Temperature fan control with an ESP32

A simple way to output a PWM signal based on temperature readings by a thermistor

What you need:

• An ESP32
• A PWM fan (or anything that accepts a PWM signal that you want to control)
• A voltage regulator for the ESP32 (if you have a 12V fan)
• An NTC thermistor (take one from a 3D printer hotend), preferrably a 100K 3950
• A 100KΩ resistor
• A pull-down resistor (1KΩ is fine)
• A 12V voltage source (or 5V for 5V fans)
• Some free time

notes:

• Check the pinout of your ESP32 to avoid headaches! Not all boards are the same!
• If you want to put everything on a breadboard, the ESP32s without silkscreen legends are thinner
• If you use 12V fans, the voltage regulators linked above are configurable. Get a thin blade and some solder
• If you use small 5V fans you don't need to step down the voltage and can just use a 5V supply for everything
• the thermistor resistor isn't necessarily 100KΩ, use the voltage divider equation to find an appropriate resistor (we'll set this up in the code). Usually a resistor with the same value as the thermistor is good

Getting Started

• download pwm_fan_control.ino from its folder and open it with the arduino IDE
• Wire everything up
• set up the code and flash it to the ESP32
• if you don't know how to flash to the ESP32 from Arduino IDE, follow this tutorial to get started
• you're done!

Table of contents

• Getting the materials
• wiring
• Setting up the code
  • Setting up the fan curve
• Calibration
  • Opening the serial monitor
  • figuring out the voltage offset
  • calibrating the thermistor

Wiring

Wiring everything up is fairly easy, as you can usually do everything on one side of the ESP32. Again, make sure to check the pinout of your board, as some have the 3V3 output on the other side!

Follow this image for reference:

• note that in this image the bottom rail in the breadboard has 12V, and the top rail is 5V. Don't mix them up!

• the pin next to the 5V input is NOT a ground! it doesn't say GND, but CMD. follow the diagram in the image above!

• By default, the fan PWM output (green wire) is on pin GPIO35, and the thermistor input (brown wire) is on pin GPIO25. Check where the pins are on your board, as some have them scrambled in different places. If you choose differents pins, you can edit the default ones in the next step. For the thermistor input, make sure to choose a pin connected to the ADC (ADCx_y in the website's image)

• The pull-down resistor (connected to the green wire) isn't strictly necessary, but if you don't use it some fans might misbehave

• The 100KΩ resistor goes between 3V and the thermistor, as in this image:

  

• You need to set up the voltage regulator for 5V. To do this, cut the little trace over the ADJ pad with an xacto knife (circled in the image), and bridge the two pads for 5V. If you plan on modifying the project a bit, remember that these voltage regulators can't boost the voltage, only step it down! If you need to go up, buy a different one.

• The Enable pin of the regulator will stay discconnected, as the regulator can stay on all of the time.

  

• Be careful with connecting the fan wires correctly! the PWM signal wire is NOT next to the power wires, but the one on the other side of the connector! be careful or you will fry your esp32!

• If you're following these instructions in order and plan on calibrating the thermistor, no need to plug the fan in yet. Do it at the end, as it's quite clumsy

Set up the code

We're almost done! Now you need to set up the code.

This README guides you through the basic settings, they'll easily get you up and running. It skips some of the settings you don't need to edit for it to work correctly, but they're all described in the code and pretty straight forward!

It is well documented, but make sure to follow this readme at least for the thermistor calibration!

• After downloading it, open pwm_fan_control.ino with the Arduino IDE
• If you have never used an ESP32 with the Arduino IDE yet, follow this tutorial to get started. Now that you see the esp32 code correctly, we can go on editing our settings.
• define the correct pins by editing these two variables, if you used the same pins as me you don't have to modify this

  //setting the I/O pins
    //the fan PWM output pin
      const int fanOutput = 32;
    //thermistor pin
      const int thrm = 25;

Now you need to define the temperatures for the fan curve!

• Keep this line uncommented if you want the fan to turn off when below the minimum temperature, or comment it if you want the fan to always spin

  #define FAN_OFF

• This variable sets the minimum temperature that makes the fan spin (if you commented FAN_OFF, below this temperature the fan just spins at the minimum speed you set later)

  const int MINTEMP = 35;

• This variable sets the maximum fan speed, it goes from 0 to 255.

  const int maxDutyCycle = 190; 

• Now you have a few different fan curves you can experiment with. I recommend just using the default one, as it's the most customizeable.

• At this temperature the fan curve reaches maxDutyCycle, and caps off

  const int MAXTEMP = 60;

• You set up your fan curve! To aid in visualizing it, here's how the default fan curve looks like:

  

• Note that if FAN_OFF is defined, when the temperature reaches MINTEMP the fan will turn off instead of staying at minDutyCycle.

• If you don't wish to calibrate the thermistor readings, you're done! you just need to flash the code to the ESP32 and you're golden! If you have difficulty flashing to the ESP32 and you did everything this tutorial tells you to do, try holding the BOOT button on the ESP32 while it is flashing. Your finger might hate you ;)

• If you want to continue, keep reading of course!

Calibration

This is needed to get decent readings out of the thermistor. You can probably skip the voltage offset part, as most ESP32s have similar ADCs, but if you're getting weird readings and can't seem to be able to get a good thermistor calibration, you might need to look into it.

