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.

Digital thermometers

I've been sick and didn't have much strength to think or write. But today I saw some of my chilli plants have sprouted (yeah, I also grow chillis) and I noticed that the temperature in the room was over 33 °C when there was sun shining.
And it got me thinking that every time that I want to do a temperature measurement, I have to look at the tiny LCD display of the digital thermometer, write down the temperature on a piece of paper, and then look at my watch and write down the time the temperature was taken. That's because I only have the cheapest thermometers available, those for $2 off ebay.

a cheap digital thermometer
This is so cheap, you should buy hundreds!

So I decided to do something about it. I want to modify one of these thermometers so I can connect it to my computer and record temperatures automatically. I am not sure how I'm going to do it yet. The most simple option would be to just connect it to the Line-In audio jack and then record the raw voltage input. A more advanced option would of course be the USB connector, but this will most likely require some kind of a driver.