How to Use LM75A: Examples, Pinouts, and Specs

The LM75A is a digital temperature sensor with an I2C interface, designed for precise temperature measurement and monitoring. It operates over a wide temperature range of -55°C to +125°C with a resolution of 0.5°C. The LM75A includes programmable over-temperature and under-temperature alarms, making it ideal for applications requiring temperature control and safety monitoring.

• HVAC systems for temperature regulation
• Over-temperature protection in electronic devices
• Industrial temperature monitoring
• Battery management systems
• Home automation and IoT devices

• Temperature Range: -55°C to +125°C
• Resolution: 0.5°C
• Supply Voltage: 2.7V to 5.5V
• Communication Protocol: I2C (up to 400 kHz)
• Accuracy: ±2°C (typical) from -25°C to +100°C
• Alert Function: Programmable over-temperature and under-temperature alerts
• Power Consumption: 300 µA (typical operating current)

The LM75A is typically available in an 8-pin package. Below is the pinout and description:

The LM75A's I2C address is determined by the states of the A2, A1, and A0 pins. The base address is 0x48, and the full address is calculated as:

Address = 0x48 + (A2 * 4) + (A1 * 2) + A0

For example:

• If A2 = 0, A1 = 0, A0 = 1, the address is 0x49.

1. Power Supply: Connect the VCC pin to a 3.3V or 5V power source and the GND pin to ground.
2. I2C Communication: Connect the SDA and SCL pins to the corresponding I2C lines of your microcontroller. Use pull-up resistors (typically 4.7 kΩ) on both lines.
3. Address Configuration: Set the A2, A1, and A0 pins to configure the I2C address. These can be connected to VCC (logic high) or GND (logic low).
4. Alert Pin (Optional): Connect the OS pin to a microcontroller GPIO pin or an external circuit to monitor over-temperature alerts.

• Use decoupling capacitors (e.g., 0.1 µF) between VCC and GND to reduce noise.
• Ensure proper pull-up resistors are used on the I2C lines for reliable communication.
• Avoid placing the LM75A near heat sources to ensure accurate temperature readings.

Below is an example of how to interface the LM75A with an Arduino UNO and read temperature data:

• Connect VCC to the Arduino's 5V pin.
• Connect GND to the Arduino's GND pin.
• Connect SDA to the Arduino's A4 pin.
• Connect SCL to the Arduino's A5 pin.
• Use 4.7 kΩ pull-up resistors on the SDA and SCL lines.

#include <Wire.h>

#define LM75A_ADDRESS 0x48 // Default I2C address of LM75A

void setup() {
  Wire.begin(); // Initialize I2C communication
  Serial.begin(9600); // Start serial communication for debugging
}

void loop() {
  float temperature = readTemperature();
  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.println(" °C");
  delay(1000); // Wait 1 second before the next reading
}

float readTemperature() {
  Wire.beginTransmission(LM75A_ADDRESS); // Start communication with LM75A
  Wire.write(0x00); // Point to the temperature register
  Wire.endTransmission();

  Wire.requestFrom(LM75A_ADDRESS, 2); // Request 2 bytes from LM75A
  if (Wire.available() == 2) {
    // Read the two bytes of temperature data
    uint8_t msb = Wire.read(); // Most significant byte
    uint8_t lsb = Wire.read(); // Least significant byte

    // Combine the bytes and convert to temperature
    int16_t rawTemp = (msb << 8) | lsb;
    rawTemp >>= 7; // Right shift to remove unused bits
    return rawTemp * 0.5; // Each bit represents 0.5°C
  }
  return NAN; // Return NaN if no data is available
}

1. No Temperature Data Received

   • Cause: Incorrect I2C address or wiring.
   • Solution: Verify the I2C address based on the A2, A1, and A0 pin configuration. Check the SDA and SCL connections and ensure pull-up resistors are in place.

2. Inaccurate Temperature Readings

   • Cause: Heat sources near the sensor or poor decoupling.
   • Solution: Place the LM75A away from heat-generating components. Add a 0.1 µF capacitor between VCC and GND.

3. Alert Pin Not Functioning

   • Cause: Incorrect configuration of the OS pin or alert thresholds.
   • Solution: Verify the alert thresholds are correctly programmed via the LM75A's configuration register.

• Q: Can the LM75A operate at 3.3V?
  A: Yes, the LM75A supports a supply voltage range of 2.7V to 5.5V.

• Q: What is the default I2C address of the LM75A?
  A: The default address is 0x48 when A2, A1, and A0 are all connected to GND.

• Q: How do I change the temperature alert thresholds?
  A: The alert thresholds can be configured by writing to the LM75A's over-temperature and hysteresis registers via I2C.

• Q: Can the LM75A measure negative temperatures?
  A: Yes, the LM75A can measure temperatures as low as -55°C. Negative temperatures are represented in two's complement format.