It easily fits into a Hammond 1551G box which is just big enough to take a 10mm cable gland it you shave some plastic off the securing nut (see picture left).
I have soldered on a small piece of header to the board and soldered that in turn to a small piece of strip board which can be screwed to the small plastic mounting pillars in the box.
I have drilled a series of 1.5mm holes into the lid so the sensor is exposed to the air.
I wired this up to the monitoring equipment with “Alarm Cable”. This contains eight different coloured strands and is shielded. (I often use wires stripped from this cable for general low voltage connections for which they are very useful!)
The sensor currently hangs from the roof of the greenhouse.
With the library from Adafruit, it is very easy to feed measurements from the sensor into a sketch.
I mentioned above the possibility of controlling a ventilation fan. Here is such a fan (far left). It is an extractor fan designed to go in the ceiling above a bathroom or similar. Here, I am using it to blow cool air in from outside the greenhouse. I think it is important to do this as, if the doors are closed, even if the ceiling vents are open, not much air will circulate.
The fan is freestanding and can be pointed in any direction. It is connected to the control unit and turned on under program control, at present, according to temperature.
Even if the automatic vents are open, if the doors are closed, without a fan, not much air will circulate and temperatures will rise to possibly dangerous levels for the plants within.