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Make a Dimmer Switch with a Pencil

Difficulty
Time Required Average (6-10 days)
Prerequisites None
Material Availability For your convenience a kit for this project is available at the Science Buddies Store. Estimated project time includes time for shipping the kit.
Cost Low ($20 - $50)
Safety Short circuits can get very hot and present a burn hazard. Never connect the positive and negative ends (red and black wires) of the battery pack directly to each other. Adult supervision is required for whittling the pencil.

Abstract

Dimmer switches let us control the brightness of a light, anywhere from completely off to full brightness. This can be nice when you want to set the brightness "just right," as opposed to a regular light switch that only lets you turn a light on or off. It turns out that you can make a dimmer switch out of an everyday object—a pencil! Try this project to find out how a dimmer switch can control the brightness of a light.

Objective

Build a dimmer switch using a pencil and determine how it affects the brightness of a light in a simple circuit.

Credits

Ben Finio, PhD, Science Buddies

Cite This Page

MLA Style

Science Buddies Staff. "Sliding Light: How to Make a Dimmer Switch" Science Buddies. Science Buddies, 19 Aug. 2016. Web. 29 Sep. 2016 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p056/electricity-electronics/dimmer-switch-basic-circuits.shtml?from=Blog>

APA Style

Science Buddies Staff. (2016, August 19). Sliding Light: How to Make a Dimmer Switch. Retrieved September 29, 2016 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p056/electricity-electronics/dimmer-switch-basic-circuits.shtml?from=Blog

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Last edit date: 2016-08-19

Introduction

Look around the room you are in. How many light switches do you see? Are any of them dimmer switches—either knobs or sliders that let you adjust the brightness of a light? Regardless of the type of switch, all lights are powered by electricity. All electrical devices have electrical current, or "flow," of electricity moving through them. The current usually flows through wires and other electrical parts, like the lightbulb in a lamp or the motor in a toy car. These parts combined are called an electrical circuit. Figure 1 shows a basic circuit with a battery and a lightbulb.

A closed circuit with a battery and a lightbulb
Figure 1. A closed circuit with a battery and a lightbulb. Electrical current is indicated by the yellow arrows.

In order for electricity to flow in a circuit, it must have a complete "loop," or path, through which to flow. In a battery-powered circuit, this loop must connect the positive end of the battery (marked with a "+" symbol) to the negative end of the battery (negative is indicated by a "-" symbol, but this is usually not printed on the battery). This is called a closed circuit, as shown in Figure 1. If any part of the loop is broken, this creates an open circuit and electricity does not flow at all, as shown in Figure 2.

open circuit -  the path for electricity to flow is broken
Figure 2. The path for electricity to flow is broken, so this is an open circuit. The lightbulb does not light up because no electricity can flow.

If the positive and negative ends of a battery are connected directly to each other—without anything like a lightbulb or motor in between them—this creates a short circuit, as shown in Figure 3. Short circuits are dangerous because they allow a lot of electrical current to flow, and can cause the battery to get very hot or even explode. You should never connect the two ends of a battery directly to each other.

A short circuit
Figure 3. A short circuit with the positive and negative ends of a battery connected directly to each other. This can cause the battery to overheat.

What types of materials are circuits made out of? Circuits have to let electrical current flow through them easily. Materials that let electricity flow through them easily are called conductors. Most metals are conductors. However, materials that prevent electricity from flowing are also important for circuits. These materials are called insulators. Most rubbers and plastics are insulators. Insulators are important because they can help protect you from an electric shock when you touch a wire, or help protect a sensitive circuit from damage. For example, the power cord for a lamp consists of metal wires inside a rubber insulator, and an electronic toy might have a plastic case that protects the circuit inside from damage.

However, not all conductors are the same. An important property of conductors is their resistance, or how much they resist the flow of electrical current. Circuit parts with a specific resistance value are called resistors (resistance is measured in ohms [Ω], but you will not need to measure ohms for this project). The resistance of a circuit determines how much current flows through it. That means that if you can make a variable resistor, you can control how much current flows through a circuit, and control the brightness of a lightbulb (the bulb will be brighter when more current flows through it). In this project, you will make a dimmer switch out of a pencil (the graphite core of a wooden pencil is a conductor) and see how it affects the brightness of a lightbulb in a battery-powered circuit.

Technical Note

Variable-resistance dimmer switches—like the one you will make in this project—were originally designed for incandescent lightbulbs, which are being replaced by compact fluorescent (CFL) and light-emitting diode (LED) bulbs. As you may have noticed when purchasing CFL and LED bulbs, many of them are not compatible with older dimmer switches. Modern dimmer switches designed for CFLs and LEDs work differently than the one described in this project.

Terms and Concepts

  • Electricity
  • Current
  • Circuit
  • Positive
  • Negative
  • Closed circuit
  • Open circuit
  • Short circuit
  • Conductor
  • Insulator
  • Resistance
  • Resistor
  • Ohm

Questions

  • What is electrical resistance?
  • How does the electrical resistance of an object depend on its size and shape?
  • Which do you think will have a higher resistance, a longer section of graphite or a shorter section? Hint: Think about water traveling through a pipe. Is it harder to push water through a long pipe or a short pipe?
  • How will the resistance of a pencil resistor affect the brightness of a lightbulb when they are connected to a battery?

