31. I2C Temperature LM75

This lesson uses: Arduino UNO R3 board, TinkerBlock TK42 LM75 module, jumper wires (or breadboard). Use Wire library for I2C to read LM75 temperature; outcome: serial continuously shows temperature (°C); warming the sensor makes the value rise.

1. What is I2C?

I2C (Inter-Integrated Circuit) is a protocol using two wires (SDA data, SCL clock) to connect multiple devices.

• SDA: data line
• SCL: clock line
• On Arduino Uno, I2C pins are fixed: SDA=A4, SCL=A5

2. Wiring

Connect TK42 LM75 to Arduino (I2C):

• GND → Arduino GND
• VCC → Arduino 5V
• SDA → Arduino A4 (I2C data)
• SCL → Arduino A5 (I2C clock)
• NC leave unconnected

3. New sketch

1. Arduino IDE: File → New (or Ctrl/Cmd + N) to create a blank sketch
2. Confirm board and port are selected (same as Lesson 01)
3. Type the code below into the default setup() and loop() yourself

4. Program structure

Same as last lesson: two fixed blocks setup() and loop().

• setup(): Include library, start I2C, start serial; runs once.
• loop(): Keep repeating “read temperature over I2C → print to serial → wait → repeat”.

Two wires (SDA, SCL) read temperature; serial will show LM75 °C about once per second.

5. Code to write

Type the code below at the top of the file and in setup() and loop() (do not copy-paste; typing it yourself helps you remember):

// Read LM75 temperature register over I2C; combine two bytes and convert to °C
#include <Wire.h>

#define LM75_ADDRESS 0x48   // LM75 I2C address (7-bit)

void setup() {
  Wire.begin();   // I2C init (SDA=A4, SCL=A5)
  Serial.begin(9600);
  Serial.println("I2C temperature sensor started");
}

void loop() {
  Wire.beginTransmission(LM75_ADDRESS);
  Wire.write(0x00);   // Temperature register address
  Wire.endTransmission();
  Wire.requestFrom(LM75_ADDRESS, 2);   // Request 2 bytes
  if (Wire.available() >= 2) {
    int highByte = Wire.read();
    int lowByte = Wire.read();
    int tempRaw = (highByte << 8) | lowByte;   // 16-bit raw
    float temperature = (tempRaw >> 5) * 0.125;  // LM75: upper 11 bits, 0.125°C per step
    Serial.print("Temperature: ");
    Serial.print(temperature);
    Serial.println("°C");
  }
  delay(1000);   // Update every second
}

Program notes

Overall idea: LM75 stores temperature in register 0x00 as 2 bytes. Send register address, then request 2 bytes; high byte first, combine to 16-bit; upper 11 bits are temperature (0.125°C per step), so (tempRaw >> 5) * 0.125 gives °C.

6. Hands-on

1. After entering the code, click Verify (✓) to compile
2. Click Upload (→) to upload to the board
3. Open Serial Monitor (Tools → Serial Monitor, baud 9600)
4. Watch serial: current temperature in °C
5. Warm the sensor with your hand and see the value change

Expected result:

Serial monitor:

I2C temperature sensor started
Temperature: 25.3°C
Temperature: 25.4°C
Temperature: 26.1°C
...

Proceed to Lesson 32.

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