Saturday, February 19, 2011

Digital thermometer modification: halfway through


Expect loads of pictures!

After I got that idea about modifying a digital thermometer I couldn't stop thinking about it. So I decided to do it right away. First I had to disassemble the thermometer. The procedure is simple and easy, only tool needed is a small flat screwdriver to press some pins and the case opens. The insides can be seen in the picture below.

Digital thermometer parts
1 - two LR44 button batteries; 2 - LCD screen; 3 - case, upper half; 4 - plastic spacer between the LCD screen and the circuit board;
5 - circuit board; 6 - case, bottom half

Next I had to find the correct pair of connections that I could use to determine the temperature. After some testing I found out that I have to connect the + source to the connection marked on the picture below.

the circuit board closeup

Once I soldered all the wires and a female 3.5 jack connector I reassembled everything and the physical work was done. Results:

wiring completed

multimeter measures 1.42 V at 30.8 °C
Multimeter reading is in Volts.
After some experimenting I found out that when temperature increases, voltage drops.

As you can see on the multimeter, I can measure the voltage without any problems. The maximum voltage would be 1.50 V so there is no harm to the computer.
The problem occours when I actually plug the jack into the computer Line-In. Suddenly the voltage drops to about 5 mV and there is no way to increase it back to the previous normal level. Voltage measurement of the computer's Line-In input:

oscilloscope measurement of the Line-In voltage reads 6 mV
No explanation for this yet


If you have any suggestions what might be causing the voltage drop and how to fix it, please let me know.

6 comments:

  1. amazing! i wish i had enough time on my hands to do this!

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  2. i'm not that good in this stuff, but i'm sure u will figure it out!

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  3. Wow that looks really cool. I hope it works out.

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  4. That looks cool! Following so I can keep up with all the nifty science stuff. Might give me ideas on what kind of little projects to do with my kids

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  5. My cousin recommended this blog and she was totally right keep up the fantastic work!


    Coffee Thermometer

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