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Sliding Light: How to Make a Dimmer Switch

Time Required Average (6-10 days)
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.


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!


To build a simple dimmer switch and to investigate the relationship between resistance in the circuit and the amount of light produced.


Kristin Strong, Science Buddies

Edited by Peter Boretsky, Lockheed Martin

Cite This Page

MLA Style

Science Buddies Staff. "Sliding Light: How to Make a Dimmer Switch" Science Buddies. Science Buddies, 6 Feb. 2016. Web. 24 July 2016 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p056.shtml?from=Blog>

APA Style

Science Buddies Staff. (2016, February 6). Sliding Light: How to Make a Dimmer Switch. Retrieved July 24, 2016 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p056.shtml?from=Blog

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Last edit date: 2016-02-06


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.

Electricity and Electronics  Science Project  This drawing shows an electrical circuit with a voltage source (symbolized by a circle with a plus and minus sign) driving a current (arrow symbol) through a resistor (jagged line symbol) around a square closed loop.
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 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. Note: you can read more about many of the concepts discussed above in the Science Buddies Electricity, Magnetism, & Electromagnetism Tutorial.

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 and Concepts

  • 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)


  • 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?


  • 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:

This source provides a discussion and an example of electrical circuits and Ohm's law:

For help creating graphs, try this website:

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Materials and Equipment

The electronics parts for this project are available from Jameco Electronics.

  • 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
  • AA batteries (4), part #198707
  • 4xAA battery holder, part #216152
  • Alligator clip leads (sold in a 10-pack, but you only need 3 for the project), part #10444
  • 6 V mini incandescent bulb, part #210008
  • Light meter; such as the Light Meter LX1010B,50,000 Lux Luxmeter with lcd display, available at Amazon.com. You can also download light meter apps for many smartphones or tablets. Search for "light meter" or "lux meter" to find one.
  • A dark room to do the experiment
  • Lab notebook
  • Graph paper

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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 2. This may take a couple of tries (with a fresh pencil each time) to get it right.

    Electricity and Electronics  Science Project This photo shows a number two pencil being held in one hand while a pocket knife scrapes off the wood with the other to expose the graphite core below.
    Figure 2. 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. Set up your circuit that you will use to test your dimmer switch, as shown in Figure 3.
    1. Insert the four AA batteries into the battery holder. Make sure the "+" signs on the batteries line up with the "+" symbols inside the battery holder.
    2. Connect a red alligator clip to the red wire from battery holder (in electronics, red wires are usually used for the "positive" connection).
    3. Connect a black alligator clip to the black wire from the battery holder (in electronics, black wires are usually used for the "negative" connection).
    4. Attach the other end of the black alligator clip to one of the light bulb leads.
    5. Attach one end of a yellow alligator clip to the other light bulb lead.
      1. Note: the color of this wire does not matter. Your alligator clip pack also came with green and white wires. You could use one of those instead.
      2. Note: the light bulb leads are very close together. Make sure the two alligator clips do not touch, or this will create a short circuit and the bulb will not light.
    6. You will connect your dimmer switch between the free ends of the red alligator clip and the yellow alligator clip.
test circuit for conductive materials
Figure 3. The test circuit for this experiment. The twist ties are not required, but they can help keep your circuit neat by bundling up the alligator clip leads.
  1. Take the free ends of the red and yellow alligator clips. Clip one of them into one end of the pencil, as shown in Figure 4. Use the other one as a "slider" by pressing it onto the graphite core at different points along the length of the pencil.
pencil graphite dimmer switch
Figure 4. Use alligator clips to connect to the graphite core of the pencil.

Preparing for Illuminance Measurements

  1. Read the instructions for your light meter so that you know how to operate it.
  2. Turn off the lights and close the window blinds, if possible, in the room where you plan to do the experiment.
  3. Turn on your light meter and take a light reading.
  4. Now, turn on the light bulb by connecting the red and yellow alligator clips directly to each other (bypassing the pencil). This turns the light bulb on "full brightness."
  5. Hold the light bulb several centimeters in front of the light meter. Pay attention to see if the light meter's reading changes. If the reading does not go up at all, there is still too much ambient light in the room affecting the light sensor's reading. You will need to move to a darker room or do the experiment at night.
  6. Pay attention to how close, and in what orientation, you hold the light bulb relative to the light meter. You will need to keep this constant for each trial. You can build a small jig (for example, out of tape and cardboard) to hold the light bulb in place if that is easier.

Testing Your Electrical Circuit

  1. Grasp the insulated (plastic part) of the free alligator clip 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 acts as your "slider." It is how you vary the resistance.
  2. Touch the "slider" (the alligator clip 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 Table 1, in your lab notebook.
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     
Table 1. Illuminance measurements data table.
  1. 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.
  2. Repeat step 3 until the entire length of the graphite core has been tested.
  3. 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?)

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  • Using a digital multimeter (such as Equus 3320 Auto-Ranging Digital Multimeter, available at Amazon.com) 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?

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