Connecting LM75A to Raspberry PI

How to connect popular LM75A Temperature sensor to Raspberry PI using I2C.

Tested HW/SW:

• Raspberry PI version (from https://raspberry-projects.com/pi/pi-hardware/raspberry-pi-pcb-versions):

  • B Model B Revision 2.0 (512MB)
  • PCB Says: Raspberry Pi (c)2011.12

• OS: Raspbian GNU/Linux 9.9 (stretch) from http://director.downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2019-04-09/2019-04-08-raspbian-stretch-lite.zip

• CJMCU-75 board with I2C LM75A Digital Temperature Sensor. Please see https://github.com/hpaluch/i2c-cjmcu-75 for instruction on this board setup (you need to wire address pins A0,A1,A2 to VCC or GND to set LM75A I2C device address.

Setup

Connect your Raspberry PI to CJMCU-75 board using this layout

           Rasp. Pi
             1 2
CJMCU-75    +---+       CJMCU-75
NC  --  3.3V|o o|5V  -- VCC
SDA --  SDA |o o|NC
SCL --  SCL |o o|GND -- GND
             ...
            other pins unused

Install these requirements (see https://www.instructables.com/id/Raspberry-Pi-I2C-Python/ for details):

sudo apt-get install i2c-tools python-smbus

Should not be needed: Add your non-privileged user (typically pi) to i2c group to have access to /dev/i2c-1 device:

usermod -G i2c -a pi

If you are already logged as pi issue:

newgrp

To ensure that you are now member of i2c group.

Now you can try to detect LM75A I2C Device:

 i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Looks OK - device found at address 0x48

Now you can run example python program:

./test-lm75a.py 1 0x48

Expecting LM75 to have I2C slave address 0x48 on I2C BUS 1
Current temperature is 26.0 Celsius
Hysteresis temperature is 75.0 Celsius
Shutdown temperature is 80.0 Celsius

Here is I2C communication in Sigrok PulseView:

Getting current temperature (Pointer Register = 0x00):

NOTE 0x001a = 26 (degrees of Celsius)

Getting hysteresis temperature (Pointer Register = 0x02):

NOTE: 0x004b = 75 (degrees of Celsius)

Bugs

N/A