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Comment by geocrasher | Hacker News Reader

Indeed, in some cases all you need is an antenna, a semiconductor of almost any kind (which can be improvised) and an audio amp. In some cases, it's all-in-one. My family used to talk about a brass headboard that you could hear Morse code from, coming from the local military base.

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To receive AM you just need a non linearity and the x^2 term iirc will do your rectification. So any element that electrically provides some kind of nonlinear behavior (ie is not a pure R, L, or C) should give some sort of modulation.

(Sorry just rereading and I meant demodulation, autocorrect got it the first time)

Calling bullshit. Semiconductors cannot be "improvised". PN junctions aren't found anywhere accidentally.

And a brass headboard isn't a crystal radio and isn't a circuit. A crystal radio needs a diode, a capacitor, a coil, a ground, an antenna, and a quartz headphone stuck in your ear. These things don't configure themselves by accident. The reason analog headphones with a long wire can do this is they have almost all of the attributes necessary to pickup AM radio. FM radio requires a vastly more complicated circuit.

The stories about hearing radio through bad tooth fillings are all untrue?

I was wondering that too, since I heard/read about it several times as a kid.

This thread is actually pretty decent, although it's still mostly speculation:

https://www.reddit.com/r/rfelectronics/comments/oh4zpy/how_d...

My wife's mother claims to have actually experienced this many years ago: AM radio from a tooth filling when driving close to a broadcast tower on a family road trip.

   Calling bullshit. Semiconductors cannot be "improvised". PN junctions aren't found anywhere accidentally.

https://en.wikipedia.org/wiki/Foxhole_radio

   One common type was made from an oxidized razor blade (rusty or flamed) with a pencil lead pressed against the blade with a safety pin. The oxide layer on the blade and the point contact of the pencil lead form a semiconductor Schottky diode and only allow current to pass in one direction.

Yep but that doesn't happen accidentally. And good luck getting a quartz crystal in a form that can be heard.

Where did anybody say accidentally? I said it could be improvised. Are you arguing just to argue?

Edit: I realize that the headboard story implies an "accidental" semiconductor. And yes that's totally possible.

It could absolutely happen accidentally. Any bad contact between two conductors can cause a non-linear junction that can detect an AM radio signal.

TFA refers to, presumably an FM signal (because of the 3.3 meter band and what's usually broadcast there), so I doubt this story a bit.

Yep I mentioned that in a comment below, an accidental semiconductor IS possible, but that wasn't my point really. I get the arguers point though, a headboard implies accidental, I wasn't thinking of it that way. But anyway, as we've established, both improvised and accidental are possible and documented.

You are aware that there are even such things as natural nuclear reactors that happen "accidentally"?

So ruling out a simple PN junction is a weird hill to die on.

Certainly a brass headboard doesn't have a semiconductor in it. But that doesn't mean the story is bullshit. Just that it's not working in the way they think it is.

Heres an example of a thistle being used to convert an AM signal into audio. Granted, they are touching the plant directly to the broadcast tower, and it's outputting enough power that it starts burning and even burns the hand of the person holding the plant.

https://www.youtube.com/watch?v=b9UO9tn4MpI

I admit, it's apocryphal. But, if there is a gap anywhere in joint in the headboard, and it oxidized, that's likely enough. Still, I believe my family. Maybe I have details wrong (likely) but it was a headboard and they heard radio from it.

>Certainly a brass headboard doesn't have a semiconductor in it.

Are you sure?

http://www.sparkbangbuzz.com/els/photocell-el.htm

https://en.wikipedia.org/wiki/Metal_rectifier

This is extremely misleading. See:

https://en.wikipedia.org/wiki/Crystal_detector

https://en.wikipedia.org/wiki/Foxhole_radio

Flagged.

in case this story is true I wonder if its diodes in the output driver? we aren't given enough information about the actual circuit that is formed. Could the ground level between Left and Right channels be shifted, due to the EMF in Left and Right earphone coil, which in turn results in a the ear-phone microphone signal between ground and mic-in ?

You don't need PN junctions for an AM radio. Accidental point-contact diodes might be enough nonlinearity (for mixing) and rectification.

Here's a fun jumping off point for a deeper dive:

http://www.sparkbangbuzz.com/

See https://www.americanradiohistory.com/Archive-Electrical-Expe... page 325 and 360. Couple of sewing needles across a carbon block. He noticed that when he removed the dry battery, he got a better signal.

These are things that would have been available to just about anybody at the time he did his experiments.

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