Variable speed fan controller for the fridge.

I am getting back into my old electronics hobby and have just ordered over £100 worth of components and tools.

The first project is a fan controller for my new fridge.

I want a temperature controlled fan that will exhaust the warm air through the top vent to keep the fridge operating efficiently and reduce battery consumption. The reason for this is I want a fan at the back to circulate the air, but I don’t like loud fan noises in the van. So I want the fan to not be running at all when it is cool and run at full belt if the fridge needs the airflow. The original idea was just to put a thermistor in series with the fan to adjust the voltage but this just doesn’t work very well as at lower temperature the fan just stalls out due to the low voltage. The new plan is to use a very cheap Atmel ATTiny85 microcontroller with a temperature sensor and the fan connected to one of the PWM outputs.

The reason to use PWM is that it outputs the full 12V required to drive the fan but pulses the output in a square waveform. You can then vary the speed by changing the ratio of on to off. This is the most efficient way to control any motor.

Whilst thinking about this and planning the software I realised that this system has a small problem. The fan will be programmed to go at a fixed speed based purely on temperature. In the UK during most seasons this would be fine. However during the summer and when abroad in hotter climes this could prove problematic. Say for instance I program the fan to be at 25% at 25C, 50% at 30C, 75% at 35C and 100% at 40C. This will work fine until the temperature reaches 25C. The fan would then be running all the time circulating warm air and having no effect.

So what I am planning to do is have 2 temperature sensors. The first which will be at the bottom of the fridge and the second further up or near the top. The microcontroller will then set the speed of the fan based on the difference in temperature. For instance if I am in the South of France and the temperature is 35C at the bottom of the fridge there is no point spinning the fan at 75% speed if the top of the fridge is at 36C.

The speed of the fan will be dictated purely by the differential in temperature.

I plan to use 2 x 140mm fans as the larger the fan the slower I need to rotate it to move the same volume of air as a smaller fan. For instance a 100mm fan will move 32 cubic feet per minute (CFM) whereas a 140mm fan will move 76 CFM. So I can run the 140mm fan much slower than the equivalent 100mm fan. As the ATTiny85 has 2×2 PWM outputs I can control up to 3 fans easily or 4 fans with a bit of difficulty. I plan on using 2 x 140mm fans so I can run them even slower to shift the same amount of air.

Anyway, I just now await the arrival of all my components.

This is the first in a series of project I have to make my van easier to live in using Arduino controllers.

Here is my shopping list so far.

1	Heatshrink			£2.90
5	dc adjustable PSU		£2.77
10	DS18B20 – 1 wire temp sensors	£7.68
1	prototype board			£2.79
1	20 x 4 LCD Display		£3.82
3	DHT11 temp/humidity sensors	£3.22
1	wire snips			£2.15
1	HC-05 bluetooth			£3.83
1460	Resistors			£7.07
200	Electrolytic Capacitors		£4.49
40	male to male jump leaders	£1.58
40	male to female jump leads	£1.74
40	Female to female jump leaders	£1.58
300	Zener Diodes			£2.64
100	diodes				£2.08
1	Soldering iron kit		£16.95
3	Digispark ATTiny85 USB sticks	£5.99
1	Arduino Uno R3 Clone		£10.00
10	ATTiny85 chips			£9.78
1	ATTINY85 Dev board		£0.99
10	Strip board			£5.78
5	TIP120 Darlington transistor	£1.98

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