What is a Humidity Sensor? || Principle of Operation of a Humidity Sensor.

Humidity Sensors

A humidity sensor (also called a hygrometer) is a device that measures the amount of water vapor in the air. It converts humidity levels into an electrical signal, which can then be displayed, recorded, or used for automation.

A humidity sensor measures the amount of moisture in the air. It is commonly used in weather monitoring, industrial applications, home automation, and agriculture.

Principle of Operation of a Humidity Sensor

A humidity sensor works by detecting changes in environmental moisture and converting it into an electrical signal. The principle varies depending on the sensor type, but the most common types operate based on capacitive, resistive, or thermal properties.

Types of Humidity Sensors

1. Capacitive Humidity Sensor (Most Common)

Working Principle:

• Consists of a dielectric material (such as polymer or ceramic) placed between two conductive plates.
• The dielectric absorbs moisture from the air, causing a change in its capacitance (the ability to store electrical charge).
• A circuit measures this capacitance change and converts it into a humidity value.

Example Sensors: DHT11, DHT22, BME280, Si7021

2. Resistive Humidity Sensor

Working Principle:

• Uses a hygroscopic (moisture-sensitive) material whose resistance changes when exposed to different humidity levels.
• As humidity increases, the material absorbs water, causing its resistance to decrease.
• The sensor circuit measures this resistance change and calculates humidity.

Example Sensors: HR202, SYH-1

3. Thermal Humidity Sensor

Working Principle:

• Uses two thermistors (temperature-sensitive resistors) - one exposed to air and one sealed.
• The difference in temperature readings between the two thermistors is used to calculate humidity.

Example Sensors: Used in industrial applications, not common in Arduino projects.

Summary of Principles

Capacitive;- Measures change in capacitance due to moisture absorption, Weather monitoring, home automation, industrial use

Resistive;- Measures change in electrical resistance caused by moisture ,HVAC systems, industrial humidity control

Thermal ;-Uses heat transfer differences to determine humidity ,High-precision industrial and meteorological applications

Applications of Humidity Sensors

✅ Weather Monitoring – Used in weather stations to track humidity and climate conditions.
✅ Home Automation – Used in smart thermostats and air purifiers.
✅ Agriculture – Helps monitor soil and air humidity for optimal plant growth.
✅ Medical Devices – Used in ventilators and CPAP machines for respiratory care.
✅ Industrial Use – Used in HVAC systems to control air quality and temperature.

Popular Humidity Sensors for Arduino

• DHT11: Low-cost, measures temperature and humidity, but less accurate.
• DHT22 (AM2302): More accurate than DHT11 and has a wider range.
• BME280: Measures humidity, temperature, and pressure with high precision.
• Si7021: Compact and accurate sensor for humidity and temperature.

Using a DHT11/DHT22 with Arduino
Connections

DHT11/DHT22 Pin = Arduino Pin

VCC =5V
Data =Digital Pin (e.g., D2)
GND =GND

Arduino code

#include <DHT.h> #define DHTPIN 2 // Digital pin connected to the sensor #define DHTTYPE DHT11 // Change to DHT22 if using it DHT dht(DHTPIN, DHTTYPE); void setup() { Serial.begin(9600); dht.begin(); } void loop() { float humidity = dht.readHumidity(); float temperature = dht.readTemperature(); if (isnan(humidity) || isnan(temperature)) { Serial.println("Failed to read from DHT sensor!"); return; } Serial.print("Humidity: "); Serial.print(humidity); Serial.print("% Temperature: "); Serial.print(temperature); Serial.println("°C"); delay(2000); }