Premium motherboards (Asus Crosshair Hero VII) have T_in 2pin header for 10k thermistor temperature monitoring. Probing around reveals internal voltage divider with ~12KOhm rTop resistor (as per attached picture).
Goals for sensor read @ T_in: 25C (aka 10K) when GPIO is off or disconnected. 60C+ (aka ~2K) when GPIO is high.
Rough calculations, voltage drop T_in-1 to T_in-2 be ~1V for ~2k rBottom equivalent resistance.
I thought I could use NPN transistor as a simple switch, plus some resistors to act like a programmable parallel resistor network (10k at low, 2k at high). Really its much more complicated.
I don't have exact datasheet, only markings on transistor C945, scavenged parts bin.
I have a background in software engineering but its been a while for transistor logic. I built attached circuit and IT WORKS! 1.08V vCollector to Gnd. Asus reports 68C at GPIO high, and 25C at GPIO low. 10Kohm resistors were chosen mostly because they were convenient in my tickle trunk.
- I fudged around with the resistors until I got what I wanted. How would I go about calculating the values?
- What if I wanted 60C(~2500ohm) instead of 68C ?
- Am I over complicating this / easier circuit to build ?