Experiments with your laser. (laser-based seismometer, seismograph, small or slow movement detection and reflected sound modulation)

Detect earthquakes and other vibrations, build an optical seismometer, and experiment with a laser listening device.

NOW YOU CAN PUT YOUR LASER TO USE AND TRY COUPLE OF EXPERIMENTS. THE FIRST ONE WILL MEASURE GROUND MOVEMENT. THE BEST KNOWN DEVICE FOR DOING THIS IS CALLED A SEISMOMETER.

You can build a laser-based seismometer that is simple, yet remarkably sensitive and accurate.

Seismometers are best known for their use in measuring the intensity of earthquakes. However, they can also detect other vibrations, such as those from highway traffic, trains, explosions, lightning bolts, or any other source that causes earth movement. Even if such movements occur some distance away, you will still be able to detect them. The heart of the seismometer device is the sensor unit. It incorporates a variation of the speaker modulator described in the last column (Electronics Now, June 1996, page 51).

Figure 1 shows all three parts of the sensor unit. It is simply a 1 1/2-inch square of rigid, aluminized mylar attached to one end of a 2-inch length of medium tension spring. Epoxy, silicon, or hot-melt glue will do the job. Fasten the other end of the spring to a 2-inch square block of 1 x 2 pine. This assembly, when mounted to a very stable foundation, will vibrate in response to any shock waves traveling through the earth.

For an indoor location, use an upright stud or pillar that is sunk into your home's foundation. Such a spot in your garage or basement will work well. Outdoors, a 6- …