Help finding sensor

I'm looking for some kind of sensor that can detect when a certain object passes a distinct point/plane. Now the thing that makes this complicated, is that many different objects will be passing this point/line, not just that certain object. Now I'm trying to look for a sensor that would detect that only object crossing a plain that is about 3ft wide and ranges about 1-5ft high, and i really need whatever goes on the object to be small (and hopefully not need a power source, that can possibly be taped or worn)

I first was thingking an IR sensor or obstruction sensor (kind of like when you walk into a store a little bell rings), but the problem there is that there will be plently of things passing the plane. (basicly back to the store scenario, would need it to detect certain people walking in, not all)

Any suggestions??

I just though about it now, would some sort of photo sensor be ok that senses when a certain color passes? I don't know if there is such thing...

A webcam and a program written in Processing should be able to detect a unique color. You might need to control the lighting.

There was a recent proof-of-concept for acquiring color images from a video camera with an Arduino. Perhaps that technology could be used to look for a particular color.

I would just use a ping sensor? OR am I oversimplifying it?

Yes a ping sensor is over simplifying it because to my knowledge it would read everything that passes through and not only certain objects. (there wouldn't be like difference in size or anything in objects passing that field)

How would I go about using a webcam though?

You have to define the characteristic of the objects that you need to discriminate. That, you haven't told us. "certain" is a tad vague.

Well right now it's nothing in particular. Im kind of aiming on to figure out just how to put a sensor, tag, or I guess even a colored cloth on anything to know if it crosses a 2D plane.

How would I go about using a webcam though?

I would look at the language called 'Processing' it is free and there is webcam support. It runs on any computer OS. Using a web cam is not something you can do with an arduino.

m kind of aiming on to figure out just how to put a sensor, tag, or I guess even a colored cloth on anything

Sounds like you haven't thought through this project. It is fundamental that you know what you want to do. This is not as easy as you think and what ever you devise is not going to be 100% reliable. Remember this is not the movies it is real life and things are seldom neat and tidy.

Would an RFID chip work?

No the range of RFID chips is very small. True there are some UHF chips with a large range but the readers are very very expensive.

Sounds like you haven't thought through this project. It is fundamental that you know what you want to do. This is not as easy as you think and what ever you devise is not going to be 100% reliable. Remember this is not the movies it is real life and things are seldom neat and tidy.

I know what I'm wanting to do, and trust me, I also do understand the reliability, I just can't think of anything that would be able to sense objects crossing a 2D plane without reading everything passing though. (and also not spending quite a bit like I would on UHF chips)

If it still sounds confusing back to that metaphor I gave earlier, on the store. A lot of stores have obstruction sensors that ring everytime someone walks in, in my case it's like I would only need it to ring when men walked in, or certain ethnicity, or age of people walked in. (by placing something on them that a sensor could read)

It is one thing knowing what you want to do and another thing knowing how to do it. The classic example is platting saw dust, quite simply you know what to do but not how to achieve it. It is all very well saying you need to detect something on an individual but what something. As I see it your options are:- 1) Something reflective (IR or light or ultrasonic) 2) Some sort of image processing 3) Some sort of inductive sensing (RFID and the like)

However, what I am saying is that this is very very hard to do and not a cheap off the shelf sort of thing. The problems with visual tags is that they can be obscured or disorientated. The problems with image processing is that what is easy for us is difficult for a computer. The problem with inductive sensing is it tends to be expensive.

Something like a shop security tag might be an option. This is not RFID as you are just sensing the presence of a tag and not reading any information from it.