Temperature Sensor Interfacing with Arduino: A Comprehensive Guide

Temperature sensor interfacing with Arduino is a crucial aspect of many IoT and automation projects, allowing for precise monitoring and control of environmental conditions. This comprehensive guide delves into the technical details of selecting, designing, calibrating, and acquiring data from temperature sensors using the Arduino platform.

Temperature Sensor Selection

The most commonly used temperature sensors for Arduino interfacing are thermistors, which are resistive temperature sensors that change resistance with temperature. Thermistors come in a variety of types, including Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC) variants. When selecting a thermistor for your Arduino project, consider the following specifications:

1. Resistance at 25°C: Thermistors are typically characterized by their resistance at 25°C, which can range from a few hundred ohms to tens of thousands of ohms, depending on the type and model.
2. Temperature Coefficient: The temperature coefficient, expressed in %/°C, indicates the rate of resistance change with temperature. NTC thermistors have a negative temperature coefficient, while PTC thermistors have a positive temperature coefficient.
3. Temperature Range: The operating temperature range of the thermistor should match the expected temperature range in your application. Common thermistor temperature ranges span from -40°C to 125°C.
4. Accuracy and Precision: The accuracy and precision of the thermistor, typically expressed in °C, determine the reliability of the temperature measurements.
5. Size and Packaging: The physical size and packaging of the thermistor should be compatible with your project’s requirements, such as mounting, heat transfer, and environmental conditions.

Table 1 provides a comparison of common thermistor types and their typical specifications:

Thermistor Type	Resistance at 25°C	Temperature Coefficient	Temperature Range
NTC Thermistor	2.2 kΩ to 100 kΩ	-3% to -5% per °C	-40°C to 125°C
PTC Thermistor	100 Ω to 1 kΩ	+3% to +6% per °C	-40°C to 125°C

Circuit Design

The circuit design for temperature sensor interfacing with Arduino involves the use of a voltage divider circuit to convert the resistance change of the thermistor into a voltage signal that can be read by the Arduino analog input pin. The voltage divider circuit consists of the thermistor and a fixed-value resistor, as shown in Figure 1.

Figure 1: Voltage Divider Circuit for Temperature Sensor Interfacing

The voltage at the analog input pin of the Arduino can be calculated using the following formula:

V_out = V_in * (R_thermistor / (R_thermistor + R_fixed))

where:
– V_out is the voltage at the analog input pin of the Arduino
– V_in is the supply voltage (typically 5V or 3.3V)
– R_thermistor is the resistance of the thermistor
– R_fixed is the resistance of the fixed-value resistor

The choice of the fixed-value resistor depends on the resistance range of the thermistor and the voltage range of the Arduino analog input. A common practice is to select a fixed-value resistor that is approximately equal to the thermistor resistance at the midpoint of the expected temperature range.

Calibration

Calibration is the process of converting the voltage signal from the temperature sensor into a temperature value. This involves the use of the Steinhart-Hart equation, which relates the resistance of the thermistor to temperature:

T = 1 / (A + B * ln(R) + C * (ln(R))^3)

where:
– T is the temperature in Kelvin
– R is the resistance of the thermistor
– A, B, and C are the Steinhart-Hart coefficients, which are specific to the thermistor model

To determine the Steinhart-Hart coefficients, you need to measure the thermistor resistance at known temperatures and then use a curve-fitting algorithm to calculate the coefficients. This process can be automated using a microcontroller or a computer program.

Once the Steinhart-Hart coefficients are known, the Arduino can use the measured voltage from the voltage divider circuit and the Steinhart-Hart equation to calculate the temperature value.

Data Acquisition

The Arduino can acquire temperature data from the sensor at a rate determined by the sampling frequency. The sampling frequency should be high enough to capture the dynamics of the temperature changes, but not so high that it overwhelms the Arduino’s processing capabilities.

The Arduino can output the temperature data through the serial port for visualization and analysis. This can be done using the built-in Serial.print() function in the Arduino programming language. The temperature data can be displayed on a computer or mobile device using a serial terminal application or integrated into a larger data acquisition and control system.

Accuracy and Precision

The accuracy and precision of the temperature measurement depend on several factors, including the sensor’s sensitivity, the resolution of the Arduino analog-to-digital converter, and the calibration process.

The accuracy of the temperature measurement can be quantified by measuring the temperature at known reference points, such as the freezing and boiling points of water, and calculating the error between the measured and reference temperatures. The precision of the measurement can be quantified by calculating the standard deviation of multiple measurements at the same temperature.

Typical accuracy and precision values for a well-designed temperature sensor interface with Arduino are:

• Accuracy: ±0.5°C to ±1.0°C
• Precision: ±0.1°C to ±0.3°C

To improve the accuracy and precision of the temperature measurements, you can consider the following strategies:

1. Sensor Selection: Choose a thermistor with a high temperature coefficient and low tolerance to minimize the impact of resistance variations.
2. Circuit Design: Optimize the voltage divider circuit to maximize the voltage change per degree of temperature change.
3. Calibration: Perform a thorough calibration process, including measuring the thermistor resistance at multiple known temperatures, to ensure accurate Steinhart-Hart coefficients.
4. Sampling and Filtering: Adjust the sampling frequency and apply digital filtering techniques to reduce noise and improve the stability of the temperature readings.

Conclusion

Temperature sensor interfacing with Arduino is a crucial aspect of many IoT and automation projects, allowing for precise monitoring and control of environmental conditions. By understanding the technical details of selecting, designing, calibrating, and acquiring data from temperature sensors, you can create reliable and accurate temperature measurement systems using the Arduino platform.

References

1. Team 14, F21-S22, “Automatic Hybrid Cooling Fan End-of-Project Report,” California State University, Sacramento, 2022. [PDF] Available at: https://www.csus.edu/indiv/t/tatror/senior_design/SD%20F21-S22/Team_14_F21S22_EOP_Report.pdf
2. DHT11-Technical-Data-Sheet-Translated-Version-1143054.pdf, “DHT11 Temperature and Humidity Sensor Datasheet,” Mouser Electronics, 2021. Available at: https://www.mouser.com/datasheet/2/758/DHT11-Technical-Data-Sheet-Translated-Version-1143054.pdf
3. 4Ch-relay.pdf, “4 Channel Relay Module Specification Sheet,” Handsontec, 2021. Available at: https://www.handsontec.com/dataspecs/4Ch-relay.pdf