Raspberry Pi Light Sensor

The Problem: GPIO Pins Cannot Read a Varying Voltage

A light-dependent resistor is a genuinely analog component - its resistance is high in darkness and drops as more light falls on it - but a Raspberry Pi has no analog input pins at all. Every GPIO pin can only tell you whether the voltage on it is above or below a single threshold, so there is no pin you can just connect an LDR to and read a light level directly the way you could on a microcontroller with a built-in analog-to-digital converter.

The Workaround: Timing a Capacitor's Charge

The trick gpiozero's LightSensor relies on is to pair the LDR with a small capacitor and measure time instead of voltage. The pin briefly drives low to fully discharge the capacitor, then switches to being an input and the code starts a stopwatch. Current trickles in through the LDR and charges the capacitor; once the rising voltage crosses the pin's high threshold, the input reads as high and the stopwatch stops. A brightly lit LDR has low resistance, so the capacitor charges quickly and the timed interval is short; in darkness the LDR's resistance is high, charging is slow, and the interval is long. LightSensor repeats this many times a second and converts the interval into a 0.0 to 1.0 reading.

• LDR wired from the 3.3V rail to a chosen GPIO pin.
• A small capacitor, commonly in the 1 to 10 microfarad range, wired from that same GPIO pin to GND.
• The two components share the single GPIO pin as their junction - no separate pins for 'in' and 'out'.

from gpiozero import LightSensor
from time import sleep

ldr = LightSensor(18)

while True:
    print(f'Light level: {ldr.value:.2f}')  # 0.0 (dark) to 1.0 (bright)
    sleep(0.5)

from gpiozero import LightSensor, LED
from signal import pause

ldr = LightSensor(18)
lamp = LED(17)

ldr.when_dark = lamp.on
ldr.when_light = lamp.off

pause()