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Design news latest “Gadget Freak” has a great how-to on making your own seismic detector –

Like Jack Godell’s coffee in “The China Syndrome,” the fluid in William Grill’s Seismic Detector reveals subtle vibrations. The entire detector is built around a laser pointer, a few hobby-type mirrors, a PIN photdiode, regulator and amplifier, enclosed in common PVC tubing and fittings. This simple and inexpensive combination makes this detector far more sensitive than Jack’s coffee.

Case #100: William Noticed a Vibration – 4/9/2007 – Design News – Link.