Thermopile temperature sensing
Notes from attempting to make a useful instrument from a thermopile
Description of a thermopile
A thermopile is a silicon-based sensor that induces a voltage when there is a temperature difference between a base-layer and a layer exposed to radiant energy. A thermopile for terrestrial uses has an infrared-pass filter to make the sensor sensitive to radiation from terrestrial bodies, roughly -50°C to 500°C.
The voltage output from a typical thermopile is a few millivolts.
Creating a system to read values from a thermopile
Since the thermopile outputs just a few millivolts while micro-controller systems like Arduino, PIC, and others can digitize voltages of up to 1 to 5 volts into 8 to 12 bits, one needs to amplify the thermopile output to digitize it meaningfully. One can use a moderately inexpensive operational amplifier, or a more expensive instrument amplifier. The latter requires less external parts like resistors and capacitors, and more readily provides linear gain.
A thermopile is often packaged with thermistor, so that the difference from the former can be added to the temperature given by the latter to give an absolute temperature reading. This addition can happen in the operational amplifier or in the micro-controller.