Overview

The LM75A is an 11-bit digital temperature sensor and thermal watchdog manufactured by NXP Semiconductors, Texas Instruments, and Maxim Integrated. Operating over an $I^2C$ bus, it measures ambient temperatures from $-55^\circ\text{C}$ to $+125^\circ\text{C}$ with a resolution of $0.125^\circ\text{C}$ ($1/8^\circ\text{C}$) and accuracy of $\pm 2.0^\circ\text{C}$.

Featuring a dedicated open-drain Over-temperature Shutdown (OS) output pin, the LM75A can operate as a standalone programmable hardware thermostat (thermal watchdog), pulling the OS pin Low when temperature exceeds the programmed T_OS limit threshold and releasing it when temperature drops below the T_HYST hysteresis threshold. It is widely used in server rack monitoring, 3D printer enclosure thermal protection, and ESPHome environmental nodes.

Quick reference

Pinout (SOIC-8 / TSSOP-8 Package & Breakout Header)

             ┌───┴───┐
         SDA ─┤ 1    8├─ VCC
         SCL ─┤ 2    7├─ A0
          OS ─┤ 3    6├─ A1
         GND ─┤ 4    5├─ A2
             └───────┘

Specifications

Register Map & Data Format

The temperature data register (0x00) returns 2 bytes in 11-bit 2's complement format (left-justified):

• Byte 1: Integer temperature bits 10–3.
• Byte 2 (Bits 7–5): Fractional temperature bits 2–0 ($0.125^\circ\text{C}$ per count).

$\text{Temperature } (^\circ\text{C}) = \frac{\text{Raw 11-Bit Signed Value}}{8} = \frac{\text{(Byte 1 } \ll 3) \mid (\text{Byte 2 } \gg 5)}{8}$

Wiring

Example

#include <Wire.h>

#define LM75A_ADDR 0x48

void setup() {
  Serial.begin(9600);
  Wire.begin();
}

void loop() {
  Wire.beginTransmission(LM75A_ADDR);
  Wire.write(0x00); // Select Temperature Register
  Wire.endTransmission();

  Wire.requestFrom(LM75A_ADDR, 2);
  if (Wire.available() >= 2) {
    int16_t raw = (Wire.read() << 8) | Wire.read();
    // Shift right by 5 to get 11-bit signed value
    raw >>= 5;
    
    // Handle 11-bit sign extension
    if (raw & (1 << 10)) {
      raw |= 0xF800; // Sign extend negative values
    }

    float tempC = raw * 0.125;
    Serial.print("LM75A Temperature: "); Serial.print(tempC); Serial.println(" °C");
  }

  delay(1000);
}

Common mistakes

• Not bit-shifting the 2nd byte right by 5 bits: The LM75A formats its 11-bit reading left-justified across 2 bytes. Reading raw 16-bit integers without right-shifting by 5 results in values multiplied by 32.
• Floating address pins $A_0, A_1, A_2$: On raw DIP/SOIC IC packages, tie $A_0, A_1, A_2$ to GND or $V_{CC}$ to lock the $I^2C$ address.

Notes

• LM75A vs TMP102: LM75A is 11-bit ($0.125^\circ\text{C}$); TMP102 is 12-bit ($0.0625^\circ\text{C}$). Both share near-identical register structures.