39 Temperature Sensor Labs: Comparing Sensor Types

39.1 Start With the Decision

Picture a freezer sensor showing 85 C just after power returns. The number is valid data but not a valid freezer reading.

39.2 Route Overview

This is part 1 of 2. Continue with Temperature Sensor Labs: Thermocouple Measurement.

39.3 Part Objectives

• Compare temperature sensor types by range, accuracy, interface, and field limits.
• Trace a DS18B20 reading through wiring, conversion, validation, and fault checks.

39.4 Start With the Measurement Story

Prove One Temperature Before Trusting the Display

Picture a freezer sensor showing 85 C just after power returns. The number is valid data but not a valid freezer reading. The lab must separate a real temperature from a startup or link fault.

A cyclic redundancy check, or CRC, is a short code used to detect changed data. General purpose input output, or GPIO, means a configurable pin on a small computer board. Both help prove the reading path.

Measure known cold, room, and warm points. Delay a read, remove the sensor, disturb the pull-up, restart the board, and record raw value, check result, address, time, unit, lag, and final decision.

This runway does not prove accuracy across every range or mounting condition. The deeper sections compare sensor types, wiring, conversion timing, calibration, error codes, and heat-flow limits.

A temperature lab is not just a wiring exercise. The useful story is heat moving through the sensor package, the reading lagging behind reality, and calibration evidence showing when the number can be trusted.

39.5 In 60 Seconds

39.6 Key Concepts

The mathematical gist. A 3.3 V, 12-bit converter steps by 0.806 mV. At the chapter’s 1.00 mA excitation, a Pt100 changes by 0.385 mV/C and therefore resolves only 2.09 C/count before amplification; a Pt1000 changes by 3.85 mV/C and reaches 0.209 C/count. The same current also causes $I^2R$ self-heating: 0.100 mW and 1.00 mW respectively.

Math Bridge · guided foundationsWhy does excitation current help and heat an RTD?Let Phoebe connect resistance slope, ADC count size, and self-heating for Pt100 and Pt1000. Learn the maths

39.7 Learning Objectives

By the end of this chapter, you will be able to:

• Interface DS18B20 sensors: Configure 1-Wire bus and read multiple temperature sensors on a single pin
• Analyse thermocouple operation: Explain how thermocouples generate temperature-dependent voltages via the Seebeck effect and calculate output voltages using the Seebeck coefficient
• Evaluate humidity sensors: Differentiate capacitive, resistive, and thermal conductivity sensing methods based on accuracy, response time, and application suitability
• Construct temperature monitoring systems: Wire and program robust temperature acquisition circuits with correct pull-up resistors and multi-sensor addressing
• Diagnose common sensor errors: Interpret the meaning of 85C, -127C, and NaN readings and apply targeted fixes for each failure mode

39.8 Prerequisites

Required Knowledge:

• Sensor Fundamentals - Sensor types and characteristics
• Electronics Basics - Circuit fundamentals
• Sensor Circuits - Signal conditioning

Hardware Requirements:

• Arduino Uno/Nano or ESP32 development board
• Breadboard and jumper wires
• DS18B20 temperature sensor (or DHT22)
• 4.7kOhm pull-up resistor (for DS18B20)
• USB cable for programming

Software Requirements:

• Arduino IDE installed and configured
• Required libraries: OneWire, DallasTemperature (for DS18B20), DHT sensor library (for DHT22)

Skills Checklist:

• Can upload code to Arduino/ESP32
• Can read basic circuit diagrams
• Can use serial monitor for debugging
• Can install Arduino libraries via Library Manager

39.9 For Beginners: Temperature Sensor Basics

39.10 Temperature Sensors Overview

Implementing a temperature sensor project follows a structured workflow. Use Figure 39.1 to see why deployment is the end of an evidence chain rather than the first time a reading is trusted. Follow the main path first, then inspect the debug loops that send failed checks back to an earlier decision.

