Diodes can be used for temperature sensing. Nick Chernyy used this principle to make a Diode based Fan Controller for his Shuttle PC.
“Figuring that I should just go build a more substantial fan controller myself, I promptly went to the lab to look for a thermistor. Unable to find one, I settled for the next best thing: a diode. After building the circuit, I tested the output with a DMM and applied various amounts of heat using a hot air gun. For the final install, I simply stuck the diode inside the CPUs heatsink/heatpipe assembly, connected the device to power and the output to the fan.”
Diode based Fan Controller
