Advice for the type of sensor to use

I want to make a puzzle where three items must be picked up in the correct order for a lock to unlock. I want to use various types of items ,for instance a letter, a key, and a pendant. I am hoping to hide the three sensors below a thin layer of wood so not to be noticed by the user.
My question is does anyone know a type of sensor that could accomplish this? I was thinking RFID would work but would require a tag and I would like to be able to use items without tags.

Maybe a photo transistor.

Could you attach a tiny rare earth magnet to each item to be detected with Hall effect switches?

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Yes, I thought of that but the magnet would be noticeable on something like a letter or piece of paper. I could make three small metal detector coils too. But once again it would work with paper.

I was thinking that also or maybe a photo diode. The only problem I see is that I would have to have a small hole in the wood above I've sensor. Hoping to stay away from that but might have to go that route.

Or simple reed switches.

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If you want to be tricky and your items can be of metal or partly of metal you could:

  1. put a coil under each part of the puzzle
  2. Ping each coil with a pulse of current (Mosfet)
  3. Read the frequency (will change when the metal is removed.

Personally I would go with a small Neo magnet and a reed switch under the layer of wood. Simple and functional.

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