Connecting and Configuring Sensors: Digital and Analog

1Giving the board senses

A school gate opens itself when a car is close; a classroom fan starts when the room gets hot; the lights of a corridor come on when it gets dark and someone walks by. Each of these needs the Arduino to measure something. Sensors come in two families. Digital sensors give the board a HIGH or LOW signal, or a timed pulse; analog sensors give a voltage that varies smoothly, which the board converts into a number. This lesson wires and programs one of each kind of sensor you are most likely to meet.

2Digital sensor 1: motion (PIR)

A passive infrared (PIR) sensor such as the HC-SR501 notices the change in infrared radiation when a warm body moves. It has three pins: VCC (to 5 V), GND, and OUT, which goes HIGH (about 3.3 V, enough for the Uno to read as HIGH) for a few seconds after motion. Reading it is the same as reading a button, without the pull-up: pinMode(2, INPUT); then digitalRead(2) == HIGH means "motion". Two small knobs on the module set the sensitivity and how long OUT stays HIGH.

3Digital sensor 2: distance (ultrasonic)

The HC-SR04 sends a burst of ultrasound when its Trig pin receives a 10 µs pulse, then holds its Echo pin HIGH until the echo returns. The sketch measures that time with pulseIn(). Sound travels at about 343 m/s in air at 20 °C, which is $0.0343\text{cm}0.0343\backslash ,\backslash text\{cm\}$ per microsecond, and it travels to the object and back, so

$$d=\frac{t\times 0.0343}{2}d\; =\; \backslash frac\{t\; \backslash times\; 0.0343\}\{2\}$$

with $tt$ in microseconds and $dd$ in centimetres.

// HC-SR04: measure distance and report it; warn when closer than 20 cm
const int TRIG_PIN = 9;
const int ECHO_PIN = 10;
const int LED_PIN = 8;

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
}

float readDistanceCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);        // a 10 microsecond pulse starts a measurement
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  unsigned long us = pulseIn(ECHO_PIN, HIGH, 30000UL);  // echo time, give up after 30 ms
  if (us == 0) return -1;              // no echo: nothing in range
  return us * 0.0343 / 2;              // sound: 0.0343 cm per microsecond, there and back
}

void loop() {
  float cm = readDistanceCm();
  if (cm < 0) {
    Serial.println("Out of range");
  } else {
    Serial.print("Distance: ");
    Serial.print(cm, 1);
    Serial.println(" cm");
  }
  digitalWrite(LED_PIN, (cm > 0 && cm < 20) ? HIGH : LOW);
  delay(200);
}

The red LED from the last lesson, still on pin 8, lights when something is closer than 20 cm. The third argument of pulseIn is a time-out: without it, a missing echo would freeze the sketch for a whole second.

4Analog sensors

An analog pin returns a number from 0 to 1023. Each step is

$$\frac{5\text{V}}{1023}\approx 4.89\text{mV}\backslash frac\{5\backslash ,\backslash text\{V\}\}\{1023\}\; \backslash approx\; 4.89\backslash ,\backslash text\{mV\}$$

so a reading $nn$ corresponds to the voltage $V=n\times \frac{5.0}{1023}V\; =\; n\; \backslash times\; \backslash frac\{5.0\}\{1023\}$.

Light (LDR). An LDR's resistance falls as light increases: very roughly 1 kΩ in bright light and 100 kΩ or more in darkness. The board cannot measure resistance directly, so the LDR is put in a voltage divider with a fixed 10 kΩ resistor, and the board measures the voltage at the junction:

$$V_{\text{out}}=5\text{V}\times \frac{R_{\text{fixed}}}{R_{\text{LDR}}+R_{\text{fixed}}}V\_\{\backslash text\{out\}\}\; =\; 5\backslash ,\backslash text\{V\}\; \backslash times\; \backslash frac\{R\_\{\backslash text\{fixed\}\}\}\{R\_\{\backslash text\{LDR\}\}\; +\; R\_\{\backslash text\{fixed\}\}\}$$

Temperature (LM35). The LM35 outputs 10 mV for every degree Celsius, so $T=V\times 100T\; =\; V\; \backslash times\; 100$ with $VV$ in volts.

// Read an LDR divider on A0 and an LM35 on A1; print raw values and units
const int LDR_PIN = A0;
const int LM35_PIN = A1;

void setup() {
  Serial.begin(9600);
}

void loop() {
  int light = analogRead(LDR_PIN);          // 0 to 1023
  int raw = analogRead(LM35_PIN);
  float volts = raw * 5.0 / 1023.0;         // reading -> voltage
  float celsius = volts * 100.0;            // LM35: 10 mV per degree
  int lightPercent = map(light, 0, 1023, 0, 100);

  Serial.print("Light: ");
  Serial.print(lightPercent);
  Serial.print(" %   Temperature: ");
  Serial.print(celsius, 1);
  Serial.println(" C");
  delay(1000);
}

map(light, 0, 1023, 0, 100) rescales the reading to a percentage, using whole-number arithmetic.

5Sensors with libraries

Some sensors send their readings as coded digital messages, which a library decodes for you. The DHT22 (and the cheaper, less accurate DHT11) measures temperature and humidity on a single data pin. Install the DHT sensor library by Adafruit from the IDE's Library Manager (it also installs the Adafruit Unified Sensor library), connect VCC to 5 V, GND to GND and DATA to pin 4 (bare sensors also need a 10 kΩ resistor from DATA to VCC; most modules have one built in), and upload:

// DHT22 temperature and humidity sensor on pin 4 (library: "DHT sensor library" by Adafruit)
#include <DHT.h>

const int DHT_PIN = 4;
DHT dht(DHT_PIN, DHT22);              // use DHT11 for the cheaper blue sensor

void setup() {
  Serial.begin(9600);
  dht.begin();
}

void loop() {
  delay(2000);                         // the DHT22 needs about 2 s between readings
  float humidity = dht.readHumidity();
  float celsius = dht.readTemperature();
  if (isnan(humidity) || isnan(celsius)) {
    Serial.println("Sensor read failed: check the wiring");
    return;
  }
  Serial.print("Humidity: ");
  Serial.print(humidity, 0);
  Serial.print(" %   Temperature: ");
  Serial.print(celsius, 1);
  Serial.println(" C");
}

All three sketches in this lesson were compiled for the Arduino Uno with arduino-cli without errors.

6Hands-on: a corridor light

1. In Tinkercad or Wokwi, wire an LDR divider to A0, a PIR sensor (or a push button standing in for it) to pin 2, and an LED with its resistor to pin 8.
2. Upload light_temp and record the LDR reading in light and in darkness (in the simulator, drag the light-level slider). Choose a threshold.
3. Write a sketch that turns the LED on for 10 seconds when it is dark and motion is detected, using millis() for the 10 seconds.
4. Add the HC-SR04 and print the distance whenever motion is detected.

Check: the LED never lights in bright light, lights within a second of motion in the dark, and goes off 10 seconds after the last motion.