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.
we are looking at 100 paper per process in a container. The sensor will detect each paper if wet.
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.
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.
Thanks guys! I'll try that. Hope it works
Choosing a humidity sensor mind the most important specifications. They are accuracy, repeatability, interchangeability, long-term stability, resistance to chemical and physical contaminants and cost effectiveness.