🛠 Capteur de lumière avec seuil et hystérésis

Objectives

Make serious use of the onboard light sensor (an LDR read on channel 5 of the analog multiplexer): scaling, moving average filtering, and hysteresis to prevent flickering around the threshold. This is the basic pattern for any analog sensor alert system, and the Esplora handles it without a single wire.

Materials

• Arduino Esplora + micro-USB cable.
• Esplora library.
• Optional: a flashlight to test the threshold.

Complete code

/* Veilleur de lumiere : la LED RVB passe au vert le jour, au rouge la nuit.
   - moyenne glissante sur 8 mesures (le mux impose des lectures sequentielles)
   - hysteresis de 60 points pour ne pas osciller autour du seuil.
   NB : le capteur est une LDR, 1023 = tres eclaire, 0 = obscurite. */
#include <Esplora.h>

const int SEUIL_ALLUMAGE = 350;   // sous ce niveau -> mode nuit
const int SEUIL_EXTINCTION = 410; // au-dessus -> mode jour (hysteresis)
const int N = 8;                  // taille de la fenetre de moyenne

unsigned long mesures[N];
size_t idx = 0;
bool jour = true;

unsigned long moyenne {
  unsigned long s = 0;
  for (size_t i = 0; i < N; i++) s += mesures[i];
  return s / N;
}

void setup {
  Serial.begin(9600);
  for (size_t i = 0; i < N; i++) mesures[i] = Esplora.readLightSensor;
}

void loop {
  mesures[idx] = Esplora.readLightSensor;
  idx = (idx + 1) % N;
  unsigned long lum = moyenne;

  if (jour && lum < SEUIL_ALLUMAGE) {
    jour = false;                       // bascule en nuit
    Esplora.writeRGB(255, 0, 0);
    Esplora.tone(440, 120);             // bip court sur le buzzer (D6)
  } else if (!jour && lum > SEUIL_EXTINCTION) {
    jour = true;                        // retour au jour
    Esplora.writeRGB(0, 255, 0);
  }

  Serial.print("lum="); Serial.print(lum);
  Serial.println(jour ? "  JOUR" : "  NUIT");
  delay(100);
}

Expected verification

In daylight, the LED is green. Cover the sensor with your finger: below ~350, the LED turns red with a short beep. Release it halfway: the LED stays red until 410 (hysteresis) — without this gap, it would toggle frantically. The moving average smooths out spikes (a hand clap in front of the sensor no longer triggers a clear measurement error).

Common errors

1. Identical threshold for rising and falling edges: permanent oscillation around the threshold; the hysteresis gap (here 60 points) is mandatory on a noisy sensor.
2. Average recalculated on an unfilled window: initialize the buffer with real measurements in setup, otherwise the first cycles are skewed by zeros.
3. Forgetting the LDR nature: the response is logarithmic with respect to actual illuminance; a threshold in ADC value is not linear in lux — readjust your thresholds for YOUR environment.
4. Buzzer whistling continuously: call Esplora.tone(f, duration) with a duration, or remember to use noTone; without a duration, the sound remains stuck.
5. Reading other sensors in between without delay: each change of channel on the multiplexer has a settling time; the library handles this, but do not recreate the reading with a direct analogRead(A4) without respecting this delay.