How exactly is the “blue” razor blade acting as a tuner in this scenario?
The coated blade/pencil lead form a point-contact diode - basically a semiconducting rectifier. Finding the best point of rectification is all the tuning that happens. Without a variable capacitor, the resonance of the aerial and coil is the only real tuning going on. These sorts of crystal radios only really isolate the strongest signal, unless you have coil taps or a variable length antenna. The design dates back to WW2.
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In some circuits a capacitor is not used, as the antenna also serves as the capacitor.
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Wow! That takes me back. We made these at school when I was a boy in the late 40s so we could listen to Radio Luxembourg on 208 meters. 11.00 PM Sunday, long after lights out, huddled under the blankets, listening to the Top Twenty – introduced by Pete Murray.
I believe that people who build crystal radios today usually have a good commercial diode handy to get it tuned in. Then they try to find a signal using the razor blade or a natural crystal. The blued surface and pin point act as a diode; I’ve read that some blue blades work OK, others not at all.
I love how the design directs kids to work with a razor blade, without even the slightest warning or suggestion of parental supervision. The past is another country.
the blade is a detector.
it would be the same as a crystal in a crystal radio. there are many dozens of circuits these so not all schemes seem to have all components but they are there.
One thing that’s important - a “blued” razor blade (or sometimes a rusty one) is required. The corrosion or oxidation on the surface of the metal is necessary to make the diode.
This a version of a foxhole radio that were commonly made by GI’s during WWII.
Here’s a youtube video of how to make one: How to Make a Foxhole Radio - YouTube
Do they still even make that kind of razor blade? I remember them, but I don’t recall seeing any recently, with all the double, triple and quad blades that the razor companies sell now.
single edged paint scraping blades exist and though not blued a bit of rust might make it work.
Ah, all the better: “Kids, if you can’t find a blue one, you can make do with a nice rusty razor blade instead.”
What was the blue coating? Probably to provide some grip or corrosion protection while clamped in the razor.
However, I question it’s real purpose as in the OP’s link they suggest to scratch the surface to get a better signal. Quite possibly, they are referencing a “blue razor blade” as a style of blade to get.
It’s “bluing.”
The oxidation is necessary to create the diode - you can’t do this with a new, clean razor blade.
ETA -
From the link above:
But according to the youtube I linked above, you can accomplish the same thing on new blade by heating it up. See the 2:01 mark on the video.
Heating the blade allows some surface oxidation to occur, causing the bluing.