You need to give us some idea of size and working range ...
measuring over 180 degrees of rotation might need a different approach to endless rotation.

Maybe you could consider a 'Gray Code' encoder wheel and a series of opto readers, depends also on resolution.

Originally Posted by gastonanthony

I'm trying to get the angular displacement of up to 180 degrees only, I'm connecting a small camera(2x2x2 inches) to the motor and need to know the angular displacement the camera has travelled

A couple of thoughts!

1) Use a servo, that way you know the absolute position at all times, you may need some mechanical gearing to give 180 degrees.

2) Use a stepper motor, with a limit switch at either one end or both, so you can identify a start position, then you can adjust it's relative position.

3) Another thought on the servo position, build you own servo (using the electronics from a servo if needed), with the pot mounted on the camera - a pot allows up to 270 degrees travel.

4) You could use a pot to simply give the position, but essentially you are recreating a servo.

Originally Posted by gastonanthony

I like the idea of creating my own servo because it's very expensive , it's price is about 30-40 dollars around here. do you know of a good tutorial explaining the block diagram and schematic of servos and how to build one?thanks

You might try looking at http://www.cpg1.freeserve.co.uk/servos/servos.htm, where they make a large servo! - mechanically a servo is just a geared motor feeding a potentiometer.

Originally Posted by gastonanthony

Am I right in thinking that the circuit in the website doesn't actually detect or know the exact anglular displacement of the motor because I can't see any feedback in the schematics presented in the website? :?:

No you're not right!, the feedback is the 5K pot labelled "feedback".

Originally Posted by gastonanthony

oh, ok, sorry I didn't see the pot because I'm not familiar with the m5166ol. I haven't used a chip like that before, do you know the approximate price of that chip?because here in my place, component prices are often three times more expensive than that available internationally because of import expenses

I've no idea where you would get one, it's one of the chips used in radio control servos, the easiest way would probably to buy a servo and strip it?.