LM75 | GUILCOR SENSORS

LM75
Temperature sensors

Industry-standard I²C temperature sensor for reliable thermal monitoring in electronics and control systems.

Maximum precision
+/- 2,0°K

Measurement range
-55 / 125°C

Resolution
9 bits

Power supply voltage
2,7 → 5,5V

Conversion speed

100mS

I2C adress
0 x 48

Typical current
250µA(active)

Price
Low

What is a LM75 sensor ?

The LM75 is a very simple digital temperature sensor, originally developed by National Semiconductor and now produced by several manufacturers (TI, NXP, ST…).

It is one of the first sensors with I²C output, still widely used for its reliability, low cost, and universal compatibility.

It directly provides the temperature in digital form on 9 bits, with a resolution of 0.5 °C and an SMBus / I²C compatible interface.

Operating principle

The LM75 measures temperature using an integrated silicon diode, whose threshold voltage decreases proportionally to the temperature.

This voltage is digitized by an internal 9-bit ADC and then transmitted via the I²C bus.

The most significant bit (MSB) indicates the sign (negative or positive temperature).

T(°C) = Binare code x 0,5

Example:

0x19 → 25 × 0.5 = 12.5 °C

Temperature curve / numerical value (12-bit resolution)

Temperature (°C)	Numerical value (hex)	Binary output (12bits)
-25	0xFE7	1111 1110 0111
0	0x0000	0000 0000 0000
25	0x019	0000 0001 1001
50	0x032	0000 0011 0010
100	0x064	0000 0110 0100
125	0x07D	0000 0111 1101

💡 Each bit represents 0.5 °C. The format is signed on 9 bits (negative values are encoded in two's complement).

Principle diagram (I2C connection)

+3.3V
 │
[LM75]
 │ SDA ─────────┐
 │ SCL ─────────┤── [I²C Microcontroller]
 │ OS (alerte) ─┘
 │ GND

💡 The A0–A2 pins allow you to change the I²C address to connect up to 8 LM75 on the same bus.

Application areas

🧠 Electronic cards and microcontrollers

⚙️ Thermal monitoring of embedded systems

🔋 Thermal management of batteries and power supplies

🧪 Basic environmental measurements (HVAC, home automation)

Should I choose a LM75 sensor ?

Strengths points

💰 Unbeatable value for money
→ It is one of the most cost-effective I²C digital sensors on the market.
⚙️ Ease of integration
→ No calibration required, compatible with native SMBus and I²C.
🔋 Low consumption
→ Less than 1 µA in standby, perfect for low-power systems.

Weaknesses points

🌡️ Limited resolution (0.5 °C)
→ Unsuitable for applications requiring fine precision (replaced by TMP117, MCP9808…).
📡 Slow communication
→ Modest sampling rate (~100 ms), insufficient for dynamic measurements.
🌬️ Sensitive to parasitic heat
→ May be influenced by the thermal dissipation of the circuit to which it is soldered.

Useful information

Here is some useful information regarding the LM75 sensors.

Main registers (7-bit address: 0x48 → 0x4B)

Adress	Registry Name	Function
0x00	Temperature Register	Contains the temperature measurement (9 signed bits)
0x01	Configuration Register	Activate the alert, sleep mode, or polarity
0x02	T_HYST Register	Hysteresis temperature (low threshold)
0x03	T_OS Register	Alert trigger temperature

Application example – Digital reading I²C

i2c_start();
i2c_write(0x48 << 1);      // Adress LM75
i2c_write(0x00);           // Temperature log
i2c_start();
i2c_write((0x48 << 1) | 1);
MSB = i2c_read_ack();
LSB = i2c_read_nack();
i2c_stop();

// Signed 9-bit data:
raw = (MSB << 1) | (LSB >> 7);
if (raw > 255) raw -= 512;  // Sign management
temp = raw * 0.5;

✅ Example:

MSB = 0x19, LSB = 0x00

→ (0x19 << 1) = 0x32 → temp = 25.0 °C

Typical integration diagram

+3.3V ───────────┐ │ [LM75] │ SDA ───────┐ │ SCL ───────┤── MCU (Arduino, ESP32, STM32…) │ OS ────────┘ (alert exit) │ GND

💡 Plan for pull-up resistors of 4.7 kΩ on SDA/SCL, and possibly a resistor on OS if the comparator mode is enabled.

Example of quick calculation

If the temperature register contains 0x19:

T=0x19×0.5=25.0°CT = 0x19 \times 0.5 = 25.0 °CT=0x19×0.5=25.0°C

If the temperature falls below the threshold defined by T_HYST, the OS pin is activated (alarm mode).

Typical precision curve