Can you use a 2020 3.3V DC Fan as direct input to an analog PIN?
My thought is that when wind blows through the fan it will generate some voltage that can be read by the analog input.
I understand there is a dedicated anemometer. I do not need precision, just need to detect whether there is wind around 30mph or higher.
Thanks.
It is not clear what type of fan you have...
Is it brushless?
If it has brushes you could add some resistors, a capacitor and a zener and a diode to make it a safe input...
My guess is that without load resistor, you will not get useful readings.
Without protection resistors and a zener, you may easily destroy your analog input. A diode might be a good idea to protect from reverse voltage (due to EMI from the motor or wind from behind).
Maybe a diode bridge to deal with wind from either direction.
I wonder if you could have 2 fans at right angles and calculate the wind direction also.... 
I’ve been working on a solution to the same problem, except my 3V DC brushed motor isn’t part of a fan. This is for an ESP32 ADC with voltage range 0 to 3.3V. There is additional protection following VOUT (diode clamps to 3.3V and 0V).
I’m in the middle of verifying that the above circuit works as per the theoretical spread sheet calculations (using superposition).
I previously used a bridge rectifier (4 x BAT85 to minimize Vf drops), but it requires about 0.25V from the motor before the ADC sees any voltage. Maybe not a problem for the OP, but I wanted to eliminate that dead spot.
No.
Yes you can, probably the easiest connection would be the Tachometer output (yellow wire), it will give you pulses in direct relationship to its RPM. Test the fan first without any connections and see if it is free enough to spin with the wind flow you want Many fans will not start but continue to coast when removing power they will slow down but keep spinning in a strong enough wind stream.
Thanks all for replies.
Yes, No, Maybe tells me it's not as simple as I thought. I learned something.
Reading the other threads on similar topic, somebody suggested a vane with a switch --which could also work- led me to a vane with a small magnet and a hall sensor. Again, only to detect the presence of wind with no regards to its speed.
Cheers.
Then another option is a hot-wire anemometer.
Leo..
For cost and form-factor reason Anemometer was never an option for this project. FYI
An old analog wind detection system is the wind chime.
instead of cotton wires use thin copper wires and you have a wind detection system.
Connect the middle clapper to GND and the chimes are connected (with a pull up) to a digital in (interrupt) pin of the Arduino. If there is wind the clapper will touch the chimes and a pulse is send to the Arduino. The frequency of the pulses indicates a relative force.
If you have 8 chimes, you could connect them individually to a separate pin (or a PCF8574) and get some information about the wind direction too.
Think it can be build with not much costs as simple metal wind-chimes are less than $10.
Final advantage is that if there is power loss, you still can hear there is wind 
