We are well past the Total 5 guesses, but I’ll answer this one because it matches MOST of the clues.
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it could be done with off the shelf products and simply encapsulated.
A better one would likely take some development, which is why we probably don’t have one yet. -
People have put in an LF and HF xLED for this purpose, and a 2in1 LF and HF xLED has been discussed and asked for before, but is not yet a thing, but as far as I am aware, a HF/LF field detector has not been done.
In my opinion, encapsulating a LF and HF xLED to make a single implant, is over complicating a straight forward side by side or end on end install.
A more viable option is for DT to make the 2in1, or design a flex version, but LF Flex make things complicated / large. -
I am not the only one who has considered it. It has been mentioned a few times.
HOWEVER
It HAS been mentioned before, but not done, due to a lack of confidence of it working.
This is where your guess falls over.
It looks like nobody managed to actually guess, but it was a fun game, and has maybe, given some people some ideas to try out.
If you think your idea, or somebody elses is worth discussing further, maybe make a post over here.