Bibliography

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Materials and Equipment Product Kit Available

These specialty items can be purchased from the Science Buddies Store:

  • Basic Circuits Kit (1). Includes:
    • AA batteries (4)
    • 3xAA battery holder
    • Alligator clip leads (3)
    • Mini lightbulb
    • Mini bulb screw base

You will also need these items, not included in the kit:

  • #2 pencils (only 1 is required for the experiment, but you will need extras in case you break it by accident)
  • Pocket knife
  • Metric ruler
  • Fine-tip permanent marker
  • Coping saw or hacksaw (adult supervision required)
  • Lab notebook

To make this a more advanced project, there are two options that will allow you to make more quantitative measurements (see the Variations section for details):

  • A lux meter will allow you to measure the exact amount of light coming from the lightbulb, instead of rating brightness on a 0–5 scale
  • A multimeter will allow you to measure the resistance of your dimmer switch in ohms, and measure the amount of current flowing through the circuit

Recommended Project Supplies

View Kit
Project Kit: $24.95

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Experimental Procedure

  1. Watch this video for an introduction to your Basic Circuits Kit.

Basic Circuits Kit: Pencil Dimmer Switch
  1. Set up your test circuit, as shown in Figure 4.
    1. Insert three AA batteries into the battery holder (your kit comes with one extra battery). Make sure the "+" symbols on the batteries line up with the "+" symbols inside the battery holder.
    2. Connect a red alligator clip to the exposed metal part of the red wire from the battery holder (in electronics, red wires are usually used for the "positive" connection).
    3. Connect a black alligator clip to the exposed metal part of 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 screws on the lightbulb holder. If necessary, use a small Phillips head screwdriver to loosen the screw slightly so it is easier to clip to.
    5. Screw the lightbulb into the bulb holder.
    6. Attach one end of the green alligator clip to the other screw on the bulb holder.
    7. You will connect your pencil resistors to the free ends of the red and green alligator clips.
    8. Test your circuit by touching the exposed metal ends of the red and green alligator clips together. This creates a closed circuit and your lightbulb should light up. If it does not light up, then check the following:
      1. Make sure the lightbulb is screwed tightly into the base.
      2. Make sure none of your alligator clip connections are loose.
      3. Make sure none of your batteries are backwards.
basic circuit kit experiment
Figure 4. The test circuit for this experiment. The twist ties are not required, but bundling up the alligator clip leads can help keep your circuit neat.
  1. Ask an adult to whittle away the wood on the side of a number 2 pencil with a pocket knife to expose the graphite core, as shown in Figure 5. This may take a couple of tries (with a fresh pencil each time) to get it right.
  2. Using a ruler and a fine-tip permanent marker, start at the tip of the pencil and make marks every 1 cm along the length of wood.
whittling pencil
Figure 5. Whittling away one side of a wooden pencil to expose the graphite core.
  1. Connect the pencil dimmer switch to your circuit.
    1. Take the free ends of the red and green alligator clips.
    2. Clip one of them onto the tip of the pencil, as shown in Figure 6. Make sure the metal jaws of the alligator clip make contact with the graphite, and not just the wood.
    3. Use the other alligator clip as a "slider" by pressing it onto the graphite core at different points along the length of the pencil.
    4. Experiment with how you can use the slider to control the brightness of the lightbulb.
    5. If your lightbulb does not light up at all, press down more firmly on the graphite with the alligator clip, or try scratching the surface of the graphite with the clip (graphite can oxidize when exposed to air, which will prevent electricity from flowing).
pencil graphite dimmer switch
Figure 6. Use alligator clips to connect to the graphite core of the pencil.
  1. Make a data table, like Table 1, in your lab notebook. You can record the brightness of the lightbulb using a 0–5 scale (where 0 is "off" and 5 is "very bright"). Or, if you purchased the lux meter, you can measure the brightness of the bulb in lux (read the operating instructions that came with the lux meter).
 Brightness
Core Length (cm)Trial 1 Trial 2Trial 3Average
0 cm     
1 cm     
2 cm     
3 cm     
4 cm     
5 cm     
Table 1. Example data table for recording brightness of the lightbulb.
  1. Start out with your slider at the 0 cm mark so the two alligator clips are actually touching each other. Record the brightness of the lightbulb in your data table.
  2. Move the slider down to the next centimeter mark and record the brightness of the bulb.
  3. Repeat this for each mark down the entire length of the pencil.
  4. Start over at the tip of the pencil and repeat two more trials of the experiment. Record all your results in your data table.
  5. Analyze your results.
    1. Calculate an average brightness for each distance. Do this by adding up the brightness for each trial and dividing by 3, the total number of trials. For example, say you recorded brightnesses of 4, 4, and 5. The average is (4+4+5)÷3=4.33. Ask an adult if you need help calculating an average.
    2. Make a graph of your results, with graphite core length on the horizontal (x) axis and bulb brightness on the vertical (y) axis.
    3. How does lightbulb brightness change as the length of the graphite core changes? Does this make sense based on what you know about resistance? Remember from the background that higher resistance makes it harder for current to flow, and the bulb will be brighter when more current flows through it.

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Variations

  • This experiment is a good demonstration of Ohm's law, which relates the current, resistance, and voltage in a circuit. Do some background research on Ohm's law and incorporate it into your project.
  • Use a multimeter to measure the resistance of your dimmer switch at various lengths in ohms, and the amount of current (in amperes) flowing through the circuit for each length. See the Science Buddies reference How to Use a Multimeter to learn more about how to measure resistance and current.
  • Use a lux meter to measure the brightness of the lightbulb (in lux) instead of rating brightness on a 0–5 scale. For the best results, do the experiment in a dark room with no other light sources.
  • What if you wanted to make multiple individual pencil resistors with fixed resistance values? See the Pencil Resistors project to learn more. You can do this project using the parts from your Basic Circuits Kit.
  • Could you use this circuit to test objects other than pencils? It turns out you can use the circuit to determine whether objects are conductors or insulators. See the Which Materials are the Best Conductors? project to learn more. You can do this project using the parts from your Basic Circuits Kit.

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