Cover sensor from above suitable for arduino?

Question

Arduino

Hello.
there is a wooden surface the size of an a4 sheet, a sensor must be built into this surface that returns information whether there is something on top or not. A sheet of paper or a pack is placed on top, you just need to determine that there is at least something. The room may be dark.
What sensor can be used for this?

Answer 1

An ultrasonic distance sensor can be used.

Answer 2

drill 2 holes at a distance of, say, a floor of 1 cm from each other, from above, closer to the side where a sheet of paper is placed between the holes, remove some wood, say 2-3 mm, insert an LED into the first, a photodiode or photoresistor into the second, when you lay down the sheet of light from the LED is reflected from the sheet and enters the photodiode, respectively, you decree a change in the level.

Answer 3

IR distance sensor is possible. As mentioned above, you can use an ultrasound sensor, but it may not respond to a sheet of paper.