Build your own Seismograph!

An activity for Earth Science, Physical Science, Technology Education, or Electronics

Grades 4-12 or College

Table of Contents Overview Building the Seismograph Building and Setting up the Sensor Setting up the Coach Activity Lesson Ideas by Discipline Return to the Earth Science Page Return to the Main Page Subscribe to our Free Newsletter

Overview

A seismograph is a sensitive mechanical device that is used to observe and record vibrations.  Scientists and engineers use seismographs in order to measure and predict earthquakes and volcanic eruptions and also to measure the effects of earthquakes and weather on buildings and structures.  With a probeware interface device (such as CoachLab, ULAB, TI-CBL, or Vernier LabPro), Coach software, and about $25.00 worth of parts you can construct a simple and sensitive seismograph that can be used to teach principles of earth science (vibrations from earth movements), physical science (oscillations and damped oscillations), technology education (civil engineering and effects of vibrations on structures), or electronics (how sensors work, ohm's law). 

This article will show you how to create such a seismograph, how to calibrate its sensors, and how to use this device in activities in your own classroom or lab. 

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Building the Seismograph

A seismograph makes use of inertia in order to operate.  Inertia is the tendency of an object at rest to stay at rest or an object that is in motion to stay in motion unless acted upon by an outside force.  A seismograph uses a flexible "reed" that can move freely if energy in the form of vibrations acts upon it to overcome its inertia.  Once the vibrations stops internal friction of the reed's materials cause the oscillation to dampen or slowly stop.  The reed in a seismograph is connected between a stable base that is fixed to the source of vibrations on one side, and to a counterweight that can move freely on the other side.  Sensors (in our case flex sensors) mounted to the seismograph can then record the extent of the vibrations acting upon the seismograph.  Any seismograph can only measure vibrations or movement in one plane.  In order to measure right and left vibrations at the same time as up and down vibrations would require multiple seismographs each mounted in the correct plane.  With Coach software it is possible to measure vibrations from four seismographs at a time (each mounted in a different plane)

For my seismograph I used "Tech Card" an educational construction material made of recycled cardboard that is designed to be easily cut and folded into whatever shapes you can imagine.  Tech Card is available through "The Science Source."  You can also use regular non-corrugated cardboard or construct your seismograph from other materials as long as you have a flexible reed that will vibrate freely in a single plane.  (Reed materials can include cardboard, thin plastic, thin metal such as steel sheet metal, or even very thin wood). 

Parts of my Tech Card Seismograph: I constructed a base with a square-upright (made of two "C" beams) as a support.  The square upright resists vibration so that most of the energy of external vibrations goes into moving the reed.  The reed is made of a single unfolded "L" beam that can freely bend and flex with vibrations.  The counterweight on the left side is made up of the remainder of the "C" beam used to make the support.  It is folded back on itself three times to add mass to the end of the reed.  Flex Sensors are mounted to each side of the reed using paperclips on each end.  Paper (or cardboard) is used as an insulator between the sensors and the paperclips.

In my design I connected the flex sensors to the reed using paperclips as a "temporary" connection.  More accurate measures of vibration can be achieved if the flex sensors are actually glued permanently to the reed.  However, unless you are making a permanent seismograph out of more stable materials than cardboard I don't recommend gluing the sensors to the reed permanently.  The paper/cardboard that covers both ends of the flex sensors are necessary as insulators so that electricity does not flow through the paperclips from one sensor to the other one thus shorting out the sensors.  Using plastic paper clips is another alternative. 

Designs may vary significantly, see "Lesson Ideas by Discipline" below.

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Building and Setting up the Sensor

Just as in our activity "Building and Testing Structures" (which also uses Tech Card materials) we again choose to use flex sensors in order to measure vibrations in this experiment.  Flex sensors are simply variable resistors that change their resistive value based on how much they are deflected from a straight line.

To learn more about the flex sensors themselves click here. (from the activity 'Building and Testing Structures")

In order to measure vibrations in the reed of our seismograph it is necessary to use two flex sensors in order to build a "vibration sensor."  The reason we must use two flex sensors is that a single flex sensor is only designed to measure deflection in one direction. 

By mounting one sensor on the right side of the reed and the other on the left side of the reed and then connecting the sensors together as a voltage divider it is possible to measure vibrations in our seismograph.  When the reed moves to the right the right side sensor deflects and measures a result while the left side outputs zero change (since it can't measure changes in that direction).  The reverse is true when the reed moves to the left.  As a result it is possible to build a sensor profile that relates the voltage output of the voltage divider to the amount of deflection of the reed at any given moment.  By recording that value vs. time we can see vibrations as they occur and compare their magnitude to each other.  (just like in a "real" seismograph).

