So I'm trying to control the temperature of a couple of thermoelectric coolers. I'm using an H-bridge and PWM signals to control the voltage accross both peltiers, and I want to sense their temperatures as I increase the applied voltage using an LM35 temperature sensor. Ultimately, I would like to implement a PID algorithm so that the temperature of the peltiers can automatically stabilize at a given temperature.
For the time being, all I'm trying to do is to get the sensor to work conjointly with the peltiers to see if it can read their temperature accurately. However I'm experiencing some difficulty with the LM35. When I send a PWM value of 0 to the H-bridge enable pin, it displays seemingly accurate values. But if I operate the peltiers on full power, the LM35 starts giving me values which are way too high. For example, I obtain these readings in the Serial monitor:
Power = 0% Temp = 26.37C
Power = 0% Temp = 26.86C
Power = 0% Temp = 26.86C
Power = 0% Temp = 25.39C
Power = 0% Temp = 25.88C
Power = 0% Temp = 25.88C
Power = 100% Temp = 87.40C
Power = 100% Temp = 104.49C
Power = 100% Temp = 106.93C
Power = 100% Temp = 106.93C
Power = 100% Temp = 106.93C
Power = 100% Temp = 104.98C
I've attached the fritzing file for the circuit and here is my code:
// Peltier temperature control
int switchpin = 13; // switch to change peltier voltage sign
int peltier1pin1 = 2; // peltier 1 logic pin 1
int peltier1pin2 = 3; // peltier 1 logic pin 2
int peltier2pin1 = 4; // peltier 2 logic pin 1
int peltier2pin2 = 5; //peltier 2 logic pin 2
int enablepin1 = 9; // H-bridge enable pin 1
int enablepin2 = 6; // H-bridge enable pin 2
int power = 0; // power level from 0 to 100%
int peltier_level = map(power, 0, 100, 0, 255); // PWM value that is given to the enable pin
int inPin = 0; // analog pin for LM35
int prevValue = 0; // previous value read that is different from current value
float celsius = 0; // temperature value
void setup()
{
Serial.begin(9600);
pinMode(switchpin,INPUT);
pinMode(peltier1pin1,OUTPUT);
pinMode(peltier1pin2,OUTPUT);
pinMode(peltier2pin1,OUTPUT);
pinMode(peltier2pin2,OUTPUT);
pinMode(enablepin1,OUTPUT);
pinMode(enablepin2,OUTPUT);
while(!Serial);
Serial.println("Power 0 to 100");
Serial.println();
}
void loop()
{
// read the state of the direction switch
boolean SwitchState = digitalRead(switchpin);
// if the switch is high, set voltage polarity across peltier:
if(SwitchState == HIGH){
digitalWrite(peltier1pin1, HIGH);
digitalWrite(peltier1pin2, LOW);
digitalWrite(peltier2pin1, HIGH);
digitalWrite(peltier2pin2, LOW);
}
// if the switch is low, switch voltage polarity:
if(SwitchState == LOW){
digitalWrite(peltier1pin1, LOW);
digitalWrite(peltier1pin2, HIGH);
digitalWrite(peltier2pin1, LOW);
digitalWrite(peltier2pin2, HIGH);
}
// read desired peltier power
if(Serial.available() > 0)
{
power = Serial.parseInt();
if(power > 100) power = 100;
if(power < 0) power = 0;
peltier_level = map(power, 0, 100, 0, 255);
}
// Write value to peltier
analogWrite(enablepin1, peltier_level);
analogWrite(enablepin2, peltier_level);
// measure peltier temperature
int value = analogRead(inPin);
if (prevValue != value) {
float millivolts = (value / 1024.0) * 5000;
celsius = millivolts / 10; // sensor output is 10mV per degree Celsius
prevValue = value; // save current value as previous value
}
Serial.print("Power = ");
Serial.print(power);
Serial.print("%");
Serial.print(" Temp = ");
Serial.print(celsius);
Serial.println("C");
delay(500);
}
I'm very new to Arduino, so I can't determine where the problem is coming from, if it's something to do with my code or the way I have connected the sensor. Any help would be great!
Cheers
peltier_controller.fzz (9.27 KB)
