Signal Sensor Processing CPU

I need help designing a circuit to control a servo motor, the model i have is an SG 90 (9g) It needs to be controlled by a sensor that receives and input from a moving object, (the object moves down), and the sensor reads the signal 1 -> 2 the servo moves up 120deg, (the object moves up) and the sensor reads the signal 2->1 the servo moves down 120 degrees. I do not know what sensor I could use to provide this signal, i am guessing the signal 1 could be a positive pole of a magnet and signal 2 the negative pole, what sensors could i use? what cpu do i need? how can i adjust the amount the servo moves if need be? what language is best to learn to code this?

SG 90 9g

From the servo data:
To control the servo you need to output a signal "PWM". It can be done in software or using a timer in the computer.
The time the signal is low is not to important. The time the signal is high is important. It needs to be 1mS to 2mS. (1.5mS is half way)
1mS=+90, 1.5mS=0, 2mS=-90.

I think there is an Arduino library that knows how to control a model aircraft servo. If not, it wouldn't be difficult to write one...

Here are three videos that might help.
Arduino servo
2
3

Do you need sensors at all? Can it just be done with a simple timer in the Arduino?

Mike.