Switch to Thermistors

(Backdated: Nov 29, 2016)

Today I finally bailed on thermocouples. I couldn’t for the life of me figure out a good way to measure the tiny voltage they produce, even with an instrumentation amp with buffered inputs and a high gain stage. In light of that, I switched to the tiny glass-bead NTC thermistors I ordered before break. I put them together in a Wheatstone bridge of 25K resistors with an instrumentation amplifier (gain of 300) across the middle. Upon powering the circuit, I found that even touching the thermistor caused it to rail low, meaning I had the inputs inverting and that my gain was way too high — a welcome relief from the thermocouple debacle! From there I tried just connecting an oscilloscope across the wheatstone bridge before the amplifier, and I found that the circuit would produce a voltage of around 1V when I applied a heat gun to the thermistor, meaning that I could probably get away with just using an Arduino’s onboard ADC to measure it.

I also looked into chip ADCs to hook up to a Raspberry Pi, but it looks like that’s really not worthwhile. I’ll end up using an Arduino for control, and a Raspberry Pi for the interface. They’ll communicate over a fixed-word-length serial connection for simplicity. Now it’s time to make some progress!