Ten Thermometers on One Wire: DS18B20 and the 1-Wire Bus

Why the DS18B20

• ±0.5 °C accuracy between −10 and +85 °C, full range −55…+125 °C.
• Sold in a waterproof probe with a 1–3 m cable — drop it in water or strap it to a pipe.
• Every sensor has a unique 64-bit address, so dozens of sensors can share one data wire.

Parts

• ESP32 DevKit
• 2–10 DS18B20 sensors (probe version recommended)
• One 4.7 kΩ resistor for the whole bus

Wiring

All sensors are wired in parallel:

For cables over 10 m, drop the resistor to 2.2 kΩ and use twisted pair.

Libraries

• OneWire
• DallasTemperature

Code

Polling is non-blocking: we start a conversion and read the result 800 ms later — the web server keeps answering instantly in the meantime.

#include <WiFi.h>
#include <WebServer.h>
#include <OneWire.h>
#include <DallasTemperature.h>

const char* WIFI_SSID = "YOUR_SSID";
const char* WIFI_PASS = "YOUR_PASSWORD";

const int ONE_WIRE_PIN = 4;
const int MAX_SENSORS = 10;

OneWire oneWire(ONE_WIRE_PIN);
DallasTemperature sensors(&oneWire);
WebServer server(80);

DeviceAddress addrs[MAX_SENSORS];
float temps[MAX_SENSORS];
int count = 0;
bool converting = false;
unsigned long requestedAt = 0;
unsigned long lastCycle = 0;

String addrToString(const uint8_t *a) {
  char buf[17];
  for (int i = 0; i < 8; i++) sprintf(buf + i * 2, "%02X", a[i]);
  return String(buf);
}

void handleData() {
  String json = "[";
  for (int i = 0; i < count; i++) {
    if (i) json += ",";
    json += "{\"id\":\"" + addrToString(addrs[i]) + "\",\"t\":";
    json += temps[i] == DEVICE_DISCONNECTED_C ? String("null") : String(temps[i], 2);
    json += "}";
  }
  json += "]";
  server.send(200, "application/json", json);
}

void setup() {
  Serial.begin(115200);
  sensors.begin();

  count = sensors.getDeviceCount();
  if (count > MAX_SENSORS) count = MAX_SENSORS;
  Serial.printf("Found %d sensor(s)\n", count);

  for (int i = 0; i < count; i++) {
    sensors.getAddress(addrs[i], i);
    sensors.setResolution(addrs[i], 12);   // 0.0625 °C, ~750 ms per reading
    temps[i] = DEVICE_DISCONNECTED_C;
    Serial.println("  " + addrToString(addrs[i]));
  }
  sensors.setWaitForConversion(false);

  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) delay(500);
  Serial.println(WiFi.localIP());

  server.on("/data", handleData);
  server.begin();
}

void loop() {
  server.handleClient();

  if (!converting && millis() - lastCycle > 5000) {
    sensors.requestTemperatures();   // all sensors convert at once
    requestedAt = millis();
    converting = true;
  }
  if (converting && millis() - requestedAt > 800) {
    for (int i = 0; i < count; i++) {
      temps[i] = sensors.getTempC(addrs[i]);
      Serial.printf("%s: %.2f °C\n", addrToString(addrs[i]).c_str(), temps[i]);
    }
    converting = false;
    lastCycle = millis();
  }
}

Which sensor is which

The order in which the library finds sensors does not match the order you wired them. So:

1. Connect the sensors and open the Serial Monitor — you'll see a list of addresses.
2. Hold one probe in your hand — its temperature goes up.
3. Label the address (e.g. 28FF4A1B... → "boiler, flow").

From then on, bind to addresses in Home Assistant or elsewhere — they never change.

Troubleshooting

• −127 °C — the sensor isn't responding: check the contact and the 4.7 kΩ resistor.
• 85 °C right after power-up — that's the default value; the conversion hasn't finished yet.
• Not all sensors found on a long line — lower the resistor and avoid a "star" layout (daisy-chain instead).

Ideas

• Heating monitoring: boiler flow/return and the temperature difference.
• Freezer watchdog with a Telegram alert above −12 °C.
• An aquarium thermostat driving a heater relay.