Before you can measure vibrations with your sensor and a probeware interface such as CoachLab or CBL you must first connect both sensors together as a voltage divider and then by experiment and by interpolation create a mathematical profile that relates the amount and direction of deflection to the voltage coming out of the voltage divider network.  In our example we will use CoachLab II's analog input.  In the first image on the right you see the two flex sensors (variable resistors) connected in series with each other to form a voltage divider network.  Five volts D.C. is applied to one side of the voltage divider network and zero volts (or ground) is connected to the other side.  The center tap of the two sensors (resistors) is the output of your sensor and will have a voltage that is in some way proportional to the amount and direction of movement of your seismograph.  Each flex sensor varies between thirty thousand Ohms and forty thousand Ohms depending on the level of flex. By experiment or by calculation (using Ohm's law) we can determine the values for the voltage output of the sensor at any given deflection.  For simplicity sake we'll pick three points, zero flex, 100% flex to the left, and 100% flex to the right. By using a volt meter and deflecting the reed of our seismograph manually we can determine that at zero flex the output of the sensor is approximately 2.500 volts.  At 100% left deflection the output is 2.857 volts.  At 100% right deflection the output is 2.143 volts.  We can now create a graph with deflection on one axis and voltage on the other and by drawing a straight line through the three points we have determined we can then interpolate any other value of voltage vs. deflection.  (see graph below) Your results may vary since each resistor is slightly different and each volt meter might be calibrated slightly differently.  Another way to get these values is through calculations and Ohm's law.

Calculating your sensor profile using Ohm's Law

To calibrate your sensor for use with CoachLab or another probeware device you can also use Ohm's law and a few facts about electricity in order to calculate values for any given deflection.

Facts:

• Ohm's Law: Voltage(total) = Current(total) * Resistance(total)
• In a series circuit the current flowing through each device is the same and can therefore be measured at any point in the circuit.
• In a series circuit the total resistance is equal to the some of each individual resistance.
• In a series circuit the voltage drop across each component can be added together to determine the total voltage drop for the entire circuit (which is also equal to the supply voltage, in this case 5 volts)
• At zero deflection each flex sensor has a value of thirty thousand Ohms.
• At maximum deflection (100%) each flex sensor has a value of forty thousand Ohms.

From these facts you can find:

• At zero deflection the total resistance of the sensor is sixty thousand ohms (30K + 30K)
• At 100% right deflection the total resistance of the sensor is seventy thousand ohms (30K + 40K)
• At 100% left deflection the total resistance of the sensor is seventy thousand ohms (40K + 30K)

Using Ohm's Law...

You can now use these calculated values to calibrate Coach Software and create a sensor profile.

NOTE: Calculated values may be different that those observed during actual testing due to the fact that each component is slightly different and their resistance and circuit voltage are not perfect.

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Setting up the Coach Activity

In order to record data from your Seismograph and a probeware interface such as CoachLab, ULAB, or TI-CBL you must do the following:

1) Create a sensor profile for your dual-flex sensor based on the values you calculated or observed above.

2) Set up the basic activity including setting up graphs, recording time, and recording frequency.

3) Connect your sensor and record data.

To learn more about creating your own sensor click here to read the article "Building your own Sensor."

To learn more about setting up and authoring activities click here to read the article "Authoring Activities."

Tips for Setting Up This CoachLab Activity:

• Using Percentage to indicate the amount of vibration allows for a wider range of tolerance in your actual sensor.  This use of Percentage as a unit of measure should be used in the sensor profile and on any graphs used to display the seismograph data.
• When setting up a graph for seismograph data it is desirable to assign ranges of -10 to +10 % to -15 to +15 % since the reed in the seismograph is rarely going to spike above these values.  By confining the graph to this range details of the vibration are easier to see.
• When setting up a graph for seismograph data it is useful to use a background grid.
• Measurement time should be confined to the duration of the experiment at hand.  If you are simply recording vibrations from footsteps or dropping weights then 30 to 60 seconds is good.  If you are using the seismograph to measure vibrations of a shake table then set the duration for slightly longer than you predict will be necessary to show damage or vibration to the structure under test.
• Measurement frequency should be set in multiples of 50 to 55 samples per second and/or some value equally divisible into 5 cycles per second.  (This is done to avoid picking up noise from 60 Hz devices and/or to match a harmonic of the 5 cycles per second that most earth quakes take on {also the resonant frequency of most structures that are 5 to 10 stories tall}).
• Experiment with settings outside of your classroom or lab before finalizing on the final recording activity used by your students.

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Lesson Ideas by Discipline

Now that you have constructed a seismograph, added a sensor to it, and created a Coach activity that can be used to observe and record data here are some ideas for using this sensor in your own classroom or lab:

Earth Science

• Demonstrating Vibrations as they travel through materials.
• Demonstrating Vibrations similar to those in earthquakes.
• Showing how Seismographs are used to predict earthquakes and volcanic eruptions.
• (Using two seismographs) Showing how vibrations change as they travel from one material to another.
• Demonstrating how different intensity vibrations effect soils, solid earth, and structures.

Physical Science/Physics

• Demonstrating Vibrations and Oscillations.
• Demonstrating Damped Oscillations.
• Demonstrating how vibrations travel through different solids.
• Relating vibrations to Earth Science.

Technology Education

• Demonstrating the role of vibrations in damaging structures.
• Designing ways to "earthquake-proof" structures.
• (Using a variable shake table) Determining the resonant frequency of a structure.

Electronics

• Teaching and demonstrating how to design and calibrate sensors
• Teaching and demonstrating Ohm's Law in action
• Teaching and demonstrating how "Flex Sensors" work
• Teaching and demonstrating how to interface an analog sensor to a digital computer

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