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Thursday, March 26, 2009

I2C Temperature Sensor

The electronic circuit is Temperature Sensor with I2C program. It is a circuit that plugs into the link port of a TI-85, TI-83, or TI-92 calculator and displays the temperature on the screen. Currently, there is only software for the TI-85, but I plan to write some for the TI-92 also. The sensor circuit draws power from the link port, so there is no need for any external batteries. The overall size of the unit will depend on the size you make it. Mine is about .75" by .5".


  • Two small switching diodes. I think that just about any kind will work.
  • I used 1N914 ones.
  • One small electrolytic capacitor. I tried 2.2uF, 10uF, and 100uF, and all of them worked.
  • An LM75CIM-5 integrated circuit (IC) made by National Semiconductor. For a more detailed description, you can read the data sheets on it onNational's WEB page at and also check theirdistributors. It is a surface mount chip, so it is pretty hard to workwith.
  • A 2.5mm stereo plug and cord. You can buy one, but I just cut a calculator link in half.
  • A kit that allows you to etch your own boards. You can buy these at Radio Shack for about $15 and they can be used more than once. You canalso use another method, but I found this to be the cheapest for workingwith surface mount devices.
  • Construction Materials. Just general things like a soldering iron,solder, etc.


  1. Come up with some way of using the surface mount IC. I etched my own PCB for it.
  2. Solder all the parts for the circuit on the board or however you chose to make the circuit. Be sure to follow the schematic for this step. And make sure the red and white wires are connected correctly accordingto the schematic. The only part you don't have to follow is how youconnect the A0-A2 pins. You can find out what pins these are in the data sheets available on National's WEB page read the next couple sections of these plans. If you connect them differently, you will have to change the chip ID which is explained below.


The software I wrote to control the I2C Temperature Sensor is very simple to understand. Simply run the ZShell program and it will display the temperature on the screen in both Celcius and Farenheit. If an error message appears on the screen, this section will help you.

The program continuously updates the temperature about twice every second. To exit the program, simply press [EXIT]. There is also a feature that can be activated by pressing [F5]. This will allow you to change the chip ID that is set by the A0 to A2 pins on the IC. If this is set wrong, it will display an error on the screen. After pressing [F5], you can change the ID number by pressing [UP] and [DOWN]. Then, simply press [ENTER] and it will bring you back into the program. This also allows you to have up to 8 chips in the same circuit if you change around the circuit a little bit and give each one a separate chip ID set by the A0 to A2 pins. If changing the chip ID does not get rid of the error, make sure the plug is plugged into the calculator all of the way. If the error is still there, check over your construction of the circuit. If you have an error and change something external, either plugging in the cord or fixing something on the circuit, be sure to restart the program or alter the chip ID to restart the program.

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