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Pencil Dimmer Switch

Build a dimmer switch using a pencil

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.

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Introduction

Electricity needs a circuit to flow. A circuit can be in three different states:

Open circuit

When the loop is not connected or is broken; when a switch is “OFF”.

Closed circuit

When the loop is connected; when a switch is “ON”.

Short circuit

When the loop doesn’t have any object in between the wires. THIS IS DANGEROUS! 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. 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.

Conducting/Resisting

Metal wires have a weird role in creating a current.

First, some metals are naturally more conductive than others. This means they allow the current to flow better if they are more conductive.

Second, metals also RESIST the flow of electricity. Metals that slow down the flow more are said to have a higher resistance.

What this means is that we have to use metal to conduct electricity, but the metal also creates resistance to the current flow. How do you think this could help us make a dimmer switch?

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Does the size of the metal affect the resistance (and the current)?

Which do you think will have a higher resistance, a longer section of graphite or a shorter section?

How will the resistance of a pencil resistor affect the brightness of a lightbulb when they are connected to a battery?

Materials

AA batteries (4)

3xAA battery holder

Alligator clip leads (3)

Mini lightbulb

Mini bulb screw base

Build the Circuit

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Set up your test circuit, as shown in Figure 4.

Connect the wires, the batteries, and the light bulb together. See the image.

Clip one wire onto the tip of the pencil. Make sure the metal jaws of the alligator clip make contact with the graphite, and not just the wood.

Use the other alligator clip as a "slider" by pressing it onto the graphite core at different points along the length of the pencil.

Experiment with how you can use the slider to control the brightness of the lightbulb.

If your lightbulb does not light up at all,

Make sure the lightbulb is screwed tightly into the base.

Make sure none of your alligator clip connections are loose.

Make sure none of your batteries are backwards.

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).

Important: throughout the project, only connect the lightbulb for long enough to assess its brightness, then disconnect it when not in use. Leaving the lightbulb connected for a long time can cause it to burn out prematurely.

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Make a data table 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").

Table (example for recording brightness)

Table (example for recording brightness)

Core Length (cm)

Brightness

Trial 1

Trial 2

Trial 3

Average

There are no rows in this table

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Start 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.

Move the slider down to the next centimeter mark and record the brightness of the bulb.

Repeat this for each mark down the entire length of the pencil.

Start over at the tip of the pencil and repeat two more trials of the experiment. Record all your results in your data table.

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Analyze

How does lightbulb brightness change as the length of the graphite core changes? 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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