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Arduino Mega · DS18B20

Arduino Mega DS18B20 Simulator: Wiring, Code and a Live Circuit

This is a DS18B20 digital temperature sensor wired to an Arduino Mega 2560 with its 4.7 k pull-up, read live with the OneWire and DallasTemperature libraries: one reading a second, to a hundredth of a degree.

Three legs and one resistor: VDD to 5V, GND, and DQ to pin 2 with the pull-up from DQ to 5V. In the editor, drag the sensor’s temperature slider and the next reading follows.

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  • 5 V logic
  • Library: DallasTemperature.h
  • DQ on pin 2
  • 4.7 k pull-up
Arduino Mega · DS18B20live0.000 s 0.00x
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The circuit itself, running on the simulator with the sketch below. Press what can be pressed; click anything else to open it in the editor.

Wiring the DS18B20 to the Arduino Mega

Every connection in the circuit above, with the board’s own pin names.

DS18B20 pinArduino Mega pinWhy
VDD (right leg)5VThe DS18B20 runs from 3.0 to 5.5 V.
DQ (middle leg)pin 2The one data wire, shared by every sensor on the bus.
GND (left leg)GNDGround, with the flat face toward you.
4.7 k resistorDQ to 5VThe pull-up the bus needs: the sensor only ever pulls the line low.

On the Arduino Mega the sensor and its pull-up go to 5V. A long cable may need a stronger pull-up, 2.2 k, to keep the edges sharp.

Pins to leave alone on the Arduino Mega

Pins 14 to 19 are the three extra serial ports and 20 and 21 are I2C, so keep them free if you will need those. An Uno shield’s I2C on A4 and A5 is not connected to anything on a Mega.

On a real bench

The Mega is long, so put the breadboard beside its double row of pins 22 to 53 and leave the left-hand headers free. The DS18B20 uses only a few of its 54 pins; a Mega earns its size when one sketch drives a keypad, a display and several sensors at once.

The code

The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.

DS18B20 on Arduino Mega pin 2
#include <OneWire.h>
#include <DallasTemperature.h>

const int ONE_WIRE_PIN = 2;   // DQ, pin 2, with 4.7 k to 5V

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

void setup() {
  Serial.begin(9600);
  sensors.begin();
  Serial.print("Sensors found: ");
  Serial.println(sensors.getDeviceCount());
}

void loop() {
  sensors.requestTemperatures();          // start a conversion on every sensor
  float celsius = sensors.getTempCByIndex(0);
  if (celsius == DEVICE_DISCONNECTED_C) {
    Serial.println("Sensor not found");
  } else {
    Serial.print("Temperature: ");
    Serial.print(celsius, 2);
    Serial.println(" C");
  }
  delay(1000);
}

OneWire oneWire(ONE_WIRE_PIN) runs the 1-Wire protocol on the pin, and DallasTemperature sensors(&oneWire) speaks to DS18B20s over it. sensors.begin() searches the bus, and getDeviceCount() says how many sensors answered.

requestTemperatures() starts a conversion on every sensor at once and waits for it: 750 ms at the default 12 bits. getTempCByIndex(0) then reads the first sensor found. DEVICE_DISCONNECTED_C, -127, means no sensor answered.

Each DS18B20 has a unique 64-bit address, so several can share this one pin. Serial.begin(9600) opens the serial monitor. Serial is the USB port; Serial1 to Serial3 are on pins 18 and 19, 16 and 17, and 14 and 15.

Common mistakes with the DS18B20 on the Arduino Mega

No pull-up resistor

Without the 4.7 k from DQ to the supply, the line never goes high, sensors.begin() finds nothing and every reading is -127. In the simulator, delete the resistor and you get exactly that.

The sensor in backwards

With the flat side facing you, the legs are GND, DQ, VDD from left to right. Reversed, a real DS18B20 gets hot fast. Waterproof probes use red for VDD, black for GND and yellow or white for DQ.

Taking the serial or I2C pins

Pins 0 and 1 are the USB serial port, 14 to 19 are Serial1 to Serial3 and 20 and 21 are I2C. the 1-Wire bus works on any of them until the sketch needs that port, so start on pin 2 and up.

Questions and answers

Can I simulate a DS18B20 with an Arduino Mega 2560?

Yes. The circuit at the top of this page is an Arduino Mega 2560 with a DS18B20, running in your browser on a simulated ATmega2560. The simulated sensor speaks real 1-Wire, timing and addresses included, so the libraries run unchanged. Press Run it in the editor to change the wiring or the code; it is free and needs no account.

Why do I get -127?

That is DEVICE_DISCONNECTED_C: no sensor answered. The pull-up is missing, DQ is on a different pin from the sketch, or the sensor is wired backwards.

Can I connect several DS18B20s to one pin?

Yes. Wire them all in parallel to pin 2, with one pull-up for the bus, and read each with getTempCByIndex(0), (1) and so on, or by its address.

What is parasite power?

A mode where VDD is tied to GND and the sensor draws power from the data line through the pull-up. It saves a wire but needs a strong pull-up during conversions; three wires, as here, is more reliable.

Build your own DS18B20 project

Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.

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