Electronic Thermometer Circuit

Posted on Feb 6, 2014

There are plenty of digital thermometers with ·1 C displays but the accuracy is about + 1 C and you can not calibrate them. I made this thermometer circuit from components that were available at the local electronics hobby shop and it was an educational experience. If you want a simple modern circuit that requires no calibrating I recommend

Electronic Thermometer Circuit
Click here to download the full size of the above Circuit.

you look at a circuit that uses the LM34 or LM35. In the circuit above T2 is a fixed current device that sinks approximately 70 Micro amps. P1 is adjusted so that this current does not vary with temperature by balancing the negative temperature coefficient of T1 with the positive coefficient of T2. R3 reduces the effect that adjusting P1 has on the set current. T4 provides a current proportional to the absolute temperature and is adjusted to equal the fixed current through T2 at 0 C. Thus at any temperature other than 0C a current must flow into or out of the voltage divider formed by R7 and R6 and provide a voltage for the meter as it flows through P3 and R5. P3 is adjusted so that at some standard temperature the 200 milli Volt meter reads correctly. Simply put T4 is a Kelvin thermometer and T2 subtracts 273. 15 to convert it to a Centigrade one. The LM334z (T4 - T2) is a readily available device that provides a current source that is directly proportional to the absolute temperature ( K ) over the range 0 to 70 °C for larger ranges there are other devices in the same family that can be substituted. Great care was taken to ensure that temperature changes to the body of the instrument do not cause changes to the readings. T2 has changes nulled by P1 but the resistors themselves change value with temperature and this is also canceled out as follows. Any change to the resistance of R1, P2 and P1, R2, R4, R3 with the...

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