Question: How Do Dimmer Switches Work?

Remember the last time you were at a movie theater and the lights dimmed smoothly when the movie was about to begin? Looks really cool right? But what makes that happen? Do this fascinating experiment to find out.

Materials Needed:

(Any of The Materials Highlighted in Blue are Clickable Links for Purchasing)

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


Fine-point marker

9 V battery

9 V battery snap connector

Alligator clip wires

Mini incandescent bulb

Light meter;  (You can  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

Project Procedure:

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

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.

3. Set up your circuit board that you will use to test your materials.

  1. Connect the snap connector to the 9 V battery.
  2. Connect a red alligator clip to the red wire from the 9 V snap connector (in electronics, red wires are usually used for the "positive" connection).
  3. Connect a black alligator clip to the black wire from the 9 V snap connector (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. 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.
  6. You will connect your pencil dimmer switch between the free ends of the red alligator clip and the yellow alligator clip.

4. Take the free ends of the red and yellow alligator clips. Clip one of them into one end of the pencil. Use the other one as a "slider" by pressing it onto the graphite core at different points along the length of the pencil.

5. Read the instructions for your light meter so that you know how to operate it.

6. Turn off the lights and close the window blinds, if possible, in the room where you plan to do the experiment.

7. Turn on your light meter and take a light reading.

8. 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."

9. 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.

10. 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.

11. 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.

12. 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        


13. 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.

14. Repeat step 13 until the entire length of the graphite core has been tested.

15. Repeat steps 12-14 two more times, so that you have a total of three trials.

16. Calculate and record the average of your illuminance measurements for each graphite core length.

17. 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 likeCreate 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?)

Posted by Isaac Fornari on 14 January, 2016 electronics projects, middle school |
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