I am trying to find an optic reflective sensor to use as a tachometer. I would like to count the number of revolutions of a 4 bladed plastic fan. Currently, I am using a sensor that detects anything that comes within a certain distance but I need a sensor that will only react to a certain object on a fan blade.
What do you mean by a "certain object" on a fan blade? Do you mean something to differentiate one blade from the other three, such as a piece of tape? Unless you can significantly change the reflective characteristics of the one blade compared to the others (and even if you do) it will be very difficult to reliably count the one and not some of the others. If you are trying to count revolutions, it would be far simpler to just count each blade and divide your count by four. You can accomplish that with a microcontroller or a simple counter IC such as the CD4018.
Is tehre a reflector sticker that you have stuck on the fan blade from whebre you need a reflection to e detected as an object.
Also you need the reading directly in RPM? and also what is the meximuAm speed of the FAN. Also the FN Blades are tilted or straight. Many other factors may be of importance while deciding a sensor for this kind of sensing. Deepak, INDIA