Yesterday, we showed the mysterious circuit here, from the May 1956 issue of QST.  The two boxes are connected by only two wires.  Yet switch #1 controls lamp #1, and switch #2 controls lamp #2.

And this was 1956, so there’s not a Raspberry Pi inside each box sending control signals back and forth.  As you probably figured out, the trick involves four diodes.  If one had examined the circuit in action, they might have noticed that the bulbs weren’t burning at quite their full intensity.  But according to the solution in the June issue, the effect wasn’t great enough for most people to notice.  Here’s the complete circuit:

When one switch is turned on, it sends positive pulsating DC, which can only pass through one of the diodes.  When the other switch is turned on, it passes negative pulses, which can only pass through the other diode.  When both switches are turned on, the AC is more or less unaffected, and both lamps light.

This 1956 version used selenium rectifiers.  The modern version could use silicon diodes, and fit inside a much smaller package.

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