Wednesday, July 20, 2016

Project #1: Thermometer based on thermocouple and STM32

Why thermometer? Because I need one.

Mainly for developing printed circuit boards at home.

Thus, I need wide temperature range, lets say from 0 to 200 degrees Celsius is a must. Wider temperature range is welcome. Precision is also a good thing, but I can't estimate the acceptable error tolerance. Hope will figure out this later.

Two more requirements come to my mind at this point. The device must be as cheap as possible, so no expensive parts. The device must be mobile, but I don't want to spend too much on batteries. Will try to implement it with small rechargeable accumulator.

As far as I know there are three primary types of electronic devices for measuring temperature:
  1. Thermistors
  2. Thermocouples
  3. IC Temperature sensors a-la LM35
The latter seems to be a good option for beginners. These are ready-to-use devices with easy interface for microprocessors. A few things stop me from using them: cheap ones seem to have low upper temperature limit (LM35 is up to 150C according to the datasheet), they are not that precise (0.5C tolerance at 25C). 

Thermistors seem to have similar problem. Cheap ones seem to have upper temperature limit up to 200C-300C. Another problem with thermistors is that they need voltage to be applied to measure resistance. Since I want to measure a temperature in toaster oven (for heating up the boards), I need a temperature sensor external to the body. Longer wires mean higher voltage need to be applied. Increased power consumption is not good for the device mobility.

I am going to use a thermocouple from ebay. It is not cheap compared to thermistors, but has very wide temperature range which is a big plus.

Based on the Wikipedia article about thermocouples I need a thermistor to measure cold junction temperature. Implementing a simple devices using thermistor is probably a good start.

Finally I am going to take STM32F030F4P6 chip to control everything. It seems to have enough pins to control everything I need plus simple HD44780 display. I have bought a couple on ebay.

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