Build a Halloween Seismometer for Under $5

I like to stop at the Dollar Tree store whenever I get a chance. I look for any new $1
trinkets that wiggle or blink or make noises because that means there may be a motor
or LED or speaker that I can repurpose. This month they had some solar dancing witches
and solar dancing skeletons for halloween. (Maybe there will be solar dancing turkeys,
santas, elves and rabbits as the seasons roll by.) It turns out the dancers make
simple, but effective and cheap, seismometers!

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"?).

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.

The Halloween seismometer has a magnet suspended at the end of a lever arm that moves
over a coil of wire. The magnet generates a small electrical current in the coil when
it moves. That current is fed into the microphone jack on a computer or digital
recorder. Computer software then analyzes the signal.

I wrote a similar IBL a while back (https://www.instructables.com/id/Build-a-Pendulum-Seismometer-for-under-15/) and you might get some ideas there on other kinds of
seismometers to build. There are a number of references in the last step of this IBL
that may help you with this 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 of the seismometer should be under $5 and the software is free.

Step 1: Parts and Tools

From Dollar Tree (Pic 1)

Solar Dancing Skeleton or Witch - $1.00
1/8" plug audio cable (See Step 4) - $1.00
4-Pack of 2-Cup Storage Containers - $1.00


Tools (Pic 2)

Small Screw Driver
Side Cutters
Soldering Iron
Electrical Solder
Fix All Glue or Hot Glue Gun

Wire Stripper (Optional)
Drill with 1/8" Bit (Optional)
Lighted Magnifier (Optional)

Step 2: Remove the Head and Body Parts

The head and body are molded plastic front and back parts that separate with a little
pressure from the screw driver. Carefully insert the screwdriver blade along the seams
and pry them apart. They are not glued together so they are quite easy to open up.

Step 3: Pop the Base Plate Free

The base plate of the Halloween has three pressure fitted attachment points. These are
not glued so you can slowly work your way around the base with the screwdriver to
remove the base (Pic 1).

You will see the magnet suspended on the pendulum arm. The solar cell, circuit board
and coil are attached to the base and are exposed as you free the base (Pic 2).

Be very careful not to break the hair-thin wires connecting the coil to the circuit
board.

Use the side cutters to clip off the top support for the head (Pic 3). Clip this off
just above the pivot that suspends the magnet arm.

Step 4: Unsolder the Circuit Board

Use your soldering iron to unsolder the two wires from the coil where they attach to
the circuit board.  Working with such tiny wire is the hardest part of this whole
project (at least for me with my big, clumsy fingers).

You won't need the solar cell or circuit board for this project so they can go in your
parts bin for some later project.

The two wires from the coil are hard to see so a lighted magnifier can be a big help
if you have one. Mine is mounted to my work bench, but a hand-held magnifier would
also work.

Step 5: Prepare the Cord and Glue in Place

You can use just about any audio cable that has a 1/8" plug on the end. This could be
from a $1 myTunes audio cable, $1 headphone or a small $1 speaker from Dollar Tree.
Clip off the device at the end of the cable leaving a cord with a 1/8" audio plug on
it.

Identify the conductor going to the tip of the 1/8" plug. Confirm this with an ohm-
meter or by plugging it into the microphone jack on your computer and using the
Audacity software (see Step 8) to identify the active lead.

Clean and apply solder to the free ends of the wires going to the plug tip and shell.

Drill a 1/8" hole in the larger end of the support in the base. You can use the hot
tip of the soldering iron to make this hole if you don't hava a drill. Insert the free
end of the cord through this hole and glue it in place.

Wait for the glue to set before proceeding to the next step.

Step 6: Solder the Coil Wires to the Cord

This was the hardest part for me. There should already be bare wires where the coil
was soldered to the circuit board but but if not then scrape the coil wires clean and
apply solder to them.

Connect one wire from the coil to each of the wires in the cord. It doesn't matter
which wire from the coil goes to the center wire of the cord and which wire from the
coil goes to the outer wire of the cord.

Test the connection of the solder joints by plugging the cord into your computer
microphone jack and starting the Audacity software. Moving the magnet across the coil
should cause some reaction in Audacity. If there is no response then recheck your
soldered connections.

Glue the base plate to the center of the inside of one of the storage container lids
(Pic 1).

Make a small notch in the upper shell of the base over where the cord will exit. You
can use the drill bit to do this or touch the soldering iron tip to the plastic to
melt a small notch (Pic 2).

Step 7: Sand Container and Seismometer Cover

Glue the storage container cover with the base plate on it back-to-back with a second
cover.

Drill or melt a 1/8" diameter notch in one of the storage containers where the audio
cable will exit. Fill a second container most of the way full of sand (Pic 1).

Press the upper base shell on the base and put the container with the notch on the
container lid with the cord extending out through the notch (Pic 2).

Press the lower lid onto the container of sand (Pic 3). This completes the seismometer
and we are ready to connect it to the computer.

Step 8: Audacity Software

I use two different pieces of software with the seismometer. 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 from the audacity.sourceforge.net site.

Install Audacity and start it up. Use the 1/8" cord to connect the seismometer to the
microphone input jack on your computer.

Check your audio settings to be sure your default microphone input 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 as described in Step 5.

Step 9: Long-Term Seismographic Software

The "Seismo" software is available free from my Web site:
www.AudioAbility.com/seismo/. It only works on Windows PCs that run Win98 or newer.
This will probably work best on a PC you can dedicate to the seismographic chore. The
computer doesn't need to be fast but it does need a microphone input jack to work.

Download the program (seismo.exe) and save it in a folder on your PC hard disc or on a
flash drive. When you first start the program it will create a subdirectory and the
default initialization file. 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 10: References

Wikipedia:
http://en.wikipedia.org/wiki/Seismometer

Discussion here:
https://www.instructables.com/community/detect-earth-movement/

veedo wrote this Instructable:
https://www.instructables.com/id/Arduino-Seismic-Activity-Monitor-Ethernet-Shield/

and Mony^ suggested this site in the comments:
http://psn.quake.net/

"gmoon" suggested this article.
http://web.ics.purdue.edu/~braile/edumod/educseis/educseis.pdf

Goephone
http://www.tenrats.org/geo.shtml