DS18B20 on Arduino: 1-Wire Temperature Sensor

At a glance

The Temperature sensor, DS18B20 in brief

What it is, and every setting the properties panel gives it.

The DS18B20 is the three-legged digital thermometer in every kit or the same chip in a steel probe, and it reports -55 to 125 degrees Celsius to a sixteenth of a degree as a number rather than a voltage. With the flat face toward you the legs are GND, DQ and VDD; VDD takes 3.0 to 5.5 volts, so it runs from an Uno’s 5 V or an ESP32’s 3.3 V, and DQ goes to any digital pin. DQ is open drain and needs one 4.7 kilohm pull-up to VDD for the whole bus; forgetting it is the classic reason a sensor is never found, and the pullup property stands in for the three-pin modules that carry one. Mokxi runs the real 1-Wire protocol bit by bit with the datasheet’s timings: the 480 microsecond reset and presence pulse, a ROM search that finds several sensors on one wire, CRC-8 checks, and conversions of 93.75 to 750 milliseconds by resolution. The OneWire and DallasTemperature libraries work as tutorials use them. Read before converting and you get 85,. Parasite power and the half-degree error a real sensor is allowed are not modeled.

Family: Sensors
Pins: 3
temperature: 22
serial
pullup: false

A circuit built around it, running and explained: the DS18B20 simulator.

Pins

3 pins

Named as the editor names them, so a wire lands where you expect it.

What it does

GND

Ground. The left leg with the flat face towards you.

DQ

The data line: open drain, and needs a pull-up.

VDD

Supply, 3.0 to 5.5 V.

How to use it

How to use the DS18B20 with an Arduino

Wire VDD to 5 V, GND to GND and DQ to pin 2, then put one 4.7 k resistor from DQ to 5 V. That single pull-up serves every sensor on the wire, so a second DS18B20 needs only its DQ joined to the same row. Without it the line floats and the library reports no devices at all.

Call sensors.begin() once, then sensors.requestTemperatures() before every read: it starts every sensor converting and waits the 750 ms twelve bits take. Read without it and you get 85, the power-up value. With several sensors, fetch each address once with getAddress() and read it by address after that, which is quicker than searching by index every time.

Part pin

Board pin

DQ

Pin 2, with 4.7 k to 5 V

VDD

5 V

GND

On an ESP32, power it from 3.3 V and pull DQ up to 3.3 V instead; the sensor runs from 3.0 V and the same libraries compile.

Arduino Uno: two DS18B20s on one wirelive0.000 s 0.00x

Click to open it in the editor

This is the simulator itself, running here. Click anything to open it in the editor.

Open this project in the editor Read the full tutorial

Reading a DS18B20 with DallasTemperature

#include <OneWire.h>
#include <DallasTemperature.h>
OneWire oneWire(2);
DallasTemperature sensors(&oneWire);
void setup() {
  Serial.begin(9600);
  sensors.begin();
}
void loop() {
  sensors.requestTemperatures();
  Serial.println(sensors.getTempCByIndex(0));
  delay(1000);
}

How it is modeled

What is true about the Temperature sensor, DS18B20, here

In its help article’s own words: what the simulation gets right, and where it stops.

Modeled

The wire, bit by bit, with the datasheet's timings: the 480 µs reset and the presence pulse after it, write slots sampled 30 µs in, read slots where a 0 holds the line low from the master's fall. The ROM commands (READ ROM, MATCH ROM, SKIP ROM, SEARCH ROM and ALARM SEARCH) and the function commands (CONVERT T, READ and WRITE SCRATCHPAD, COPY SCRATCHPAD, RECALL E2, READ POWER SUPPLY), with the Dallas CRC-8 on the ROM code and on the scratchpad.

The search works because the wire does. Several sensors on one net answer a SEARCH ROM together and the open-drain line ANDs what they send, exactly as on the bench, so search() finds every one of them. Nothing in the part arbitrates.

Resolution costs time. Nine bits (half a degree) convert in 93.75 ms and twelve bits (a sixteenth) in 750 ms, the datasheet's worst case, and the undefined low bits of a coarse reading read zero. Read slots after CONVERT T return 0 until it is done.

Before the first conversion the temperature register holds +85 °C, as a real one does at power-up. A sketch that reads without converting prints 85, which is the most famous number in the datasheet.

Not modeled

Parasite power. A real DS18B20 can run with VDD tied to ground, living off the data line, if the master holds the line hard high during a conversion. Here a DS18B20 with no VDD is off the bus, and READ POWER SUPPLY always answers "external".

No error in the reading. The part reports temperature exactly, to its resolution, with none of the ±0.5 °C a real one is allowed, and it does not warm itself up. The EEPROM keeps the alarm limits and resolution for the run, not between runs.

From DS18B20 1-Wire thermometer, in full.

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