Using Raspberry Pi to Control a PWM Fan and Monitor its Speed

Nov 25 2019 9 Minutes read (About 1408 words)

A lot of people uses fan to cool down their Raspberry Pi 4, but the fan’s noise is quiet problem (in a quiet room). In most case pi didn’t need that amount of cooling performance, so we can use PWM (Pulse Width Modulation) to control fan speed and reduce the noise.

Wiring

I use Noctua NF-A4 5V PWM, a 40mm, 5V PWM fan. Here’s the wiring diagram:

Yellow: 5V
Black: GND
Green: RPM signal
Blue: PWM signal input

If your fan doesn’t support PWM control, then you can see #138 Variable Speed Cooling Fan for Raspberry Pi using PWM and PID controller or PWM Regulated Fan Based on CPU Temperature for Raspberry Pi to use a BJT transistor to use PWM control.

The fan’s RPM signal is an OC(Open-Collector) circuit design (in most fan), so you’ll need a pull-up resistor to measure output waveform according to Noctua PWM white paper.

Warning! The Pi has 3.3V GPIO, so your pull-up resistor must be connect to 3.3V ONLY! You will fry your Pi if Vcc is connected to 5V.

The pull-up resistor that I use is 1kΩ 1/4W. The white paper says the maximum current is 5mA. According to Ohm’s Law: $V=IR$, the resistor should be 660Ω or larger to protect your fan.

Most micro-controller or SBC already have a decent circuit to generate PWM signal, you don’t need special circuit to generate it.

I use a Molex 2510 4PIN (2.54mm) connector to connect the fan. (Need to cut out some fool-proof board to fit.)

Here’s a photo testing hardware and program on Pi 3B.`

Control Script

PWM GPIO Test

I use Wiring Pi to test PWM signal. If you don’t want to test it, jump to Use PWM to Control Fan Speed.

The Pi4 haven’t update to latest version of wiring-pi yet, need to update it manually.

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cd /tmp
wget https://project-downloads.drogon.net/wiringpi-latest.deb
sudo dpkg -i wiringpi-latest.deb

Version check：

gpio -v

The latest version when I write this post is 2.52

Read GPIO status：

gpio readall

Set pin 1 to PWM mode (using wiring-pi pin number: Pi GPIO Pinout).

gpio mode 1 pwm

Set PWM signal (number from 0~1023)

gpio pwm 1 [number]

Check the fan is working as you expect.

Use PWM to Control Fan Speed

The control script is on: DriftKingTW/Raspberry-Pi-PWM-Fan-Control

I use GPIO18 which is the hardware PWM(12/13 and 18/19) and all other pins are software PWM. Hardware PWM and the headphone jack use the same circuits, don’t use them at the smae time.

mkdir -p Scripts

cd Scripts/

Download the python script:

1	wget https://raw.githubusercontent.com/DriftKingTW/Raspberry-Pi-PWM-Fan-Control/master/fan_control.py

python fan_control.py

And make it auto start:

Edit rc.local

1	sudo vim /etc/rc.local

Add the following line before exit 0 :

1	python /home/pi/Scripts/fan_control.py &

The route might be different for you. Make sure you’re using absolute path. The ‘&’ symbol means this script runs in background.

If you don’t want to know the theory part, than go ahead to: Reading RPM Signal

PWM uses square wave’s duty-cycle to reduce the average value of voltage, if the duty cycle is 50%, the fan will spin at 50% of it’s full speed; duty cycle 75%, fan 75%, and so on. More detail: Pulse-width modulation

And we can take a look at the control script:

import RPi.GPIO as GPIO
import time
import signal
import sys
import os

# Configuration
FAN_PIN = 18            # BCM pin used to drive PWM fan
WAIT_TIME = 1           # [s] Time to wait between each refresh
PWM_FREQ = 25           # [kHz] 25kHz for Noctua PWM control

# Configurable temperature and fan speed
MIN_TEMP = 40
MAX_TEMP = 70
FAN_LOW = 40
FAN_HIGH = 100
FAN_OFF = 0
FAN_MAX = 100

# Get CPU's temperature
def getCpuTemperature():
    res = os.popen('vcgencmd measure_temp').readline()
    temp =(res.replace("temp=","").replace("'C\n",""))
    #print("temp is {0}".format(temp)) # Uncomment for testing
    return temp

# Set fan speed
def setFanSpeed(speed):
    fan.start(speed)
    return()

# Handle fan speed
def handleFanSpeed():
    temp = float(getCpuTemperature())
    # Turn off the fan if temperature is below MIN_TEMP
    if temp < MIN_TEMP:
        setFanSpeed(FAN_OFF)
        #print("Fan OFF") # Uncomment for testing
    # Set fan speed to MAXIMUM if the temperature is above MAX_TEMP
    elif temp > MAX_TEMP:
        setFanSpeed(FAN_MAX)
        #print("Fan MAX") # Uncomment for testing
    # Caculate dynamic fan speed
    else:
        step = (FAN_HIGH - FAN_LOW)/(MAX_TEMP - MIN_TEMP)   
        temp -= MIN_TEMP
        setFanSpeed(FAN_LOW + ( round(temp) * step ))
        #print(FAN_LOW + ( round(temp) * step )) # Uncomment for testing
    return ()

try:
    # Setup GPIO pin
    GPIO.setwarnings(False)
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(FAN_PIN, GPIO.OUT, initial=GPIO.LOW)
    fan = GPIO.PWM(FAN_PIN,PWM_FREQ)
    setFanSpeed(FAN_OFF)
    # Handle fan speed every WAIT_TIME sec
    while True:
        handleFanSpeed()
        time.sleep(WAIT_TIME)

except KeyboardInterrupt: # trap a CTRL+C keyboard interrupt
    setFanSpeed(FAN_HIGH)
    #GPIO.cleanup() # resets all GPIO ports used by this function

Reading RPM Signal

Github: DriftKingTW/Raspberry-Pi-PWM-Fan-Control

1	wget https://raw.githubusercontent.com/DriftKingTW/Raspberry-Pi-PWM-Fan-Control/master/read_fan_speed.py

Run script and you will get RPM value every second:

python read_fan_speed.py

import RPi.GPIO as GPIO
import time

# Pin configuration
TACH = 24       # Fan's tachometer output pin
PULSE = 2       # Noctua fans puts out two pluses per revolution
WAIT_TIME = 1   # [s] Time to wait between each refresh

# Setup GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
GPIO.setup(TACH, GPIO.IN, pull_up_down=GPIO.PUD_UP) # Pull up to 3.3V

# Setup variables
t = time.time()
rpm = 0

# Caculate pulse frequency and RPM
def fell(n):
    global t
    global rpm

    dt = time.time() - t
    if dt < 0.005: return # Reject spuriously short pulses

    freq = 1 / dt
    rpm = (freq / PULSE) * 60
    t = time.time()

# Add event to detect
GPIO.add_event_detect(TACH, GPIO.FALLING, fell)

try:
    while True:
        print "%.f RPM" % rpm
        rpm = 0
        time.sleep(1)   # Detect every second

except KeyboardInterrupt: # trap a CTRL+C keyboard interrupt
    GPIO.cleanup() # resets all GPIO ports used by this function