At this point I'm not going to design for under hood temperatures. The Arduino boards just cant take it. Possibly in the future I may create a design without the Arduino. Maybe I will just use the main chip and fully integrate everything onto a PCB board.
For some back ground and some videos on the application you can visit this thread.
For a discussion on mounting the Nano33 you can visit this thread.
At this point I have paired it down to three possible circuits that will do the job.
Circuite #1 Drive an PC817 opticoupler directly from the arduino. The opticoupler is a LED that shines on to a photo transistor which completes an independent circuit grounding the fan wire. The beauty of this is that the fan circuit is not in contact with the Arduino circuit. Electricity cannot get from the fan circuitry to the Arduino circuitry. There is a physical air gap between the two. The down side of this circuit, is the power required to drive the opti is about 11ma. This is within the 15ma current limits of the Arduino Nano33BLE which is the version I will probably use. But I don't know how that 15ma was derived as no environmental specifications or Mean Time Between Failure (MTBF) data is presented in the data sheet. 15 ma might be good for someone with a drone to peek in the neighbors window and bring the drone home before the battery runs out, but what about taking a 15 hour road trip. Better find something to reduce that power. Other not so good thing is the opti has a fairly low MTBF of 1000hr. That's rated at 50ma 100% duty cycle. Which in the real world driving at 11ma and 50% duty cycle and 2 hours a day 5 days a week equates to somewhere over 5 years. The way I dive this car 30 years.
Circuit 2 Drive fan using two transistors. The beauty of this is, the ultra low power requirement of the PWM IO pin ~1/2ma and the failure rate is so low on these I cant even find the MTBF. The bad is the fan and Arduino circuit can see each other.
Circuite 3 Drive the fan using 1 transistor and an opticoupler. Good news is the PWM IO pin of the Arduino is down to an ultra low 1/2 ma. Bad news is the MTBF is still relatively low. The one thing about the opticoupler is it is a 4 pin component that can be fit into an IC socket and be made replaceable. It can be carried around as a spare along with fuses.
So what Am I going to go with. I'm leaning to circuit 3
In any case I have been playing around with PCB board layouts, so far based on circuit 3
I found this sight that makes PCB boards. Pretty amazing that for $5 I can get 10 boards made. Needless to say I wont be making my own.
I still have a long way to go on this but it is coming together. First thing I need to do is verify all this stuff works at 3.3V. All testing has been done on 5V to date but I believe its just a matter of changing resistors.
I also still need to provide mounting holes in the board to mount it to a case.