IoT Engineering Course using ESP32 Wifi Robots

📘Variables & Data Types

🎯 Lesson Objective

By the end of this lesson, students will:

Understand what a variable is
Understand how memory stores data
Learn different data types used in ESP32/Arduino
Understand size and range of data types
Learn when to use which data type
Understand common beginner mistakes

1️⃣ What is a Variable?

A variable is a named storage location in memory that holds a value.

In simple words:

A variable is a container that stores data.

Example:

int temperature = 25;

Here:

temperature is the variable name
25 is the value stored

2️⃣ Why Variables Are Important in IoT

IoT systems constantly handle data like:

Temperature values
Gas sensor readings
Water levels
Mode settings
Button states

All this information must be stored somewhere.

That storage is done using variables.

3️⃣ How Memory Works (Simple Understanding)

ESP32 has RAM (memory).

When you declare:

int temperature = 25;

The system:

Reserves memory space
Stores the number 25
Labels it as “temperature”

Whenever you use temperature,
ESP32 reads that memory location.

4️⃣ Variable Syntax

General format:

dataType variableName = value;

Example:

float voltage = 3.3;
bool relayState = true;

Structure:

dataType → What kind of data
variableName → Name you choose
value → Stored value

5️⃣ Common Data Types in ESP32 / Arduino

🔹 1. int (Integer)

Stores whole numbers.

Example:

int temperature = 30;

Range (32-bit ESP32):

-2,147,483,648 to 2,147,483,647

Used for:

Sensor values
Counters
Digital states

🔹 2. float (Decimal Numbers)

Stores decimal values.

Example:

float voltage = 3.3;

Used for:

Temperature with decimals
Voltage
Sensor calibration

Important:
Float uses more memory than int.

🔹 3. double

On ESP32, double is same as float (4 bytes).

In advanced systems, double gives higher precision.

For this course:
Use float.

🔹 4. char (Single Character)

Stores a single character.

Example:

char grade = ‘A’;

Used for:

Serial input
Character commands

🔹 5. bool (Boolean)

Stores only two values:

true
false

Example:

bool pumpState = true;

Used for:

ON/OFF state
Mode selection
Flag variables

Very important for automation logic.

🔹 6. String

Stores text.

Example:

String message = “Gas Detected”;

Used for:

Display messages
Blynk labels
Serial printing

Note:
String uses more memory.

6️⃣ Choosing the Right Data Type

Using correct data type is important.

Example:

Temperature:
Use float if decimal needed.

Gas sensor:
Use int (ADC value 0–4095).

Relay state:
Use bool.

Using wrong type wastes memory or causes errors.

7️⃣ Variable Naming Rules

Variable names:

✔ Must start with letter or underscore
✔ Cannot contain spaces
✔ Cannot use special symbols
✔ Cannot use reserved keywords

Correct:

int waterLevel;
bool manualMode;

Wrong:

int 1temp;
float gas-value;

8️⃣ Global vs Local Variables

🔹 Global Variable

Declared outside functions.

Accessible everywhere.

Example:

int temperature = 25;

void setup() {
}

void loop() {
}

Used when:
Data needed in multiple functions.

🔹 Local Variable

Declared inside a function.

Accessible only inside that function.

Example:

void loop() {
int value = 10;
}

Used for temporary storage.

9️⃣ Updating Variables

Variables can change value.

Example:

int counter = 0;

void loop() {
counter = counter + 1;
}

This increases counter each time loop runs.

Very important in IoT logic.

🔟 Real IoT Example

Smart AC Example:

float temperature = 28.5;
float setPoint = 30.0;
bool acState = false;

void loop() {
if (temperature > setPoint) {
acState = true;
} else {
acState = false;
}
}

Here:

temperature → sensor data
setPoint → user input
acState → system decision

All controlled by variables.

1️⃣1️⃣ Memory Size of Data Types (ESP32)

Data Type	Size
bool	1 byte
char	1 byte
int	4 bytes
float	4 bytes
double	4 bytes
String	dynamic

ESP32 has limited RAM.

Using correct data types improves performance.

1️⃣2️⃣ Common Beginner Mistakes

❌ Using float when int is enough
❌ Forgetting to initialize variable
❌ Declaring variable inside loop unnecessarily
❌ Using wrong data type
❌ Confusing = and ==

Remember:

= → Assignment
== → Comparison

Example:

if (temperature == 30)

Not:

if (temperature = 30)

1️⃣3️⃣ Constants

Sometimes values should not change.

Use:

const int ledPin = 2;

This prevents accidental changes.

Used for:

Pin numbers
Fixed thresholds

1️⃣4️⃣ Variable Scope Importance

Understanding scope helps in:

Avoiding bugs
Reducing memory usage
Writing clean code

Good programming = controlled variables.

📌 Lesson Summary

In this lesson, we learned:

What is a variable
How memory stores data
int, float, char, bool, String
Global vs local variables
Updating values
Choosing correct data types
Common mistakes

Variables are the backbone of logic.

Without variables → No IoT system works.