Ciencia

T H E A M AT E U R S C I E N T I S T
by Shawn Carlson

The New Backyard Seismology

E

arthquakes have long held a
special fascination for science
buffs. In the past, this column
has described seismographs no fewer
than eight times, so you might think
there would be little more to say. But
something exciting has just happened
that makes this topic well worth revisiting. Using abreakthrough technology,
amateurs can now, for about $100, easily build seismographs that are robust
and that approach professional quality.
The breakthrough is a remarkable micromachined accelerometer on a silicon
chip. Made by Analog Devices in Norwood, Mass. (telephone number: 800262-5643, ext. 3), the ADXL05 chip
costs about $20 and can detect fantastically small accelerations—less than fivethe arrangement. This design ignores
the constant acceleration of gravity, so
you do not have to level your seismograph precisely. The device is, however,
quite efficient at detecting shaking rates
from 0.1 hertz (one shake every 10 seconds) up to 100 hertz, which is the frequency range in which an earthquake
releases most of its fury.
Unfortunately, the ADXL05 chip can
be destroyed by thestray, static electrical charges that build up on our bodies.
We often have up to 4,000 volts sitting
on our fingertips, just waiting to be discharged to any convenient ground. We
usually do not even feel these static discharges, because the amount of current
that flows is relatively low. But those
tiny pulses of energy can fry these deliBINDING POSTS

WATERPROOF
PROJECT BOXACCELEROMETERS

RUBBER WASHERS

“L” BRACE
CIRCUIT BOARD

BRYAN CHRISTIE

HEAVY BASE

RUBBER WASHERS

SEISMOGRAPH USING THREE ADXL05 ACCELEROMETER CHIPS
mounted perpendicularly to one another (along the x, y and z axes) can detect
the tiniest of tremors. Not all the components are shown.

thousandths of 1 g (g is the acceleration
caused by the earth’s gravity and equals
a change in velocityof 9.8 meters per
second every second). That sensitivity is
sufficient to detect extremely tiny tremors. The largest acceleration it can detect is 5 g, which is just about the limit
of the largest earthquakes recorded.
The chip requires several resistors and
capacitors to function as a seismograph.
The circuit diagram on page 102 shows
100

cate chips. Never handle them without
keepingyourself discharged. Tying a
wire between your wrist and something
grounded, such as a metal pipe under
the sink, is a good way to prevent static
disasters. Also, dropping the chip on a
hard surface may break the inner mechanism, so lay a soft towel over your
workbench when building the circuit.
My seismograph uses three ADXL05
chips; two measure horizontal accelera-

tions, and onemeasures vertical accelerations. Mount these chips on ordinary
circuit boards. The tab on the chip points
along its sensitive axis. Mount two chips
such that these tabs face away from each
other at 90 degrees—one forward, the
other facing right (that is, so they span
the x-y plane). Affix a separate piece of
circuit board at a right angle to the first,
making it stand upright [ see illustrationon this page ], then mount the third accelerometer with its tab facing straight
up to record vertical accelerations.
House the completed unit in a weatherproof box and orient your seismograph
so that the y axis points to true (not magnetic) north and the x axis points due
east. (Hiking supply stores sell local topographic maps that indicate the difference between true and magnetic north.)Ideally, the seismograph should be bolted directly onto a concrete foundation
that descends at least one meter into the
ground. Failing that, bolt it to a weighted base and rest it on a surface—preferably concrete—away from foot and automobile traffic.
The chip’s output is 2.5 volts at rest.
A 1-g acceleration changes it by 0.4 volt.
In general, the acceleration (a ) along the
chip’s sensitive...