I'm late to the party, but how about just using an ATTiny85 uP?
It's a microcontroller, so I'd have to flash it which (for me) makes it way more complex... Plus I'd have to start from scratch again (which I already did twice before posting here). This fan control circuit is not the main feature of this PCB (I'm doing an
ATX mod to a Macintosh Quadra 950), and spending this much time just on the fan controller itself is already pretty inefficient, but at least I learned something along the way.
I initially created a dumb controller with just a potentiometer, then realized that this limited me to 2W fans, which led me to design a (flawed) comparator circuit to compare the original ATX PSU fan signal to an Analog-converted PWM lower threshold fan signal (set using that potentiometer). I then realized that it would be impossible to compare (and convert to analog for the comparison) the original ATX PSU fan signal (if it's PWM controlled, the frequency isn't standardized, unlike ATX "motherboard" fan signals which typically work at 20-25kHz). This led me to design a brand new temperature controlled fan PWM controller (the one we're talking about in this thread) which basically "doubles" the level of security (compared to my very first design, see above). The ATX PSU should still have a working overheating protection (I won't remove its thermistor) but because of this mod, it looses its fan control... so I needed to add it back in. And the LED circuit is only meant to warn the user of a problem... It should never light up...
After this small interlude/explanation, let's go back to the matter at hand:I found this website which IMO explains really well how to design Schmitt trigger circuits (it's in German. I live there so I understand it no problem, YMMV)
http://www.krucker.ch/DiverseDok/Schmitt%20Trigger.pdfExample/Beispiel 3 is exactly what I've been trying to design. Using Excel, I was able to create a basic yet effective calculator.
NB: Usatp/tn are the high/low output voltages, Utp/tn are the upper/lower thresholds.In my case, I'll just change R2 to 1M (instead of 100k), Utp= 10.32V and Utn= 10.07V (I still need to recheck those values with the NTC of my choosing.) I left the value of the pull up resistor at 10k as it just needs to be of a high enough value...
Here is the new schematic (I also changed the position of the LED, from what I've read, it should be after the PNP transistor, right?):
NB: The blue Resistor text corresponds to the resistor ID on the tutorial schematic (first picture). What do you think? Will this work now?