I want to build a mailbox - but for packages - with notification alert. What I already have is a large plastic box with top cover and a sensor (tilt switch) attached to the lid telling my Arduino if the lid is open or closed.
But I need to also have some kind of sensor inside the box which is really checking for the existence of a package. My problem with IR or Ultrasonic sensors is, that the package inside the box can be really small and tiny so the ultrasonic or IR beam is not interrupted and the sensor will not recognize the package. So I'm thinking about a pressure sensor - a very (!) sensitive pressure sensor, because the packages may not only be small, it also can have only small weight.
I already tried the method with two cardboard layers and aluminum foil between. But this is not a reliable solution for me since sometimes the cardboard just stays connected with the other cardboard side and so on... not really a professional way to detect if there is a package...
Do you have another idea how to detect, if there is a package (small and lightweight) inside of the box - more reliably?
A even more perfect solution would even tell me the weight of box content (analog value)... This would help me if someone was putting packages inside the box or if packages were removed.
So what is the operating environment of this thing? Long term? Outdoor/indoor? School project? Do you need to know "how many" things are in/out, or just that something went in/out?
If this was just a short term project, I'd consider something like a gas pressure sensor connected to an inflated tire inner tube with the box sitting on that. That's also assuming you're really just looking for "something in or out" kind of deltas, and you need to remember your environment will affect your readings (with temperature being a huge potential delta), so you'll want to compensate for that in software (unless this thing is in a pretty strictly controlled environment, anyway).
If you need longer term and/or outdoor capability, you might consider having the box sit on a shelf mounted on a metal "lever" with an embedded strain gauge.
I remember somebody (Ben Heck?) posted a YouTube video showing a very similar project. Worth a look through his video archive, because it looked like a well designed solution and I think he published the hardware and software designs for it.