Intro: Build a Pendulum Seismometer for Under $15
Instructables is such a super source of information! A while back I was looking for
ideas for a seismometer I wanted to build and ran across the IBLs that used a piezo
element from a $1 intruder alarm as a microphone. There was my seismometer transducer.
What a deal for a buck!
A seismometer is a sensing device that detects movement of the earth's crust. These
movements are often called earthquakes when they are large enough to be felt. Since
there are not a lot of earthquakes you can feel in Georgia, I wanted a seismometer
that could detect even very small movements ("micro-quakes?").
This seismometer detects when there is a quake. The inertia of a weight suspended at
the end of a lever arm flexes the piezo element. This flexing generates a small
electrical current. That current is fed into the microphone jack on a computer or
digital recorder. Computer software then analyzes the results.
I use the term "seismometer" for the detector and the term "seismograph" for the
combined detector plus the analyzing/recording software. Some people use the term
"geophone" for the detector as the detector is an "earth microphone". I am not a
geologist so decide for yourself what terms you want to use.
There are a number of references in the last step that may help you with the project.
This might make a good science project if you live in a seismically active area.
There is a bit of soldering involved in this project, but it is otherwise pretty
simple. The cost should be $10-$15 and the software is free.
Step 1: Parts and Tools
From Dollar Tree (Pic 1)
myTunes 1/8" RCA plug audio cable - $1.00
Intruder Alert - $1.00
myTunes 1/8" audio extension cable - $1.00
1 Oz. Steel Egg Sinkers (fishing supplies) - $2
Panel-mount RCA jacks (RadioShack #274-346) $4
Tools and Supplies (Pic 2)
Drill with 3/16" and 1/4" bits
Wire coat hanger
10-24 X 5/8" nut and bolt pack (Lowes #62096) $1
A tall, narrow plastic jar about 6-8" tall by 2" or more in diameter
(a 28 oz peanut butter jar would work)
Optional parts from Lowes or Home Depot (See Step 5)
7/8" spade drill bit
5' of 2" PVC Thick-walled Pipe (Lowes) $5
2 each 2" test caps (Lowes #23406) $2
2 each 3/16" X 1 1/4" fender washer (Lowes #2576) $1
1 1/8" X 23/32" rubber grommet (Lowes #139378) $1
Step 2: Harvest the Piezo Element
We will harvest the piezo element as "friger" described. See the last step in this IBL
for the link to his IBL.
1. Remove the battery compartment cover and pry off the round plastic cover (Pic 1).
2. Press down on the edges of the piezo element with a screw driver to free it from
its mounting (Pic 1 Blue Arrows).
3. Unscrew the small screw (Pic 1 Red Arrow) and remove the back cover.
4. Lift out the piezo element and printed circuit board (Pic 2).
5. Unsolder the two wires from the round black transformer (Pic 2 Arrow).
6. Discard all but the piezo element (Pic 3).
Step 3: Prepare the Piezo Element
Polish the areas on the thin metal disk at 12:00 (both sides) and 6:00 (back side).
(The two wires stick out and away from the disk at either 3:00 or 9:00.) You can use a
small piece of sandpaper or scrape it with the edge of a knife.
Tin (coat with melted solder) the three polished areas. Be sure not to heat any area
close to the central raised zone of piezo material as it will melt if you are not
Solder a 10-24 bolt to the edge of the piezo element at the 12:00 position (Pic 1).
The way I have found to do this is to secure the bolt upright in pliers with a rubber
band around the handles. Melt solder into the slot in the head of the bolt until it is
filled. Then hold the metal edge of the piezo element, heat the solder in the slot
until it melts, and position the edge of the disk down in the solder as it cools.
Straighten a 4" length of coat hanger wire for the lever arm. Scrape it clean about 2"
at each end of the wire. (The wire shold be just short enough that it will clear the
bottom of the jar when everything is mounted and the lid is screwed on.) With a 1 Oz
weight this seismometer will resonate at about 4 Hz. (I read someplace that
seismometers should resonate at 6 Hz or lower.)
Solder one end of the coat hanger wire to the back (smooth) side of the piezo element
disk at the 6:00 position (Pic 2). The wire should be in line with the threaded part
of the bolt.
Enlarge the sinker hole with a 7/64" drill bit and slide the sinker on the wire.
Solder the sinker to the end of the wire (Pic 3).
Be careful handling the completed transducer. If you flex/bend the metal disk too much
you will crack the coating of piezo material. This will make the piezo element
Step 4: Mount the Transducer in the Container
Drill a 3/16" hole at the center of the lid of the plastic jar.
Drill a 1/4" hole between the center and the edge of the lid.
Put a nut on the 3/16" bolt about half way down its length. Insert the bolt through
the center hole of the lid and add a nut on the outside to secure the transducer in
place. Turn the bolt so the wires are directed toward the 1/4" hole before tightening
the nuts securely. Add a drop of glue on each nut to lock it in place.
