Download Sliding Light: How to Make a Dimmer Switch Abstract So, you`ve got

Sliding Light: How to Make a Dimmer
Switch
Abstract
So, you've got your popcorn and are settled into your seat at the movie. The lights dim—it's
show time! But wait a second. Did you ever wonder how those lights dim so smoothly? It just
wouldn't be the same if the lights suddenly snapped off, would it? In this electronics science
fair project, you'll investigate dimmer switches, and even build a simple model of one. Try
this project and light up your room, and your mind!
Objective
To build a simple dimmer switch and to investigate the relationship between resistance in the
circuit and the amount of light produced.
Introduction
Do you have a table in your home that is the "center of attention"? Does it get used for
everything, from homework and birthday parties to dinner and board games? If so, then the
lighting above this table is the perfect candidate for a dimmer switch, a switch that you can
adjust so that the lighting is dimmer or brighter. When you blow out your birthday candles,
you'd like dim light for the most dramatic effect, but when you're doing your homework, you
want the light at its brightest, so you can see clearly. The dimmer switch is the solution to
these variable lighting needs.
What is a dimmer switch? In its oldest and simplest form, a dimmer switch is a variable
resistor. A resistor is an electrical element or component that resists (or opposes) the flow
of electrical current (the flow of electrons) in an electrical circuit. It is called variable
because you can adjust the amount of resistance that it has. When a variable resistor is used in
a circuit to vary the brightness of a lightbulb, you make the resistance greater when you want
a dimmer light setting and you make the resistance smaller when you want a brighter light
setting. Resistance is increased by increasing the length of the path of resistive material
through which the electrons have to flow, and is decreased by decreasing the length of the
path.
Resistance is calculated in an electrical circuit, like the one below, through the use of Ohm's
law, and is measured in the units of ohms.
Figure 1. This illustration shows a voltage source driving a current through
a resistor in an electrical circuit. The three quantities—voltage, current, and
resistance—are related through Ohm's law.
In the 1820's, Georg Ohm discovered that the electrical resistance of an object determines the
amount of electrical current flowing through it for a given potential difference or voltage
across the object, and is described by the following equation:
Equation 1:
Resistance =
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Voltage
Current
Resistance is in ohms (Ω).
Voltage is in volts (V).
Current is in amperes (A).
One way to think of potential difference is to imagine a pipe with water in it (instead of a
wire with electric current in it). The water pressure difference between two points in the pipe
is, in some ways, like the potential difference between two points in an electrical circuit.
In this electronics science fair project, you will build a simple model of a dimmer switch and
test it in an electrical circuit, a pathway of electrical elements in which electric current flows.
The elements in your circuit will be a battery, which will provide a direct-current (DC)
voltage source; a small lightbulb, which will be the model for the lighting above your table;
and a shaved pencil, which will be the variable resistor model for the dimmer switch. All
these elements will be connected, in series, with wire to form a loop, just as people holding
hands can form a circle.
The inner core of a pencil is graphite, which is a good conductor of electricity, but not as
good as the copper wire that connects the elements of your circuit together. The graphite does
have some resistance, so it will act as the resistor in the electrical circuit. You will vary its
resistance (the length of the resistive pathway) and measure the resulting illuminance from
the lightbulb with a light meter. So dim the lights and set the table for a science fair project!
Terms, Concepts, and Questions to Start Background Research
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Dimmer switch
Variable resistor
Electrical element
Current
Electrons
Electrical circuit
Ohm's law
Ohm (Ω)
Potential difference
Voltage (V)
Ampere (A)
Direct-current voltage source
Series
Graphite
Conductor
Illuminance
Lux (lx)
Questions
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In what kinds of situations is a dimmer switch useful?
What electrical element forms the simplest dimmer switches?
What happens when the length of the resistive path changes?
Why can a pencil be used as a variable resistor?
Bibliography
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Lawrence Hall of Science. Grade 4 Science Resources. Learning More About
Magnetism and Electricity. Delta Education, 2007, p. 68.
This source provides an animation of a dimmer switch:
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Harris, T. (2008). How Dimmer Switches Work. Retrieved December 11, 2008, from
http://home.howstuffworks.com/dimmer-switch.htm
This source provides a discussion and an example of electrical circuits and Ohm's law:
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Stutz, M. (2000). How voltage, current, and resistance relate. Retrieved December
12, 2008, from http://www.allaboutcircuits.com/vol_1/chpt_2/1.html
For help creating graphs, try this website:
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National Center for Education Statistics. (n.d.). Create a Graph. Retrieved November
12, 2008, from http://nces.ed.gov/nceskids/CreateAGraph/default.aspx
Materials and Equipment
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Adult helper
Pencils, number 2 (1 package). Note: Only 1 pencil is required for the experiment, but
buy several to account for breakage when forming the resistor.
Pocket knife
Ruler
Fine-point marker
Battery, 9-V
Light source; examples are shown in Figure 2, such as
o Small flashlight bulb, along with its lightbulb holder; holder is available at
stores that sell electronic supplies
o Christmas tree mini lightbulb
Figure 2. This photo shows examples of possible light sources for your
circuit.
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Alligator clips, insulated (3 sets)
o Optional alternative to one set of alligator clips: Bell wire, 18-gauge
o Optional if using bell wire: Wire stripper tool
Piece of white paper, 8.5 x 11 inches
Light meter; available at stores that sell camera supplies
Lab notebook
Graph paper
Experimental Procedure
Note Before Beginning: This science fair project requires you to hook up one or more
devices in an electrical circuit. Basic help can be found in the Electronics Primer.
