Detecting Falling Pieces of Paper

Hi all,

I've got a problem I want to solve on my bug list and my searches haven't brought up any answers. I've been using a Parallax pir (http://www.parallax.com/product/555-28027) to detect motion. They are very easy to use and very reliable so I can recommend them.

One thing I can't detect though is falling sheets of paper. One application I've been asked about is detecting when children drop wishes written on paper into a wishing well. The pir isn't sensitive enough to pick this up so I'm wondering if anyone has any ideas? Ideally I don't want to add anything to the paper to detect motion so using RFID etc won't really work, want I really want is an ultra sensitive motion sensor…

Thanks for any advice!

Perhaps ultra sonic sensor might work, but it might not. http://arduino.cc/en/Tutorial/Ping Using laser beams perhaps ? Is the whishing well dark below ? you can use a number of IR leds, and detect the reflectance or the interruption of IR. After a second thought, I think the IR is most likely to work.

You don't want to disappoint children, perhaps you can combine sensors. Or add a sign for the children that sometimes a larger paper is needed. Will rain have influence ? or is it indoors ?

Thanks for that. I did think about interrupting IR sensor signals but then I thought I would need too many to cover the entire hole; there would be dead spots between beams. Maybe this would be the same with sonar. But then, maybe I can create some kind of funnel so that pieces of paper have to pass between a sensor. This will be indoors but it would be good to think about other situations too...