LightSensor() no longer works under Raspbian Bookworm / Linux 6.6.20

[danny-andersen]

Operating system: Raspbian Bookworm
Python version: 3.11.2
Pi model: Pi Zero 2W
GPIO Zero version: 2.0
Pin factory used: lgpio version 0.2.2.0

Recent upgrade of Bookworm that uplifted the Linux vernal from 6.1.0 to 6.6.20, the LightSensor() class no longer works. Simple test programme below, the when_light() callback is never called and the LightSensor value always returns 0.0, regardless of the light level.

from gpiozero import LightSensor
from time import sleep

ldr = LightSensor(17, charge_time_limit=0.04)
ldr.when_light = lambda : print("Light")
while True:
	print(f"LDR: {ldr.value}")
	sleep(1)

Same circuit was used before and after. Also proved that the circuit still works by testing using OutputDevice() and InputDevice() to discharge the capacitor and timing how long it takes to charge based on whether it is light or not.

[bennuttall]

@waveform80?

[8119436437056971]

Looks as if there is a pulldown enabled when switching the GPIO to input.
Measured the voltage level on the capacitor; when input is floating then voltage should raise to Vcc, but the voltage raises to a limit much lower than Vcc.

Checked with bookworm on a Pi5.

[nkreipke]

Can confirm this for both Pi Zero 2W and Pi 4. Not sure about the pull down thing though, the capacitor voltage peaks at around 3.3V as expected (measured at the GPIO pin).

Previously, the capacitor immediately got discharged when the positive edge was detected. After upgrading, the library now always waits until the charge_time_limit has run out, failing to detect the edge.

I've also had this issue on ArchLinux ARM. As RPi.GPIO still works there, I was able to fix it by uninstalling lgpio and letting gpiozero fall back to RPi.GPIO. The issue seems to be specific to lgpio.

Unfortunately, RPi.GPIO is unusable on Bookworm because of this bug, so there seems to be no way to actually get it to work, which is kind of disappointing.