Some problrms about Movement sensor

Submitted by kymyc on Tue, 02/19/2019 - 08:34

I have a project with a very specific usage in mind.
A coin falls through a hole that is just big enough for it, into a room with an acrylic glass wall on it.

I want to detect when the coin falls through. I'll be only one coin at a time, and there shouldn't be one following so closely that a sensor should have problems picking it out.

Since it lands in a "room" I'm not keen on using sensors that tend to be very broad, such as PIR. It may trip from someone walking around since there is a clear section, or from coin movement in the room after it drops through the hole. I could use some sort of cone or similar to limit its "sight" but it's not a very permanent solution

I've thought about a small switch, but I'm having trouble imagining how I could fit it. There's also a range of different weights to the coins and I'm afraid that it's possible the coin is light enough that it may not trip the switch.

Laser based detection would be optimal, but I've only found tutorials where you need to correctly position the laser diode to shine on a photoresistor. For this purpose that's too fiddly and some might see lasers as a risk when eyesight is concerned. (It's not that I personally wouldn't use lasers, but for what I am making it isn't optimal)

I'm thinking of photoelectric sensors ( in some form. Something like this would be perfect, but it costs a lot and uses proprietary interfaces to work normally. I'm also not confident in my knowledge of photoelectric sensors that this would work. I've been looking at this. But I'm not confident in how they work, or if it's the same case as the PIR, with a very broad detection range.

For size reference, this is not a gigantic machine. It's meant to sit on a desk and it's about 30x30x25 cm/12x12x10 in (last number being height)

Is there something I could try? I intend to use an arduino mega to power the other electronics of this project.