Temperature and humidity sensors

Team Member

Chen Andi、Chen Miaoge、Chen Yingnan、Fang Ye

Background

With the advancement of smart home systems, agricultural monitoring, and industrial environmental control, real-time monitoring of ambient temperature and humidity has become increasingly important. Variations in temperature and humidity significantly impact human daily activities, industrial production, and the storage conditions of various products. Traditional temperature and humidity measurement instruments are often expensive or lack real-time data display capabilities. Therefore, designing a low-cost, real-time temperature and humidity monitoring system based on the DHT11 sensor and Arduino holds significant practical and educational value.

Objective

The objective of this project is to design and implement a real-time temperature and humidity monitoring system based on Arduino and the DHT11 sensor, capable of continuously acquiring environmental temperature and humidity data and displaying the results on an LCD1602 screen in an intuitive manner. The system is designed to fulfill the following functions:

• Real-time Data Acquisition: Accurately measures the current ambient temperature and humidity.
• Dynamic Data Display: Continuously updates the LCD screen to reflect the latest measurement data.
• Low Cost and High Scalability: Suitable for various applications such as smart home systems, agriculture, and warehouse environments, with potential extensions to wireless data transmission and other advanced functionalities.

Theory

1.Temperature Measurement

1.1 Definition of Temperature

Temperature is a fundamental physical quantity used to characterize the thermal state of a system, determining the direction of heat flow between two bodies. It is typically measured in degrees Celsius (°C), Kelvin (K), or Fahrenheit (°F), depending on the context. In the field of temperature measurement, various devices, including thermocouples, thermistors, and pyrometers, are used to determine temperature across different ranges. Temperature is defined as the measure of the average kinetic energy of the particles in a substance, and it plays a critical role in various natural and technological processes. At a molecular level, temperature correlates with the movement of atoms and molecules: the higher the temperature, the faster the motion of the particles. In the context of thermistors, temperature is directly related to resistance through a thermoresistive effect. Specifically, NTC (Negative Temperature Coefficient) thermistors show a decrease in resistance with an increase in temperature, making them useful for temperature measurement and control in electronics, industrial processes, and other applications requiring precision temperature data.

1.2 NTC Thermistors

Thermistors are electronic components whose resistance changes significantly with temperature. In the case of NTC thermistors, as temperature increases, resistance decreases exponentially. The relationship between temperature T (in Kelvin) and resistance R is often described by the Steinhart-Hart equation, which is widely used to model the behavior of thermistors:

Where R0 is the resistance at a reference temperature T0 ,B is the material constant of the thermistor, T is the temperature of interest.Figure 1 Resistance vs. Temperature Curve for 10 kΩ NTC Thermistor at 25°C NTC Thermistors exhibit a negative temperature coefficient (NTC), meaning their resistance decreases with an increase in temperature. They are used in various applications such as temperature measurement and compensation in electronics. These thermistors are particularly useful in electronics, air conditioning, and automotive applications due to their high sensitivity and low cost.

2.Humidity Measurement

2.1 Relative Humidity

Relative Humidity (R.H.) is one of the most commonly measured parameters in humidity measurement and plays a critical role in various industrial and commercial applications. It is defined as the ratio of the partial pressure of water vapor in the air (Pw​) to the saturation vapor pressure (Ps​) at a specific temperature, expressed as a percentage:

Where Pw​ is the actual water vapor pressure in the air, and Ps is the saturation vapor pressure at the ambient temperature. "When temperature and pressure change, the corresponding water vapor pressure also changes, so the relative humidity will change as well. Relative humidity provides a measure of how much water vapor is present in the air relative to the maximum amount of water vapor the air can hold at a given temperature.

2.2 Resistive Humidity Sensors

Resistive humidity sensors operate based on changes in the conductivity of the sensor material as it absorbs water vapor. The resistance of the sensor decreases as the humidity increases, which is the primary transduction mechanism. These sensors typically utilize materials with significant changes in conductivity in response to moisture content, making them suitable for a variety of humidity sensing applications.

Equipment Required

• DHT11
• Electronic display
• Arduino
• DuPont Line
• battery-9V

Experiment Steps

Circuit Connection

1.DHT11 Connection

• VCC (DHT11) → 5V (Arduino UNO)

The VCC pin of the sensor is connected to the 5V power supply pin on the Arduino to provide power.

• GND (DHT11) → GND (Arduino UNO)

The ground pin of the sensor is connected to the ground pin on the Arduino.

• OUT (DHT11) → Digital Pin 8 (Arduino UNO)

The data output pin of the DHT11 is connected to digital pin 8 on the Arduino for reading sensor data.

2.Power Connection

A 9V battery provides power to the Arduino UNO through a DC barrel jack adapter, which converts the voltage for safe operation.

DHT11 Sensor Connected to Arduino UNO

Programming and Uploading Code

1.Installing Libraries

• In Arduino IDE, install the DHT Sensor Library by Adafruit and LiquidCrystal_I2C libraries.

2.Writing Code (Code details omitted)

• Include necessary libraries (DHT).
• Define the DHT11 sensor pin and type.
• In the loop() function, read temperature and humidity data and serial monitor.

3.Uploading Code

• Connect Arduino to the computer via USB cable.
• Select the correct board model and port.

 #include "DHT.h"
 #define DHTPIN 8       // Pin where the sensor is connected
 #define DHTTYPE DHT11  // Sensor type
 DHT dht(DHTPIN, DHTTYPE);
 void setup() {
 Serial.begin(9600); // Initialize the serial monitor
 dht.begin();        // Initialize the sensor
 }
 void loop() {
 float humidity = dht.readHumidity();            // Read humidity
 float temperatureC = dht.readTemperature();     // Temperature in Celsius
 float temperatureF = dht.readTemperature(true); // Temperature in Fahrenheit
 float temperatureK = temperatureC + 273.15;     // Temperature in Kelvin
 if (isnan(humidity) || isnan(temperatureC)) {
   Serial.println("Error reading data!");
   return;
 }
 // Output data to the serial monitor
 Serial.print("Temperature: ");
 Serial.print(temperatureC);
 Serial.print("°C, ");
 Serial.print(temperatureF);
 Serial.print("°F, ");
 Serial.print(temperatureK);
 Serial.println("K");
 Serial.print("Humidity: ");
 Serial.print(humidity);
 Serial.println("%");
 delay(2000); // Delay between readings
 }

Experiment Operation

Disconnect USB Power and Connect 9V Battery.

Observe Temperature and Humidity Data on the Display and Serial Monitor.

References

[1]BU Y, LUO X, CHEN Y. Temperature and Humidity Acquisition System Based on Sensor DHT11[J]. Computer and Modernization, 2013, 1(11): 133. [2]Srivastava D, Kesarwani A, Dubey S. Measurement of Temperature and Humidity by using Arduino Tool and DHT11[J]. International Research Journal of Engineering and Technology (IRJET), 2018, 5(12): 876-878. [3]Lee C Y, Lee G B. Humidity sensors: a review[J]. Sensor Letters, 2005, 3(1-2): 1-15. [4]Aleksić O S, Nikolić P M. Recent advances in NTC thick film thermistor properties and applications[J]. Facta universitatis-series: Electronics and Energetics, 2017, 30(3): 267-284.