Opening the serial monitor

For the next two steps we're gonna need to see the values that the ESP is reading. We'll be able to by using the serial monitor of the Arduino IDE

• In the code, uncomment DEBUG to enable it, in line 4

  #define DEBUG

• Now flash the code to the ESP32. This also lets you know if you made any mistakes in the fan curve configuration!

• With the ESP32 still connected, in the IDE go to tools > Serial Monitor. Select Baud 115200 and you'll see a flow of numbers!

  

• This is your friend. The columns of numbers you see are respectively the temperature reading, the voltage reading and the resistance of the thermistor, and the PWM output (from 0 to 255) to the fan.

Calibrate the voltage readings

This is done to have slightly more accurate voltage readings, as as you might remember the built-in ADC is pretty bad. You'll need a voltage source that can output from 0 to 3.3V, in steps of 0.1V or less, and a multimeter. This is an easy but tedious process, so arm yourself with patience!

My ESP32 reads consistently about 0.1V too low, until it gets to 2.7V where it starts reading too high. We don't really care about those voltages (3.3V would mean a thermistor resistance of 0, so an infinite temperature. At those temperatures we're pretty certain that the fan will be at full speed!), so we can just add 0.1V to all the readings. Your ESP32 will probably be similar, so if you find this process to be too difficult you can use the default value and skip to the next section.

• Remove the thermistor cable from pin 25 (or your thermistor pin if you're not using the default configuration), and connect your voltage source to the pin

• Open the serial monitor, and start looking at the second value, which is the voltage reading

• You're gonna end up with a lot of numbers, to make life easier you can use excel or an equivalent program.

• Set a voltage between 0.0 and 3.3V, measure it with a multimeter, write it down and read the votlage in the serial monitor. Note it down too.

• Set a different voltage, and repeat the process.

• Ideally you want to sweep all the voltage range from 0 to 3.3V, but that takes time. The values we care about are approximately between 0.5V and 2-2.5V

• You will end up with something like this, from there you can easily see the difference between the readings and the actual voltage. You can take the most common value, or the average, and use that as the offset!

  

• Now go to the variables for the thermistor resistance calculation section, and write that number down.

  //variables for the thermistor resistance calculation
    const float V_OFFSET = 0.1;

• You're done! Let's get to the next section.

Now you have to calibrate the thermistor!

This is an important step, as as I said a few times already the ESP32 ADC is not precise. The default value works, but don't expect to get a precise temperature reading from it! A laptop here is pretty handy, as you'll have to move around a bit while getting some readings. A kitchen thermometer is pretty handy too, but not strictly necessary if you don't have one.

• Open the serial monitor, we'll need the resistance readings (third value, xx.xxK)

• Note that the resistance is given to you in kiloOhms, while the website we're about to use asks for Ohms. Just multiply the numbers you'll get by 1000!

• Open this website and keep it open in a tab, it's the calculator for the values we'll put in the code.

• Get some water in a pot on a stove, and some water from the fridge with ice in it. We want three reference points at 100C, 0C and... 36C, your body temperature!

• Now you want to submerge the probe in the boiling water, and read the resistance value in the serial monitor. Don't put the probe too far down or you'll read the temperature of the metal and your reading won't be accurate! Wait for the resistance value to stabilize but be quick, you don't want to ruin your thermistor. Take note of the value somewhere.

• Do the same in the ice water, after you're fairly sure the temperature stabilized at 0C, note that value down too, and now stick the probe into your mouth. If you have a thermometer here use it, to get a more precise reading. put the probe in the same spot where you took your temperature with the thermometer!

• As you might have guessed, this is more of an art than a science (if you don't have calibrated temperature references, of course). After some messing around, you'll know you got it right!

• Go back to the calculator, and focus on the "Please input resistance-temperature pairs" section.

  • Under "T (°C)", put 0, 36.5, 100, or the values you got with your thermometer (T1 is the cold water, T2, is your body temperature, T3 is the boiling water)

  • Under "R (Ω)", put the readings you got from the serial monitor respectively for icy water, your body temperature, and the boiling water (R1, R2, R3).

    

  • A rule of thumb to see if you got readings that make sense is to see if the blue and the yellow curves in the graph are close. They'll never be exactly the same, but they shouldn't be too far apart.

• Now, go down to the "Calculated β model coefficients" section. Keep note of the two numbers you see.

  

• We can go back to the code! Under the variables for the thermistor resistance calculation section, you need to edit R_Tk and BCoeff with the first and second value we took from the calculator respectively. (R_Tk is R(25°C)=[ ]Ω and BCoeff isβ=[ ]K). If they happen to be whole numbers, still write them as decimals! (write x.0)

  //variables for the thermistor resistance calculation
    const float R_Tk = 115025.41;
    const float BCoeff = 3433.3;

• Flash the code to the ESP32, and try to measure various things looking at the first value on the left. That's your temperature!

• If you're getting non-sensical readings, you'll have to do everything again (if you didn't use a thermometer try to use one, and experiment with the water temperatures. Try water from the fridge instead of icy water, and water that's not quite boiling yet, for example.

• If everything looks good, you're done! Go back to line 4, comment DEBUG back and flash the code to the ESP once again. The hard part is over!

Usage

You're free to do what you want with it. I use it to drive a fan on a solar inverter that should be passive but gets way too hot when pulling a consistent load, but PWM is used pretty much anywhere. Use it to drive LEDs, fans, servos, you name it! have fun!

License

GNU GPLv3.0