PM12-04N sensor use with arduino

Hi all,

I am quite beginner on that and I would appreciate your help on my project.

I have this sensor http://www.photosensor.com.tw/Proximity%20Sensor/PM12-04N.htm

and I want use it to measure rpm of a metallic wheel. Basically the sensor has 3 wires: +/- and signal.

I am using the sensor powered on PC power source at 12V. If in front of sensor is the wheel then i have 12V on signal wire. If in front of it is nothing metallic (i have a whole in the wheel) then the signal pin is 0V.

My questions are:

1 - How can i get into arduino the sensor signal which is 0 or 12V (I know arduino support max 5V)

2 - How can i measure frequency (rpm) in this case ? I saw some examples which measure frequency but are exactly opposite than my setup (they have high instead low on my case)

Thank you very much in advance

If is needed i can put some pictures

Use a voltage divider (two resistore) to tune outputvoltage to range 3.5 .. 5V Connect signal to pin 2 (or 3) on the UNO. Attach interrupt 0 (or 1) to detect each pulse.

There is examples.. several .. in this forum