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[on:]

Hi everyone,

I'm on a thesis group working on a project regarding reverse paper vending machine. The idea is not new to most people, but that is the only proposal that was approved. The way it works is:

1.) There will be a container where the paper will be inserted. (Size is limited to only legal size 8.5 x 14)
2.) The machine will detect if the inserted item is a paper
3.) If yes, it will detect if the paper on top is wet. (Now here is our problem)

we are trying to look for a sensor that can detect if the paper is wet under a budget. We are looking at $60-80.

P.S.
we are looking at 100 paper per process in a container. The sensor will detect each paper if wet.

Thanks!

[Reply #1 on:]

humidity sensors like the DHT22 measure humidity of the air. That implies that the moist of the paper must first evaporate before it can be sensed.

I would go for a contact sensor and measure the resistance (the assumption is that wet paper has a lower R than dry paper).

A simple contact sensor could be two nails (or tea spoons) some wire and a resistor - effectively create a voltage divider.
cheap enough to give it a try.

[Reply #2 on:]

Measuring the resistance with two metal pins as robtillaart wrote is the most reliable.
I think this is sometimes used with a high frequency signal. A DC current relies on the contact between the metal pins and the paper, but with a higher frequency and larger metal plates, there is a lot of change with wet paper.

If you have all the same papers and printed with the same amount, a wet paper will transfer more light through the paper. But that is highly inaccurate.

[Reply #3 on:]

Thanks guys! I'll try that. Hope it works