Acquire a temperature.

The challenge

Read a real temperature without blocking during conversion. The DS18B20 is digital: D4 exchanges 1-Wire messages; it does not read a voltage proportional to temperature.

Equipment: DS18B20, 4.7 kΩ pull-up resistor, UNO R4 and wires. The TO-92 package makes the pinout visible; a wired probe requires documentation for its wires.

Wiring

Three-wire power, not parasite mode. The DS18B20 accepts this voltage; not every sensor accepts 5 V.

Install OneWire by Paul Stoffregen and DallasTemperature by Miles Burton in Library Manager. Versions used for verification: OneWire 2.3.8 and DallasTemperature 4.0.6.

The complete program

#include <OneWire.h>
#include <DallasTemperature.h>
#include <math.h>

constexpr uint8_t BROCHE_DQ = 4;
constexpr unsigned long MESURE_MS = 5000;
constexpr unsigned long CONVERSION_MS = 800; // Marge au-dela des 750 ms a 12 bits.
OneWire bus(BROCHE_DQ);
DallasTemperature capteurs(&bus);
DeviceAddress adresse;
bool conversion = false;
bool adresseConnue = false;
bool mesureValide = false;
float temperatureC = 0.0f;
unsigned long debutConversion = 0;
unsigned long dernierCycle = 0;
unsigned long derniereMesure = 0;

void lancerMesure() {
  // Recherche du premier capteur : reconnexion possible apres debranchement.
  adresseConnue = capteurs.getAddress(adresse, 0);
  if (!adresseConnue) {
    mesureValide = false;
    Serial.println("Erreur : DS18B20 absent");
    return;
  }
  capteurs.setResolution(adresse, 12);
  capteurs.requestTemperatures();
  debutConversion = millis();
  conversion = true;
}

void gererMesure() {
  const unsigned long maintenant = millis();
  if (conversion && maintenant - debutConversion >= CONVERSION_MS) {
    conversion = false;
    const float t = capteurs.getTempC(adresse);
    mesureValide = isfinite(t) && t != DEVICE_DISCONNECTED_C && t >= -55 && t <= 125;
    if (mesureValide) {
      temperatureC = t;
      derniereMesure = millis();
      Serial.print("Temperature : "); Serial.print(t, 2); Serial.println(" deg C");
    } else {
      Serial.println("Erreur : lecture DS18B20 impossible");
    }
  }
  if (!conversion && maintenant - dernierCycle >= MESURE_MS) {
    dernierCycle = maintenant;
    lancerMesure();
  }
}

void setup() {
  Serial.begin(115200);
  capteurs.begin();
  capteurs.setWaitForConversion(false); // Pas d'attente delay(750).
  dernierCycle = millis();
  lancerMesure();
}

void loop() {
  gererMesure(); // La boucle reste disponible pendant la conversion.
}

A small state machine

At 12 bits, conversion can take up to 750 ms. The code allows 800 ms and starts a cycle every 5 s. setWaitForConversion(false) avoids this wait in the library; 1-Wire transactions remain synchronous and take a short time.

The program searches for a sensor, requests conversion, then reads its address. It rejects absence, non-finite values and out-of-range values. The first sensor is enough for the model; multiple probes need explicit addresses.

Two decimal places do not guarantee 0.01 °C accuracy. A value of 85 °C is not always wrong; a persistent startup value calls for checking conversion and wiring.