[phyrrus9]

So, my neighbor gave me some 2 speed fan that came out of god knows what 1990s car, and I decided its time I get rid of that POS mechanical one.



As you can see, its sort of just frankensteined in there, but it works.

Well, I was thinking, why would I run this thing on switches when I can just make myself a temperature sensing circuit and tune it to the temp I want it to go full blast on? What I came up with is this:



Don't worry, I can't read my hand writing either. Basically I'm using an NTC that I'll tape to the lower rad hose to drive the voltage on a p-FET low and control a SPDT relay with it. I'll wire the source for the entire circuit to a SPST relay from the car's ignition. If everything goes well, the low speed will kick on when I start the car, and then the high speed will kick on somewhere between 30C and 45C. I used a 20k RV so that I could fine tune it to what works best. The voltage graph out of the first stage (into the FET) looks like this, if you're interested.



So, any thoughts?