Proximity sensor?

[Martin Czech]

:::Sounds very like a device invented in the 20s by a Russian living in America
:::call Leon Theremin.
:::He built a device that used a pair of high frequency oscillators and an
:::antenna, that would generate a sound when someone walked past it.
:::The idea was it could be mounted in a doorway as an automatic 'doorbell'.
:::It turned out to have musical applications too.
:::
:::You can build a cheap one with cmos logic oscillators, and use a rectifier
:::to detect the audio output / presence of a body.
:::

Yes , later on Theremin (french ancestors: Thermin) was kidnapped by the
KGB and he "disappeared" in some Gulag. He then developped his invention
in order to control people, ie. motion, position etc.  He also was forced
to construct espionage devices.

m.c.