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Project Summary

Difficulty	6
Time required	Average (about one week)
Prerequisites	None
Material Availability	Readily available
Cost	Low ($20 - $50)
Safety	You will need an adult's assistance to use the pocket knife. Adult supervision is required.

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Abstract

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:

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

• 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

• 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

• 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:

• 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:

• 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:

• 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

• 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
  • Small flashlight bulb, along with its lightbulb holder; holder is available at stores that sell electronic supplies
  • Christmas tree mini lightbulb

  Figure 2. This photo shows examples of possible light sources for your circuit.

• Alligator clips, insulated (3 sets)
  • Optional alternative to one set of alligator clips: Bell wire, 18-gauge
  • 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

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: 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:
   1. Screw the bulb into the lightbulb holder.
   2. Connect one terminal of the 9-V battery to one of the terminals on the lightbulb holder with one set of alligator clips.
   3. 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."
   4. 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:
   1. Connect one terminal of the 9-V battery to one of the wires on the mini lightbulb with one set of alligator clips.
   2. 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."
   3. 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

• 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?

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

Kristin Strong, Science Buddies

Last edit date: 2009-03-17 09:00:00

Career Focus

If you like this project, you might enjoy exploring careers in Electricity & Electronics.

	Electrician Electricians are the people who bring electricity to our homes, schools, businesses, public spaces, and streets—lighting up our world, keeping the indoor temperature comfortable, and powering TVs, computers, and all sorts of machines that make life better. Electricians install and maintain the wiring and equipment that carries electricity, and they also fix electrical machines.			Electrical and Electronics Engineer Just as a potter forms clay, or a steel worker molds molten steel, electrical and electronics engineers gather and shape electricity and use it to make products that transmit power or transmit information. Electrical and electronics engineers may specialize in one of the millions of products that make or use electricity, like cell phones, electric motors, microwaves, medical instruments, airline navigation system, or handheld games.
	Electrical Engineering Technician Electrical engineering technicians help design, test, and manufacture electrical and electronic equipment. These people are part of the team of engineers and research scientists that keep our high-tech world going and moving forward.			Commercial and Industrial Designer Have you always loved art? Do you have a good eye for beauty, balance, and form? How would you like to see your designs show up in toy stores? Or in a sporting goods store? Or at a car dealer? Commercial and industrial designers create the shape and form of every type of manufactured good that you can think of—from toys, sporting goods, and medical equipment to high technology products, furniture, toothbrushes, and toasters. They design the form of new products that are as beautiful and pleasing to look at as they are functional.
	Semiconductor Processor What do traffic lights, lasers, and microchips have in common? They are made from special materials called semiconductors. Semiconductors have helped revolutionize technology. If you enjoy hands-on work and are interested in participating in cutting-edge semiconductor technology, then a career as a semiconductor processor maybe of interest to you!

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