Mount the RCA jack in the 1/4" hole and connect the two wires to it. The wire from the
edge of the metal disk goes to the lug under the nut. The wire from the central piezo
zone goes to the center lug of the jack.
Screw the lid on the jar.
Step 5: A Production Model Seismograph
The plastic jar described above is a working seismometer in a see-through container
that is good for demonstrations. When I am making a "production model" seismometer I
usually put it in a sturdier container and add a bit more isolation from sounds in the
environment. I make mine from PVC pipe and I add a rubber grommet isolation system.
The parts needed for this container are all from a hardware supply company like Lowes,
Home Depot or Tractor Supply.
I start by building the transducer as described above except that I use a 6" long
piece of wire for the lever arm. This lowers the resonant frequency to about 3 Hz with
a 1 oz. weight.
Then I drill a 7/8" hole at the center of one of the 2" test caps. Next drill a 1/4"
hole near the edge of the test cap.
Insert the grommet in the 7/8" hole and an RCA jack in the 1/4" hole.
Put one of the fender washers on the bolt of the transducer. Put the bolt up through
the grommet and add another fender washer on top. Use a 10-24 nut to hold the whole
Be sure the transducer is centered in the grommet and the wires aim at the RCA jack.
Tighten the nut enough to keep everything in place and put a drop of glue on the nut.
Cut an 8" length of the 2" PVC pipe with the hacksaw. Smooth down the edges and be
sure the cut is straight across so the pipe will stand up straight.
I make a small square base out of wood and secure the blank test cap to the base. You
can glue the test cap to the base or drill a hole through the cap and use a wood
Put the transducer cap in the top of the pipe and check that the sinker end of the
transducer will clear the bottom test cap. I usually glue the pipe to the bottom test
cap but leave the top cap unglued.
Note: It takes a couple of hours for the tension on the rubber grommet to reach
equilibrium. During that time the output of the seismometer will be overwhelmed with
scratching and hissing sounds. This should settle down and go away in time.
Step 6: Audacity Software
I use two different pieces of software with the seismometers. One is for short-term
testing and observations. The other is for long-term seismographic recording.
You probably have an audio recording tool that came with your operating system. This
will work for simple testing. I prefer to use the free Audacity software which can be
downloaded for most OSs at audacity.sourceforge.net.
Install Audacity and start it up. Use the 1/8" to RCA cable to connect the seismometer
to the microphone input jack on your computer.
Check your audio settings to be sure your default microphone is the mic jack and not
the built in microphone (if you have one). Set the mic volume to maximum, turn off the
AGC (if you have AGC) and be sure the mic input is not muted.
Click the record button in Audacity and wiggle the seismometer to see if Audacity
detects the movement. (You might have to try the red RCA plug and then the white RCA
plug to see which one works with your computer.)
Step 7: Select a Location
Pick the lowest floor or basement of the building if you can. You want solid contact
with the ground and a minimum of extraneous noise like slamming doors or vibrating
You will probably want the extension 1/8" cable if you needed for the computer to be
located up on a table.
Try to stay away from areas with heavy magnetic fields like electric motors or
Position the seismometer with the flat surface of the piezo element facing the
direction most likely to have movement. The seismometer is less sensitive to the side
of the disk as compared to the face of the disk.
Step 8: Long-Term Seismographic Software
Seismo software is available free from my Web site.
It only works on Windows PCs that run Win98 or newer. This will probably work best on a
PC you can dedicate to the seismograph chore. It doesn't need to be fast but it does
need a microphone input to work.
Download the program (seismo.exe) and save it in a folder on your PC hard disc or on a
When you first start the program it will create a subdirectory and the default
Double-click on seismo.exe to start it. A small window will appear in the upper-right
corner of your screen.
Observe the normal background levels being detected by the monitor. Set the trigger
level to be about twice the background level as a starting point. You can set this by
editing the seismo.ini file trigger level setting.
You can have the software keep a log file or not by changing the logfile setting to Y
or N in the seismo.ini file.
Step 9: Options
I have plans to build a seismometer with a horizontal lever arm. In the diagram the
two red arrows are a low-friction pivot and the Yellow arrow represents a weight out
at the end of a long arm (18" ?). The blue line represents a wire to keep the lever
arm level but allow movement from side to side.
If you have a computer that has a stereo microphone input you could make two
seismometers and mount them with the face of each disk at right angles to each other.
That would let you record the X-axis (North/South) movement on one channel and the Y-
axis (East/West) movement on the other channel.
You can change the resonant frequency of your seismometer by lengthening or shortening
the lever arm. It would be interesting to see if this would change the recordings you
I made a rough vibration table for testing the seismomters out of two pieces of 3/4"
plywood and a 2" thick sheet of foam. You can place a seismometer on it and move the
top piece of plywood in a smooth and controled fashion to calibrate your system.
You don't have to use a computer to record the seismometer output. You could record
the signal with a portable digital recorder or even a smartphone and then analyze the
recording later using Audacity.
Step 10: References
veedo wrote this Instructable:
and Mony^ suggested this site in the comments:
"gmoon" suggested this article.
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