However, if you don't have experience in putting together electrical circuits you may find it
helpful to have someone who can answer questions and help you troubleshoot if your
project isn't working. A science teacher or parent may be a good resource. If you need to
find another mentor, try asking a local electrician, electrical engineer, or person whose
hobbies involve building things like model airplanes, trains, or cars. You may also need to
work your way up to this project by starting with an electronics project that has a lower
level of difficulty.
Making Your Dimmer Switch
1. Ask an adult to whittle away the wood on the side of a number 2 pencil with a pocket
knife to expose approximately 9–14 cm of the graphite core within, as shown in
Figure 3. This may take a couple of tries (with a fresh pencil each time) to get it right.
Figure 3. This photo shows how to have an adult expose the graphite
core of the pencil.
2. Using the ruler and a fine-point marker, make marks every 1 cm along the length of
wood, next to the graphite core, and label the marks, starting with "0," at one end of
the exposed graphite.
Making Your Electrical Circuit
Caution: Before making your electrical circuit, please note these safety issues:
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Be careful to never directly connect the two terminals of a battery together with a
wire. That is called "short-circuiting" the battery and is dangerous.
Make sure your hands are clean and dry before working with any electrical circuit.
Remove all jewelry and tie back any loose hair or clothing.
Touch only the insulated (plastic parts) of the wires after the circuit loop is formed.
Keep one hand behind your back, or in your pocket, when working with a live
circuit.
1. If using the flashlight bulb as your light source:
a. Screw the bulb into the lightbulb holder.
b. Connect one terminal of the 9-V battery to one of the terminals on the
lightbulb holder with one set of alligator clips.
c. Connect one end of the second set of alligator clips to the other free terminal
of the 9-V battery. Connect the other end of the second set of alligator clips to
the spot on the exposed graphite core marked "0."
d. Connect one end of the third set of alligator clips to the other free terminal of
the lightbulb holder. Leave the other end of the alligator clips free and
unattached to hold against the graphite core at different locations. Your
electrical circuit should now look similar to Figure 4.
Figure 4. This drawing shows an electrical circuit containing a 9-V battery as a DC
voltage source, a number 2 pencil with an exposed graphite core as the dimmer
switch model, and a small lightbulb and holder as the overhead light model.
2. If using the mini lightbulb as your light source:
a. Connect one terminal of the 9-V battery to one of the wires on the mini
lightbulb with one set of alligator clips.
b. Connect one end of the second set of alligator clips to the other free terminal
of the 9-V battery. Connect the other end of the second set of alligator clips to
the spot on the exposed graphite core marked "0."
c. Make a connection to the free wire on the mini lightbulb:
 If you use the third set of alligator clips, connect one end of the
alligator clips to the free wire on the mini lightbulb, and leave the other
end of the alligator clips free and unattached to hold against the
graphite core when you do your testing.
 If you use the bell wire instead, cut a piece of bell wire with the wire
stripper tool, at least a foot long (a longer length will make it easier to
test), and with the wire strippers, strip the plastic off both ends to
expose the copper underneath. Connect the free wire on the mini
lightbulb to one end of the bell wire. To make the connection, you can
twist the two wires together or use a single alligator clip to clip them
together. Leave the other end of the bell wire free and unattached to
hold against different points along the graphite core. Your electrical
circuit should now look similar to Figure 5.
Figure 5. This photo shows the electrical circuit when a mini lightbulb is used as the light
source.
Preparing for Illuminance Measurements
1. Read the instructions for your light meter so that you know how to operate it.
2. Place a piece of white paper beside the lightbulb and point the light meter's sensor at
the paper. Be sure there is nothing between the lightbulb and the paper that could cast
a shadow.
Testing Your Electrical Circuit
1. Using one hand, and keeping the other behind your back or in your pocket, grasp the
insulated (plastic part) of the free alligator clip or bell wire coming from the light
source and touch the tip to various points along the graphite core and observe what
happens to the lightbulb and the light meter readings. This free alligator clip or bell
wire acts as your "slider." It is how you vary the resistance.
2. Touch the "slider" (the alligator clip or bell wire coming from the light source) against
the exposed graphite core above the 0-cm mark. You should now have two points of
contact at the 0 mark, so that the resistance is now 0. Record the illuminance
measurement (in units lux) from the light meter in a data table, like the one below, in
your lab notebook.
Illuminance Measurements Data Table
Core length (cm)
Trial 1 (lux)
Trial 2 (lux)
Trial 3 (lux)
Average of trials (lux)
0 cm
1 cm
2 cm
3 cm
4 cm
5 cm
3. Move the slider up 1 cm to the next mark, but leave the alligator clip coming from the
9-V battery at the 0 mark, and record the illuminance measurement from the light
meter in your data table.
4. Repeat step 3 until the entire length of the graphite core has been tested.
5. Repeat steps 2–4 two more times, so that you have a total of three trials.
Analyzing the Data Table
1. Calculate and record the average of your illuminance measurements for each graphite
core length.
2. Plot the graphite core length (in cm) on the x-axis and the illuminance (in lux) on the
y-axis. You can make the line graph by hand or use a website like Create a Graph to
make the graph on the computer and print it. What happened to the illuminance as the
graphite pathway in the circuit increased? Was the relationship between the length of
the pathway and the illuminance linear? (Does your graph form a line?)
Variations
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Using the digital multimeter and Ohm's law, calculate the resistance of each length of
the graphite core and plot the resistances on the x-axis (in ohms) and the illuminance
on the y-axis (in lux). Also calculate and plot the current (in amperes) in the circuit on
the y-axis for each length of the graphite core (or each resistance) on the x-axis. As
the resistance goes up, what happens to the current?