Light Sensors

A photoresistor (also called an LDR, a light-dependent resistor) is a resistor that changes with light. In the dark it resists a lot; in bright light it hardly resists at all. Paired with an ordinary resistor, it becomes an analog input, just like a potentiometer, except the "knob" is turned by the light in the room.

You'll need:

• 1 × photoresistor (e.g. GL5528)
• 1 × 10 kΩ resistor (brown, black, orange)
• 3 × jumper wires
• The LED from the LEDs lesson (for the night light example)

Hardware

The ESP32 can't measure resistance directly, only voltage. So we build a voltage divider: the photoresistor and a fixed 10 kΩ resistor in a chain between 3V3 and GND, with the ESP32 measuring the voltage at the point where they join.

Think of the two resistors as having a tug-of-war over that middle point:

• Bright light: the photoresistor's resistance drops, 3V3 wins the tug-of-war, and the voltage at the pin goes up.
• Darkness: the photoresistor's resistance rises, GND wins, and the voltage goes down.

Wiring

From	To
ESP32 3V3	One leg of the photoresistor (either way round)
Other leg of the photoresistor	ESP32 GPIO 36 (often labelled VP)
ESP32 GPIO 36	One end of the 10 kΩ resistor
Other end of the 10 kΩ resistor	ESP32 GND

In practice, plug the photoresistor and the resistor into the same breadboard row, then run one wire from that row to GPIO 36.

Software

Reading the sensor works exactly like reading a potentiometer:

ldr_read.py

# Read a light sensor (photoresistor) and print how bright it is
import time
from machine import ADC, Pin

LDR_PIN = 36   # often labelled VP on the board

ldr = ADC(Pin(LDR_PIN), atten=ADC.ATTN_11DB)

while True:
    raw = ldr.read_u16()          # 0 (dark) to 65535 (very bright)
    percent = raw * 100 // 65535
    print("Light:", raw, "=", percent, "%")
    time.sleep(0.5)

Run it, then cover the sensor with your hand and shine your phone's torch on it. Watch how far the numbers move.

• ADC(Pin(LDR_PIN), atten=ADC.ATTN_11DB) sets up GPIO 36 to measure the full 0–3.3 V range.
• read_u16() gives 0–65535. Bigger means brighter.

Try it in the REPL

>>> from machine import ADC, Pin
>>> ldr = ADC(Pin(36), atten=ADC.ATTN_11DB)
>>> ldr.read_u16()
23417

Run the last line a few times with your hand over the sensor, and a few times without.

Calibrating

Every room is different: your readings might only swing between 8,000 and 40,000. Calibrating means measuring the darkest and brightest values first, then scaling everything to fit between them:

ldr_calibrate.py

# Calibrate the light sensor, then show a 0-100 % light level
# that uses the full range of YOUR room's lighting
import time
from machine import ADC, Pin

LDR_PIN = 36

ldr = ADC(Pin(LDR_PIN), atten=ADC.ATTN_11DB)

# For 5 seconds, cover the sensor and shine a light on it
# so the code can learn the darkest and brightest readings
print("Calibrating for 5 seconds: cover the sensor, then uncover it!")
darkest = 65535
brightest = 0
start = time.ticks_ms()
while time.ticks_diff(time.ticks_ms(), start) < 5000:
    reading = ldr.read_u16()
    darkest = min(darkest, reading)
    brightest = max(brightest, reading)
print("Darkest:", darkest, " Brightest:", brightest)

if brightest - darkest < 1000:
    brightest = darkest + 1000   # not much change seen: avoid dividing by ~0

while True:
    reading = ldr.read_u16()
    level = (reading - darkest) * 100 // (brightest - darkest)
    level = max(0, min(100, level))   # keep it between 0 and 100
    print("Light level:", level, "%")
    time.sleep(0.3)

• During the first 5 seconds, min() and max() keep track of the darkest and brightest readings seen so far.
• After that, each reading is scaled so the darkest value becomes 0 % and the brightest becomes 100 %. Readings outside that range are clamped.

An automatic night light

Now make a decision based on the light: switch the LED on GPIO 27 on when it gets dark.

ldr_night_light.py

# Turn the LED on automatically when it gets dark
import time
from machine import ADC, Pin

LDR_PIN = 36
LED_PIN = 27
DARK_BELOW = 15000   # adjust using the readings from ldr_read.py

ldr = ADC(Pin(LDR_PIN), atten=ADC.ATTN_11DB)
led = Pin(LED_PIN, Pin.OUT)

while True:
    reading = ldr.read_u16()
    if reading < DARK_BELOW:
        led.on()
    else:
        led.off()
    time.sleep(0.2)

Use ldr_read.py to see what your room reads with the lights on and off, then set DARK_BELOW somewhere between the two.

Challenge

The night light flickers if the light level hovers right around DARK_BELOW. Fix it by using two thresholds: turn the LED on below one value, but only turn it off again above a higher one.

Show a hint

Keep the LED's current state in mind:

if led.value() == 0 and reading < 15000:
    led.on()
elif led.value() == 1 and reading > 20000:
    led.off()

The gap between the two numbers is called hysteresis. Your home thermostat uses the same trick so the heating doesn't click on and off every few seconds.

Next up: Temperature & Humidity: measure the weather indoors.