read_fan_speed.py

#!/usr/bin/env python3

import RPi.GPIO as GPIO
import sys
import time


class Fan:
    
    rpm = 0

    def __init__(self, tach, pulse=2):
        self.tach = tach
        self.pulse = pulse

    def __str__(self):
        return f"tach: {self.tach}, pulse: {self.pulse}, rpm: {self.rpm}"


# pin configuration per fan
FANS = {
    24: Fan(24)
}

WAIT_TIME = 1

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

# setup tach settings
for pin, fan in FANS.items():
    GPIO.setup(fan.tach, GPIO.IN, pull_up_down=GPIO.PUD_UP) # pull up to 3.3v


t = time.time()

# caculate pulse frequency and RPM
def fell(pin):
    global t

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

    freq = 1 / dt
    FANS[pin].rpm = (freq / FANS[pin].pulse) * 60
    t = time.time()


# add event to detect
for pin, fan in FANS.items():
    GPIO.add_event_detect(fan.tach, GPIO.FALLING, fell)


try:
    while True:
        for pin, fan in FANS.items():
            print(f"pin: {pin} - {fan.rpm} RPM")
            fan.rpm = 0
        time.sleep(1)


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

# vim: ts=4 sw=4 expandtab