Temperature Sensor: DS18B20

Working Principle

The DS18B20 is a digital temperature sensor from Maxim Integrated. Inside its TO-92 package sits a silicon bandgap temperature sensor whose output voltage is proportional to absolute temperature. An on-chip 12-bit ADC converts that voltage directly into a digital value transmitted over a proprietary 1-Wire bus.

Analogy: Imagine a mercury thermometer that, instead of expanding a column of liquid, sends a text message with the exact temperature. The DS18B20 is that self-reporting thermometer.

Electrical Characteristics

Parameter	Value
Supply Voltage	3.0 V – 5.5 V
Temperature Range	−55 °C to +125 °C
Accuracy (−10 to +85 °C)	± 0.5 °C
Resolution	9–12 bit (configurable)
Conversion Time (12-bit)	750 ms max
Interface	1-Wire (single data pin + GND)
Parasitic Power	Supported (data line powers device)

Interfacing with an MCU

The 1-Wire protocol requires only one GPIO pin plus a 4.7 kΩ pull-up resistor to V_CC. Multiple DS18B20 sensors can share the same data line because each has a unique 64-bit ROM address.

Calibration

The DS18B20 is factory-calibrated, but for applications needing < 0.5 °C accuracy:

• Ice-point calibration: Submerge in ice-water slurry (0.0 °C); record offset.
• Two-point calibration: Use ice water and boiling water; derive gain and offset.
• Software correction: T_corrected = T_raw - offset

Code Example

/*
 * DS18B20 Temperature Reader — Arduino
 * Requires: OneWire library, DallasTemperature library
 * Wiring: DQ → Pin 2, 4.7kΩ pull-up to 5V
 */
#include <OneWire.h>
#include <DallasTemperature.h>

#define ONE_WIRE_BUS 2          /* Data pin */

OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature sensors(&oneWire);

void setup(void) {
    Serial.begin(9600);
    sensors.begin();            /* Detect all sensors on bus */
    Serial.print("Found ");
    Serial.print(sensors.getDeviceCount());
    Serial.println(" sensor(s).");
}

void loop(void) {
    sensors.requestTemperatures();          /* Trigger conversion */
    float tempC = sensors.getTempCByIndex(0);

    if (tempC == DEVICE_DISCONNECTED_C) {
        Serial.println("Error: sensor disconnected!");
    } else {
        Serial.print("Temperature: ");
        Serial.print(tempC);
        Serial.println(" °C");
    }
    delay(1000);
}

Real-World Applications

Limitations

• Slow conversion: 750 ms at 12-bit — unsuitable for high-speed control loops.
• Limited range: Max 125 °C; use RTDs or thermocouples for furnace temperatures.
• Long cable runs: 1-Wire timing degrades beyond ~20 m without active pull-up drivers.
• Parasitic power fragility: Heavy bus loads can brown-out parasitic-powered sensors.