Keeping a Python script for PWM running and just changing duty cycle

Question

I am currently trying to connect a laser to my 3D printer and give a TTL signal manipulated by an M106 command.

The "official" way didn't work out for me so right now I am trying to control it by my Raspberry Pi 3B+ with Octoprint on it. I am using a plugin called Fan Speed Mirror to transform the Fan Speed (M106) to a PWM signal on a GPIO pin.

Therefore I have copied this bash script, which is calling the python script and giving it the M106 number (at least I think that's what it does, please note that I know only basic programming stuff and near to nothing about python or bash).

fan.sh script:

#!/bin/bash
killall fan.py
echo $1 $2 $3 >>/home/pi/fan.txt
python /home/pi/fan.py $1
exit $?

fan.py script:

#!/usr/bin/python3
import sys
import time
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD)
GPIO.setup(11, GPIO.OUT)
s = int(sys.argv[1])
dc = s*100/255
p = GPIO.PWM(11,1000) # channel=11 frequency=1kHz
p.start(dc)
time.sleep(100000000000000000000)

As you can see I am trying to run the PWM signal as long as no new M106 command is entered.

But that's not really working out.

Instead it runs the first M106 command and then stops the gcode.

I already asked the developer of the plugin here: https://github.com/b-morgan/OctoPrint-FanSpeedMirror/issues/3

But I was hoping to get some input here as well. I hope that what I am trying to achieve is even possible.

Why use a bash script to run a python script. Python is a scripting language? — Milliways, Dec 28 '21 at 21:14

score 0 · Answer 1 · answered Dec 28 '21 at 16:08

0

If I guess properly at the question it may be solved by changing

python /home/pi/fan.py $1

to

python /home/pi/fan.py $1 &

answered Dec 28 '21 at 16:08

joan

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Milliways · Accepted Answer · 2021-12-28T23:26:47.847

If I was going to do this I would use hardware PWM which continues to run after the program exits.

Hardware PWM is available in my Pi.GPIO (which is a superset of RPi.GPIO). It is also available in pigpio

The following modification to your program should work (I have not tested it) and avoid the strange sleep and need to kill processes, although it is rather inelegant.

Call directly - DO NOT use any scripts.

#!/usr/bin/python3
import sys
from signal import pause
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD)
GPIO.setup(11, GPIO.OUT)
GPIO.setwarnings(False)
s = int(sys.argv[1])
dc = s*100/255
p = GPIO.PWM(11,1000) # channel=11 frequency=1kHz
p.start(dc)
pause()

PS You should not get into the bad habit of using GPIO.BOARD - use BCM numbers like practically every other program.

NOTE RPi.GPIO PWM is software timed and imprecise - it is more suitable for dimming LEDs, whether this matters depends on your hardware.

I copied your script and tried to call it directly but that didn't work somehow. But then I called it by using the batch script with the added '&' as joan suggested and now it works perfectly as intended. Thank you both very much!! — Gibby1999, Dec 29 '21 at 20:23

Keeping a Python script for PWM running and just changing duty cycle

2 Answers2