LM75 temperature sensor (CJMCU-75): specs and pinout
A module sold under an LM75 name, with an I2C interface, three address pins and an over-temperature output. The chip variant on this board is not confirmed; the figures here are the TI LM75B family figures and may differ for other LM75 variants.
Specs
| Spec | Value | Source |
|---|---|---|
| Supply voltage (+VS) | 3 V to 5.5 V (LM75B) | source 6.3 Recommended Operating Conditions, p.4 |
| Interface | 2-wire I2C, up to 400 kHz (LM75B) | source 3 Description, p.1 |
| I2C address | 1001 followed by pins A2, A1, A0 (7-bit); up to eight devices on one bus (LM75B) | source Serial Bus Interface, slave address text |
| Temperature resolution | 9 bits (LM75B) | source 3 Description, p.1 and electrical characteristics |
| Temperature accuracy | plus or minus 2 C maximum from -25 C to 100 C; plus or minus 3 C from -55 C to 125 C (LM75B) | source 3 Description, p.1 |
| Specified temperature range | -55 C to 125 C (LM75B) | source 6.3 Recommended Operating Conditions, p.4 |
| Supply current | 280 uA typical operating; 4 uA typical in shutdown (LM75B) | source 1 Features, p.1 |
| Over-temperature output (O.S.) | Open-drain, active low, programmable limit and hysteresis (LM75B) | source 3 Description and Pin Functions |
Power
| Value | Source | |
|---|---|---|
| Supply | 3 V to 5.5 V (LM75B) | source 1 Features, p.1 |
| Current | 280 uA typical operating | source 1 Features, p.1 |
Pinout
Chip variant on this board not confirmed; figures are for the TI LM75B. The module board is not covered by an official document. Its regulator, pull-up resistors, address jumpers, level handling and header order are not documented by a source; check the printed labels on the board. The numbers on this page are for the chip.
| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| 1 | SDA | I2C serial bi-directional data line, open drain | source Pin Functions, p.3 (8-pin chip) | |
| 2 | SCL | I2C clock input, open drain | source Pin Functions, p.3 (8-pin chip) | |
| 3 | O.S. | Over-temperature shutdown output, open drain | source Pin Functions, p.3 (8-pin chip) | |
| 4 | GND | Power supply ground | source Pin Functions, p.3 (8-pin chip) | |
| 5 | A2 | User-set I2C address input; tie to ground or +VS | source Pin Functions, p.3 (8-pin chip) | |
| 6 | A1 | User-set I2C address input; tie to ground or +VS | source Pin Functions, p.3 (8-pin chip) | |
| 7 | A0 | User-set I2C address input; tie to ground or +VS | source Pin Functions, p.3 (8-pin chip) | |
| 8 | +VS | 3 V to 5.5 V | Positive supply voltage input; 100 nF bypass capacitor recommended | source Pin Functions, p.3 (8-pin chip) |
Build with it
Project ideas for the LM75 temperature sensor (CJMCU-75). Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.
- Temperature display: The chip runs from 3 V to 5.5 V on I2C, so it works on the ESP32 3.3 V bus with the OLED. Set the address pins first.
- Over-temperature indicator LED: Before wiring, check on the board that the O.S. pin is brought out and labelled; if it is not, use the I2C readings instead. The O.S. output is open drain with a programmable limit, so an LED with a resistor from 3.3 V can light when it goes low; keep the current to about 4 mA or less, the data sheet's test condition (LM75B). Learning project.
- Two sensor comparison: The two chips use different I2C addresses (LM75 starts at 1001, HTU21D uses 0x40), so they share one bus with the OLED. Comparing their temperature readings is a learning exercise.
Common mistakes
- Leaving address pins floating: A2, A1 and A0 set the address and the data sheet recommends tying them to ground or +VS (source: link, Serial Bus Interface).
- No pull-ups: SDA, SCL and the O.S. output are open-drain and need pull-up resistors (source: link, Pin Functions).
- Reading too fast: accessing the chip continuously without waiting at least one conversion time prevents the temperature register from updating (source: link, electrical characteristics note 5).
Not covered by the official source
- The module name says LM75; the exact chip variant on the module is not documented by a source, so the LM75B document was used.
- The 7-bit address is given as the fixed four-bit prefix plus A2 A1 A0 as the data sheet states it.
- Resolution, accuracy and supply figures are LM75B figures; other LM75 variants (for example an 11-bit LM75A) differ.
Sources
- link (LM75B, LM75C Digital Temperature Sensor and Thermal Watchdog With Two-Wire Interface, SNIS153D, July 2009, revised October 2015; chip maker)