# The Learning Circuit 16: Temperature Activated Fan!

Version 10
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 In a previous segment, DaftMike builds a thermometer using an Arduino and a thermistor. He expands on this in order to show you how to build a temperature controlled fan for your desk. It monitors the room temperature to see if it gets too hot.  When it gets hot, the Arduino is used to turn on a fan so that you can cool off!

Parts & Products:

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DaftMike goes over the code from last time.  It reads a thermistor and prints the temperature to the serial monitor.  He moves all the equations he wrote earlier into four separate functions.  A function is like its own little program.  In Arduino code there is always a setup and a loop function.  The setup is at the start and only runs once.  The loop comes after that and repeats continuously.  Taking the equations from last time, he re-writes them so that they each have their own separate function. The main loop uses these functions to execute the code. Structuring your program this way makes it easier for you to reuse parts of your code. For example, if you wanted to add another temperature sensor then all you would need to do is call these functions again and pass them a different value.  It saves you time as well as some program memory.  For it to work, he’ll need to add a couple more functions.  One for turning the fan on and one for turning the fan off.  He’ll also need to set up the built-in LED as an output to stand in for the fan during testing.  When that is done, he tests the code.

```/********************************************************
element14.com/thelearningcircuit
Episode 16: Temperature Activated Fan
********************************************************/

// Voltage divider equation parameters
const int Vin = 1023;
const long R2 = 100000;
// If you want to measure your specific R2 resistor, you can input it's actual value here for potentially better accuracy

// Steinhart–Hart equation parameters
const long R0 = 100000;     // 100K thermistor
const int B = 3974;         // Beta value (K)
const float T0 = 298.15;    // 25°C

// Analog Pin used to read the thermistor
const int thermistorPin = A0;

// Number of readings used for average

const int testLED = 13;

// Temperature thresholds for the fan in Celcius (to use Farenheit change 'T' for 'Tf' in the 'Temperature test')
int highThreshold = 28;
int lowThreshold = 27;

bool fanStatus;

/**********************************************************************************************************************/
void setup() {
Serial.begin(9600);             // opens serial port, 9600 baud

for (int i = 2; i < 7; i++) {   // setup pins 2-6 as outputs and make them low
pinMode(i, OUTPUT);
digitalWrite(i, LOW);
}

pinMode(testLED, OUTPUT);       // setup the on-board LED as an output

}

/**********************************************************************************************************************/
void loop() {
// Call our functions to get the temperature readings
float R1 = getR1Value(Vout);
float T = getTemperature(R1);
float Tf = celciusToFarenheit(T);

// Print the temperature in Celcius and Farenheit to the serial monitor
Serial.print(T); Serial.print("C | ");   Serial.print(Tf); Serial.println("F");

// Temperature test for turning the fan on/off (change 'T' to 'Tf to use Farenheit)
if (T > highThreshold && !fanStatus) {
fanOn();
}
else if (T < lowThreshold && fanStatus) {
fanOff();
}

}

/**********************************************************************************************************************/
float readSensorAverage(int analogPin) {    // take multiple readings from our thermistor and return the average value
float Vout;
for (int i = 0; i < numReadings; i++) {
delay(10);                              // allow the ADC to settle between measurements
}
Vout /= numReadings;                      // divide the running total by the number of readings to get an average
return Vout;

}

float getR1Value(float Vout) {              // Voltage Divider equation to calculate R1
float R1 = (R2 * (Vin - Vout)) / Vout;
return R1;
}

float getTemperature(float R1) {            // Steinhart–Hart equation β(Beta) parameter version
float T = (1.0 / ((1.0 / T0) + (log((R1 / R0)) / B))) - 273.15;
return T;   // T is in Celcius
}

float celciusToFarenheit(float Tc) {        // Celcius to Farenheit conversion
float Tf = (Tc * 9.0) / 5.0 + 32;
return Tf;

}

void fanOn() {
// this sequence turns our remote controlled socket on
digitalWrite(2, HIGH);
digitalWrite(3, HIGH);
digitalWrite(4, HIGH);
digitalWrite(5, HIGH);
delay(100);
digitalWrite(6, HIGH);
delay(250);
digitalWrite(6, LOW);

Serial.println("turning the fan on...");
digitalWrite(testLED, HIGH);
fanStatus = HIGH;

turnOffRemotePins ();

delay(1000);

}

void fanOff() {
// this sequence turns our remote controlled socket off
digitalWrite(2, HIGH);
digitalWrite(3, HIGH);
digitalWrite(4, HIGH);
digitalWrite(5, LOW);
delay(100);
digitalWrite(6, HIGH);
delay(250);
digitalWrite(6, LOW);

Serial.println("turning the fan off...");
digitalWrite(testLED, LOW);
fanStatus = LOW;

turnOffRemotePins ();

delay(1000);

}

void turnOffRemotePins () {         // save power by 'turning off' the remote pins
for (int i = 2; i < 7; i++) {
digitalWrite(i, LOW);
}
}

/*********************************************************************************************************************
ENER314 RF-Transmitter Board datasheet:
https://energenie4u.co.uk/res/pdfs/ENER314%20UM.pdf

Steinhart–Hart equation β(Beta)
https://en.wikipedia.org/wiki/Thermistor#B_or_%CE%B2_parameter_equation
*********************************************************************************************************************/```

After testing the code, he notice that when the temperature gets close to the trigger point it flashes.  This is not what he wants.  The way to fix this is to add what’s called hysteresis.  Hysteresis is the dependence of the state of a system on its history. It’s when the output of your system, in this case the fan or the LED for now, lags behind the state of your input, which in this case is the room temperature. Returning to the code, instead of a single trigger point, he’s set some thresholds to check. There’s a high threshold and a low threshold. As long as one number is lower than the other you can use code to check if the temperature is higher than the high threshold and switch the fan on for true.  You can then use a conditional statement to perform a check to see if the temperature is lower than the low threshold to turn the fan off.  He proceeds to test the new code.  Once the code is fixed, you can replace your test LED with a real fan. Arduino pins can only supply a small amount of current, around 20 milliamps.  This is good for LED but you’re going to need additional circuitry for something more powerful like a fan.  You could use a relay.  A relay is a type of switch that’s controlled by a magnetic coil.  In this case, DaftMike wants to use an existing fan that plugs into the mains so he uses a remote controlled outlet, originally designed for Raspberry Pi.