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Geophones are dandy motion
sensors. They are metal cans containing a powerful magnet, suspended with springs
inside a coil of wire. The magnet functions as a floating mass. The
slightest vertical motion of the device causes the coil to move relative to the magnet.
This induces a small voltage in the wire coil. These devices have been used for
decades by oil and gas companies to conduct surveys of underground rock and strata,
using induced seismic shocks. Although millions of them have been manufactured over the
years they are often hard to find. You can make your own geophone by suspending a
strong magnet with a rubber band, so it hangs inside a coil of wire containing several
hundred turns. Any vertical movement will produce a detectable voltage in the coil. To
reduce false signals produced by air movements, I suggest that the assembly be housed
inside a pipe or some other container. |
