i need sensor for counting rotation of dc motor. actually i am doing dc motor rotating with provided rotation only.
means it should rotate only predefine number only. and going to implement with PWM. but confuse of sensor.

 

Do you need a simple RPM detector or position feedback?
There are two ways for a RPM or tach, and that is a a DC tachometer or small DC motor driven off the shaft, the digital version can be in the form of a simple optical slot detector or a small Honeywell type proximity sensor using a small magnet.
If you require precise position feedback then you need something in the form of a quadrature encoder or equivalent.
Max.

 

yes i need RPM but not precise.which kinda encoder you are talking about.

 

If you need RPM monitoring without concern for direction or position one of the Opto slot sensors is often used such as H22A1 for example, there are many others.
A magnetic method is using a Honeywell SS44xx series of hall effect devices.
If you require a count up/down and/or position, then you either need a dual sensor or a Incremental Quadrature encoder for example.
Max.

 

IF i use H22A1 then what is external circuit needed for reducing noise.

 

If a stable DC is supplied then what noise are you concerned with?
If the supply cable is of any significant length, you could always place a small electrolytic at the sensor.
Max.