Reading soldering iron thermistor

i have taken a 24vdc soldering iron apart and found only 4 wires. 2 for the thermistor "thats the symbol they used" and 2 for the heating element.

when i read the thermistor at room temp i read about 4 ohm. as it heats up the resistance increases and decreases with temp. i have confirmed this on my multi meter between 4-100 ohm.

i built a arduino multimeter using a 200ohm resistor as the r1 in this project

i can successfully read a 200ohm resistor and shorting reads about 0.15ohm. however when i try to read the thermistor of the iron on or off if just read 1.5 -1.9 ohm. however at the same time i can watch the resistance increases on my multimeter but the arduino is not reading it. im using 5v as ref.

Why cant i read the thermistor?

9 posts were merged into an existing topic: Reading soldering iron temp