Light Sensor: Build a Night Light

Why it’s here

Compare a sensor reading with a threshold.

How it works

The threshold is a decision boundary. In this example, a lower reading means less light, so the LED turns on below the threshold. A real voltage divider may produce the opposite direction depending on its wiring.

Try it

1. Open this experiment
2. Run with a threshold of 500. Move the sensor reading from 499 to 500 and then 501; record exactly when the LED switches.

Connections

• A0 — LDR voltage divider
• D13 — LED through resistor

Example sketch

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int light = analogRead(A0);
  digitalWrite(13, light < 500);
  Serial.println(light);
}

Check your understanding

Is the LED on when the reading equals the threshold?