Making a temperature controlled heat pad

For my next project i want to make a temperature controlled heat pad that is able to regulate itself using an Arduino. I would like the pad to be able to maintain as steady heat as possible i think i have the basic code for this correct but would like too see if there are any suggestions on how i could make the temperature control better ideally it shouldnt fluctuate by more than + or - 1 degree Fahrenheit. The heater is supposed to light an LED when its at the correct temperature. Any ideas or suggestions would be greatly appreciated.
what i have so far:

int tempPin = A0;        
int tempReading; 
int ledPin = 2;
int heatingPin = 3;
int aref_voltage= 5;

float desiredTempC = 28; // which temperature to maintain

void setup() {
  Serial.begin(9600);

  pinMode( ledPin, OUTPUT);
  
  pinMode( heatingPin, OUTPUT);

  pinMode( tempPin, INPUT);
}

void loop() {
  tempReading = analogRead(tempPin);
   // converting that reading to voltage, which is based off the reference voltage
  float voltage = tempReading * aref_voltage;
  voltage /= 1024.0; 
 
  float temperatureC = (voltage - 0.5) * 100 ;
  
   if(temperatureC < desiredTempC){
    pinMode(heatingPin, HIGH);
  }
  else{
    pinMode(heatingPin, LOW);
    pinMode(ledPin, HIGH);
  }
    
   
  delay(1000);
}

Also would it be possible to simultaneously control two heat pads?

Also would it be possible to simultaneously control two heat pads?

Controlling multiple pads is no problem if you have a separate temperature sensor and output relay for for each pad.

With a simple on/off controller, the first thing I would do is to define a dead band, or hysteresis band, around the set point, of say +/- .5 degree. You will turn the heater off when you are at the higher value, and turn it back on when you drop back to the lower limit. Without this dead band, you can be cycling on/off based on noise and non significant variations.

Typically, most heating controls will have the indicator light on when they are heating, i.e. applying power.

If you wind up with significant overshoot or undershoot with this simple scheme, you can move to a time proportioning band where the on/off time is a function of the difference from setpoint. Much will depend upon the response of your system to the heater. You are best off getting started, keep it simple, and see how your system operates. You can optimize your control strategy ( on/off, simple time proportioning, or full PID) from there.

What is the heat pad for and what temp range ? What ambient temp ?

the temperature needs to be kept at about 104 degrees and i would like to control them sepreately if they are small enough and don't consume too much power would i be able to directly use the 5v output or just a regular pin output. i think a relay will be too bulky for my design or maybe even some transistors would be able to get the job done.

if they are small enough and don't consume too much power would i be able to directly use the 5v output or just a regular pin output

You are not going to be able to heat very much with the direct output of the Arduino.

You mentioned "heat pads", and you did not answer Boardburner 2 question "What are they"? What are you actually trying to do in your project.

Your heaters are going to have a watt density or total wattage associated with them. Your arduiono is never going to be able to output enough power to control such a device unless it's the size of your finger nail. You need a solid state relay for an operation like this if you heater is powered by 110 to 220 VAC. A mechanical relay will cycle too frequently and eventually stop making contact due to carbon build up or simple wear. If you heater is small enough and powered by DC current you might get away with a mosfet, but I really know nothing about them. Someone else here can likely guide you on that or do some google or youtube searches on them.

I have two system running on simple on/off control. One system maintains a +or- of 0.3deg F. The other system is maintained at +1.5deg to -.6 deg. PID would likely produce a bit more consistency, but the material being heated is constantly in flux and this is accurate enough for my needs. I have not used a dead band in my systems, just a trigger temperature. Both systems us SSRs switching 110VAC on one device and 220VAC on another. Heaters are rated at 10A to 15A and cover an area of about 12in by 80in.

With small heat pads especially the mechanical ararangement can be important as well. A pid controller will help here but if its liquid things like is there a stirrer make a big difference.

Also there are some self governing heat tapes and pads out there which do not need active control.

They depend on ambient temp though for effective regulation

BoardBurner 2 do you have any idea where you can find these self governing heat pads ive been looking all over but cant seem to find them. Being that they are self governing i would assume that they would be able to fulfill the purpose.

Ill have a look tomorrow.

They tend to be application specific and temp specific.

im not exactly sure how they work either.

One i saw was for culture dishes.

You could look at heating tape and wire, sometimes used on pipework.

Could be pricey though as minimum orders apply.

Thank you for your help i greatly appreciate it. If you find them and can post a link that would be great. Thanks again

http://www.heat-trace.co.uk/Products/Self-Regulating_Heaters

Its possible the pads were made with that.

Certainly felt paralell wires when pressed

Im assuming you meant 104 deg farenheit here though.

You did not specify units although its im plied from thplus minus limits

This looks like what i saw

http://www.harvardapparatus.com/webapp/wcs/stores/servlet/haisku2_10001_11051_39405_-1_HAI_ProductDetail_N_37610_37626#specificationstab

No evidence of the rectal probe though

:fearful:

Ok iv discovered a little more. Ones i saw are manufactured for a specific purpose .

Self regulating heat pads and tapes are not adjustable the polymer chemistry of the element fixes the max temp.

One i mentioned is very close to your temp requirment though.

104 is the top end of culture requirements used for some specific diseases and tests.

Not much detail but its fixed size , i guess about A3 paper size.

Size relates to power consumption which will dictate the size and type of your power supply and power switch.

You have not said yet what the specs are , small could mean anything.

If i installed football pitch heaters small would be a bed blanket.

Donek has done it .

Try to supply more information please.

Thank you for all the help as far as size is concerned they are roughly as large as a golf ball.