Anemometer & Speedometer sensor

Hi guys, i want to build a LOW COST anemometer.

I have dismantled an old Hard Disk, and i've made 3 holes on the side of the central axis of the drive (sorry for my english!), to screw the 3 lives.
The cups are from olds plastic eggs from Egg's day.

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I've spent €0.60 until now, to buy the lives.

Now the problem: I have a cyclocomputer (5 yo) and doesn't have the sensor.
I hope the sensor works with the hall effect, it has 2 contacts on the rear side, and they emit a small tension;

i don't know how to build a sensor or how i can procurate it (in an elecronic shop, i've said: give me a FET Transistor and they did not know what it was...)

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Please give me an hand!

Update: making the two pins connected at regular intervals, the computer "detect" the speed

How i can interrupt the tension with a magnet?

You place a reed relay across the pins. Let the spinner turn a magnet close enough to the reed relay so the contacts close as the magnet passes. You'll probably need a weight to balance the magnet on the shaft.