Earth Movement Sensor

A seismometer is an intriguing device. It's hard to believe that a sensor could be sensitive enough to detect ground vibrations halfway around the earth. A seismometer or earth-movement sensor, is shown in Fig. 1

The seismometer is an uncomplicated but sensitive magnetic pendulum detector. While this sensor will not pick up earth- quakes on the opposite side of the globe it has been used to pick up nuclear underground testing a few states away. The heart of the pendulum detector is a 3-foot-long x 3-inch diameter PVC tube that houses the pendulum detector. The pendulum must be free to swing unaffected by air currents or movements of the PVC outer tube. The pendulum must be placed in a draft-free, quiet, stable location to operate correctly. The easiest solution to these requirements is a PVC housing mounted to a large round plywood base that is anchored to a concrete slab Place a cap on the top end of the PVC tube. Next, drill a hole near the cap to accept a long screw that goes through the tube as shown to adjust the pendulum height above the detector. Drill a second hole near the bottom of the tube to provide viewing and sensor alignment. Place a small clear plastic cover over this hole to prevent air currents from affecting the pendulum. Attach a large round plywood base treated with a protective coating, to the PVC. Use three right-angle brackets to attach the tube to the plywood base. Secure the wooden base to a concrete slab to prevent the wooden base and sensor from "dancing" about on the floor in the case of a strong motion event.

The electronics of the earth-movement sensor include two Hall-effect sensors (see Fig. 2). Place tile two or three-lead Hall sensors at the base of the PVC tube assembly. Place both sensors flat-side down next to each other. Place one face up, i.e., writing up, and flip over the other sensor and connect the leads as shown. Sensor A has a positive output while sensor B produces a negative output with a zero magnetic field. At the center position of the pendulum, the two outputs cancel. These two outputs provide a difference signal, which is further amplified via a TI TLC251 op amp. The gain of the op amp is set for 20. Connect the op-amp's output to a zero-centered voltmeter or bridge circuit. The earth movement will be displayed as a plus and minus swing of the meter.

Next, attach a small magnet to a length of fine string or thread. Wind the free end of the thread around the long screw at the top of the PVC tube. Allow the magnet to hang freely over the Hall sensors at the base of the PVC tube. Adjust the pendulum length so that the magnet is just over but not touching the sensors. Most seismic sensors use a damping scheme to quench excessive swinging movement. This can be accomplished either mechanically or electronically. One approach to damping the pendulum is to place a glass petri dish, filled with mineral oil on top of the Hall sensors. Adjust the pendulum so that the magnet cart swing freely in the mineral oil. The earth-movement sensor is now ready to sense ground movement. This sensor arrangement is very sensitive and therefore it should be located in a quiet site away from active hiving spaces, because it will pick up passing trucks and general nearby movement.

The electronics portion of the earth-movement sensor can also be used as a compass to detect magnetic north. Mount the sensors and the electronics in a small plastic box. Affix a compass rose to the top of the plastic box. Mount the sensors next to each other perpendicular to the top of the box; i.e., the first side of the sensors are perpendicular to the around. The meter could then be read as the box is rotated. An indication will be given as the box is moved through north, and the compass rose will indicate the other directions. An LED could be substituted for the meter in this application.

Earth-Movement Sensor Parts List

Quantity	Part	Description
2	R1, R2	4.7 KΩ, ¼W resistor
1	R3, R10	100 KΩ, ¼W resistor
1	R4	1 MΩ, ¼W resistor
1	R5	24 KΩ, ¼W resistor
1	R6	50 KΩ, ¼W trim pot
2	R7, R8	20 KΩ, ¼W resistor
1	R9	330 Ω, ¼W resistor
1	D1	LED
2	U1, U2	UCS3503 Hall cells (Allegro Microsystems)
1	M	100-0-100 μA meter
1	BZ	Piezo buzzer

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