sat, 27-oct-2012, 13:46
project build

One of our servers at work has a power supply that is cooled with two small, loud fans—loud enough that the noise is annoying for the occupants of the office suite shared by the server. What I’d like to do is muffle the sound by enclosing the rear of the server (or potentially just the power supply exhaust) with an actively vented box. Since the fan on the box will be larger, it should be able to move the same amount of air with less noise.

But messing around with the way servers cool themselves is tricky, and since the server isn’t easily accessible, I’d like to be able to monitor what’s going on until I confirm my setup is able to keep the server cool.

I’ve built quite a few temperature monitoring circuits, but in this project I’ll need to be able to control the speed of the vent fan based on the temperature differential inside and outside the box. This is complicated because the Arduino runs on 5 volts and the fan I’m starting with requires 12 volts. I’ll use a Darlington transistor, which can be triggered using the low power available from an Arduino pin, but which can carry the voltage and current of the fan. For higher voltages and currents, or for controlling devices that run on alternating current, I’d need to use some form of relay.

To measure the temperature differential between the inside and outside of the enclosure, I’m using two DS18B20 temperature sensors, each wired to the end of a length of Cat5e cable (and wrapped in silicone “Rescue Tape” as an experiment). The transistor is a TIP120, and a 1N4001 diode across the positive and negative leads of the fan protect it from reverse voltages when the fan is turned off but is still spinning. A 2.2KΩ resistor protects the trigger pin on the Arduino.

Here’s the diagram:

circuit diagram

The Arduino is currently programmed to turn the fan on full speed when the temperature from the sensor inside the box is more than six degrees higher than the temperature outside the box. The fan runs at half speed until there’s less than a two degree differential, at which point the fan shuts off. These targets are hard-coded in the software, but it wouldn’t be too difficult to change the code to read from the serial buffer so that the thresholds could be changed while it’s running.

Here’s the setup code:

#include <OneWire.h>
#include <DallasTemperature.h>

#define ONE_WIRE_BUS 8

#define fanPin 9

OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature sensors(&oneWire);

DeviceAddress thermOne = { 0x28,0x7A,0xBA,0x0A,0x02,0x00,0x00,0xD1 };
DeviceAddress thermTwo = { 0x28,0x8B,0xFD,0x0A,0x02,0x00,0x00,0x96 };

void setup() {
    pinMode(fanPin, OUTPUT);
    digitalWrite(fanPin, LOW);


    sensors.setResolution(thermOne, TEMPERATURE_PRECISION);
    sensors.setResolution(thermTwo, TEMPERATURE_PRECISION);

void printTemperature(DeviceAddress deviceAddress) {
    float tempC = sensors.getTempC(deviceAddress);

And the loop that does the real work:

void loop() {
    float thermOneF = DallasTemperature::toFahrenheit(sensors.getTempC(thermOne));
    float thermTwoF = DallasTemperature::toFahrenheit(sensors.getTempC(thermTwo));
    float diff = thermOneF - thermTwoF;
    if ((diff) > 6.0) {
        digitalWrite(fanPin, HIGH);
    } else if (diff > 2.0) {
        analogWrite(fanPin, 127);
    } else {
        digitalWrite(fanPin, LOW);

When I’m turning the fan off or on, I’m using digitalWrite, but when running the fan at half-speed, because I’m using a pulse-width-modulation (PWM) digital pin, I can set the pin to a value between 0 (off all the time) and 255 (on all the time) and cause the fan to run at whatever speed I want. I’m also dumping the temperatures and fan speed setting to the serial port so I can read those values later and evaluate how well the setup is working.

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