This project is not as a permanent solution for a digital thermometer, but a quick to build one just in case you cant find your dedicated one or if it stops working. It quickly displays a rough estimate, but precision values can also be obtained if you do a little math! Whats more, it uses just 2 components and costs around $3 overall!! The only drawback is that you have to use a table to find your temperature, but as I said, this is not a permanent and dedicated thermometer.
Step 1: Components Required
A Thermistor - NTC 10K - Its an electronic component, basically a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. 10k indicates that the resistance of the thermistor would be 10k ohms at 25 degrees. You could find it online or get it from some place like radio-shack. This thing costs less than 25 cents in India.
Some wire (optional) - This is only if you need to measure the temperature of something more flexibly. You could use any wire which is not very thin, as they have a greater resistance and will affect the reading a little bit. Something which is as thick as the legs of the NTC or thicker is just fine. If you need just a feet or so of wire, you could use thinner wires, but this wont be any problem here for now. Just make sure that you connect it such that the meter gets a good contact with the NTC.
Step 2: Measuring
The table shown will tell you the temperature from the resistance reading on the multimeter. Find the range between which your resistance ends up and check the corresponding temperature, as simple as that!!!
You can measure room temperature, body temperature or anything u like, just make sure you don't touch the metal pins of the NTC or let it come in contact with any liquids which will ruin your readings.
If you need it to measure the temperature of something which is flat, say a metal tumbler, you need to get the NTC to have a better contact with the metal. For doing so, don't grind or remove the plastic from the NTC as the pins are pretty close to the outer regions internally, instead find a small metal piece, preferably a flat heatsink, put a small dab of thermal compound on one side of the NTC, place it on the heatsink, and then glue the edges so that it remains in place. Dont let the glue come in between the NTC and the heatsink.
Step 3: Precision Temperature Calculation (optional)
R = Ro exp( Beta/T - Beta/To)
R - Thermistor resistance at T (K) = Reading from Multimeter
T - Thermistor temperature (K) = To be found, substract 273 from result.
Ro - Nominal resistance at To (K) = 10000
To - Temperature where Ro is measured = 25+273 = 298
Beta - Thermistor material constant = ~3960 for NTC 10K
However this method involves solving an exponential function which is not easy for most of us, so a less accurate second method for approximation by me has been given below.
T= T1 + (Rlow-R)/(Rlow-Rhigh)
T is the temperature reading we need
T1 is the min temperature reading corresponding to the range we find when looking at the table, for eg, for 9560 ohms, T1 is 26 degrees.
Rlow is the lower end of the resistance range where we find our resistance R in. It will be a value higher than R
R is the reading of our multimeter
Rhigh is the higher end of the resistance range where we find our resistance R in. It will be a value higher than R
example we get a reading 9.56 on the multimeter on the 20K range
so 9.56x1000= 9560 ohms
9560 comes between 26 and 27 degrees on the table, so T1 = 26 degrees
Rlow among that range is 9612 corresponding to 26 degrees which is higher than our 9560
R is 9560 itself
Rhigh is 9224 which is the resistance for 27 degrees and the higher end of our range
so T= 26+ (9612-9560)/(9612-9224)
T= 26.134 degrees !!