🛠 Thermomètre de lumière : LDR, seuils et hystérésis

Objectives

Set up a real analog sensor (LDR photoresistor in a voltage divider), calibrate a threshold in real-world conditions using the Serial Monitor, and avoid oscillation around the threshold thanks to hysteresis. Intermediate level: moving from "reading" to "interpreting".

Hardware

• 1 Arduino UNO R3 + USB-B cable
• 1 LDR + 1 10 kΩ resistor (voltage divider: 5 V — LDR — A1 — 10 kΩ — GND)
• 1 LED + 330 Ω on pin 12 (darkness indicator)
• A variable light source (desk lamp) for calibration.

Complete code

// LDR sur A1 (pont diviseur avec 10 k), LED témoin sur pin 12.
const uint8_t LDR = A1;
const uint8_t TEMOIN = 12;

// Hystérésis : deux seuils distincts pour éviter le clignotement.
const int SEUIL_OBSCURITE = 300;   // en dessous : nuit
const int SEUIL_JOUR      = 400;   // au-dessus : jour
bool nuit = false;

void setup {
  pinMode(TEMOIN, OUTPUT);
  Serial.begin(115200);
}

void loop {
  int v = analogRead(LDR);          // 0 = noir, 1023 = pleine lumière (selon montage)

  if (!nuit && v < SEUIL_OBSCURITE) {
    nuit = true;
    Serial.print(F("Nuit -> ")); Serial.println(v);
  } else if (nuit && v > SEUIL_JOUR) {
    nuit = false;
    Serial.print(F("Jour -> ")); Serial.println(v);
  }

  digitalWrite(TEMOIN, nuit ? HIGH : LOW);
  Serial.print(F("ldr=")); Serial.println(v);
  delay(100);
}

Expected verification

The Serial Monitor (115200) displays a stable value that increases when you bring the lamp closer and decreases when you cover the LDR with your hand. The indicator LED turns on below ~300 and only turns off above ~400: between the two, no state change — hysteresis is working.

Common errors

• Voltage divider wired backwards (10 kΩ on top, LDR on bottom): the logic is inverted, thresholds "don't work" — always calibrate by looking at the actual values first.
• A single threshold without hysteresis: the LED flickers constantly when light levels hover near the threshold (ADC noise ±5 units).
• Forgetting F for Serial.print strings: each literal consumes the precious 2 KB of SRAM on the ATmega328P.
• delay(100) too long: you miss fast transitions; too short, you flood the Monitor.
• LDR without a fixed resistor: connecting the LDR alone between A1 and 5 V never yields 0, the divider is essential.