Read Figure 39.1 from research through wiring, code, test, calibration, and deployment. A failed electrical check returns to wiring; an implausible or biased value returns to code, reference testing, or calibration. Those loops matter because firmware cannot compensate for every wiring fault, and calibration cannot legitimise an unstable interface. The workflow supplies the running structure for each device lab below.

Before applying power, turn that workflow into a concrete inspection with Figure 39.2. Read the pre-power column first, then the first-test checks, and finally the symptom-to-cause troubleshooting path.

In Figure 39.2, voltage, ground, GPIO choice, and pull-up presence are acceptance conditions before energising the circuit. The first reading is then compared with a known reference rather than judged by plausibility alone. Finally, no response, wrong values, instability, and drift point to different investigations. This checklist turns the overview’s broad loop into evidence you can record for every sensor in the chapter.

The overview gives you the workflow. The first detailed device is the DS18B20 because it turns wiring, addressing, timing, and error handling into one compact lab.

39.11 DS18B20 (1-Wire Digital Temperature)

The DS18B20 is one of the most popular digital temperature sensors for IoT projects. It uses the 1-Wire protocol, which means multiple sensors can share a single GPIO pin — each sensor has a factory-programmed unique 64-bit address that distinguishes it on the bus.

Specifications:

• Temperature Range: -55C to 125C (+/-0.5C accuracy in -10C to +85C range; +/-2C outside that range)
• Interface: 1-Wire (multiple sensors on one pin)
• Resolution: 9-12 bit configurable (93.75 ms to 750 ms conversion time)
• Power: 3.0-5.5V, parasitic power mode available

Before placing multiple devices on one GPIO, inspect Figure to connect the small three-lead package with its internal digital identity. The shared wire works because addressing and conversion timing are explicit parts of the protocol.

Read Figure, identify power, ground, and data at the package, then connect the single data lead to the unique 64-bit address stored inside each sensor. That relationship explains how discovery, conversion delay, and per-device reads must be verified before several DS18B20s can share the bus reliably.

What to Observe Before Coding:

The DS18B20 is useful because many sensors can share one GPIO pin. The first behavior to check is discovery: how many devices are on the 1-Wire bus, and does each one have a unique address?

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

#define ONE_WIRE_BUS 4  // GPIO4

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

// Store number of devices
int numberOfDevices;
DeviceAddress tempDeviceAddress;

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

  sensors.begin();

  // Get number of devices on the bus
  numberOfDevices = sensors.getDeviceCount();
  Serial.print("Found ");
  Serial.print(numberOfDevices);
  Serial.println(" temperature sensors");

  // Print addresses
  for(int i=0; i<numberOfDevices; i++) {
    if(sensors.getAddress(tempDeviceAddress, i)) {
      Serial.print("Sensor ");
      Serial.print(i);
      Serial.print(" Address: ");
      printAddress(tempDeviceAddress);
      Serial.println();
    }
  }
}

void loop() {
  sensors.requestTemperatures();

  for(int i=0; i<numberOfDevices; i++) {
    if(sensors.getAddress(tempDeviceAddress, i)) {
      float tempC = sensors.getTempC(tempDeviceAddress);

      // Check for error values before printing
      if(tempC == DEVICE_DISCONNECTED_C) {
        Serial.print("Sensor ");
        Serial.print(i);
        Serial.println(": ERROR - disconnected");
      } else {
        Serial.print("Sensor ");
        Serial.print(i);
        Serial.print(": ");
        Serial.print(tempC);
        Serial.println("C");
      }
    }
  }

  delay(1000);
}

void printAddress(DeviceAddress deviceAddress) {
  for (uint8_t i = 0; i < 8; i++) {
    if (deviceAddress[i] < 16) Serial.print("0");
    Serial.print(deviceAddress[i], HEX);
  }
}

39.12 Continue to the Next Part

Carry this evidence into Temperature Sensor Labs: Thermocouple Measurement, which begins with Thermocouple-Based Temperature Sensors.