The Potentiometer

One-sentence summary

A potentiometer is an adjustable electronic part whose resistance we can change by turning a knob or sliding a lever.

Why does it matter?

The buttons and switches from the previous lesson give us only two states: on or off. We can turn a light on or off, but we cannot say "a little brighter" or "a little dimmer."

In real life we often want to adjust things gradually. Turning up music slowly, lowering the brightness of a lamp or reducing the speed of a fan are all examples. A potentiometer is the part that does this gradual adjusting.

Once you understand the potentiometer, you can do more than just switch a circuit on and off; you can tune its behaviour with your own hand. This is a basic and widely used skill in robotics and electronics.

What is a potentiometer?

A resistor is a part that makes it harder for electric current to flow. The larger the resistance, the less current passes through. An ordinary resistor has a fixed value; it does not change after it is made.

A potentiometer is a resistor whose value can be changed. When you turn its knob or slide its lever, the resistance grows or shrinks. It is often shortened to "pot."

Two everyday examples:

• A volume knob: When you turn the volume knob on a radio or speaker, a potentiometer turns inside it. As the resistance changes, the loudness of the sound changes.
• A joystick: The analog stick on a game controller has two potentiometers inside it. Depending on which way you push the stick, the values of these potentiometers change, and the game learns how fast your character should move.

Three legs

An ordinary resistor has two legs. A potentiometer has three legs, and this is what makes it special.

   Left end      Middle leg        Right end
    (1)             (2)              (3)
     |               |                |
     +==== resistance track ==========+
                     ^
                sliding contact
                    (wiper)

• Legs 1 and 3: These are the two ends of the resistance track. The total resistance between them is always fixed (for example, 10 kΩ).
• The middle leg (2): This is called the sliding contact, or the wiper. When you turn the knob, this contact slides left and right along the resistance track.

The key idea is this: the resistance between the middle leg and one end changes as you turn the knob. When the wiper moves closer to one side, the resistance on that side becomes smaller and the resistance on the other side becomes larger.

Rotary and slider types

Potentiometers come in two common types:

• Rotary: You turn a round knob. Volume knobs are usually like this.
• Slider: You slide a lever back and forth. The long sliders on audio mixers are this type.

Both do the same job; only the way you adjust them is different.

Adjusting the brightness of an LED

Now let us put the potentiometer to work: we will adjust the brightness of an LED by hand.

First, a safe circuit: the current-limiting resistor

If you connect an LED straight to a battery, too much current flows through it and the LED can burn out. That is why we always place a current-limiting resistor next to it.

Let us do a simple calculation. Ohm's law links current, voltage and resistance:

Resistance (R) = Voltage (V) ÷ Current (I)

In our circuit we will use two AA batteries (2×AA = 3 V). A red LED needs about 2 V across it to light up. That leaves the voltage across the resistor:

3 V (battery) − 2 V (LED) = 1 V (across the resistor)

Let us choose a safe current for the LED: 5 mA, which is 0.005 amperes. Now we find the resistance:

R = 1 V ÷ 0.005 A = 200 ohms

The nearest standard value is 220 Ω, so we use that. This resistor keeps the LED safe at all times.

Adding the potentiometer

Now we add a 10 kΩ potentiometer next to the fixed resistor. Here we use the potentiometer as a two-leg adjustable resistor: we connect its middle leg and one end leg into the circuit.

[+ Battery 3V] ── [220 Ω resistor] ── [Potentiometer] ── [LED] ── [− Battery]
                                            ^
                                turn the knob → resistance changes

How it works:

• When the potentiometer is at its lowest resistance (near 0 Ω), the total resistance is just 220 Ω. The current is highest and the LED is brightest.
• As you turn the potentiometer and increase its resistance, the total resistance rises. If you set it to 10 kΩ, the total becomes about 10,220 Ω, the current becomes very small and the LED glows faintly.

So by turning the knob you can adjust the LED gradually, from bright to dim. Because the 220 Ω fixed resistor always stays in the circuit, the LED is never exposed to a dangerous current.

Hands-on mini task

Do this task together with an adult. Parts needed: a 2×AA battery holder, a 220 Ω resistor, a 10 kΩ rotary potentiometer, a red LED, and connecting wires or a breadboard.

1. Before building the circuit, draw the schematic above on a sheet of paper.
2. The LED's long leg goes toward the plus (+) side and the short leg toward the minus (−) side. This is the polarity, the direction of the legs.
3. Connect in this order: battery → 220 Ω resistor → the potentiometer's middle leg → one of the potentiometer's end legs → LED → battery.
4. Turn the potentiometer fully to one side and look at the brightness of the LED.
5. Slowly turn it the other way. Watch how the LED dims.

Ask yourself:

• Which way do you turn the knob to make the LED brighter?
• Does the LED ever go completely dark, or does it still glow faintly at the lowest setting? Why?

Common mistakes

Skipping the current-limiting resistor

Trusting the potentiometer alone and leaving out the 220 Ω fixed resistor is dangerous. When the potentiometer reaches its lowest position its resistance is almost zero, and the LED can burn from a high current. Always keep the fixed resistor in the circuit.

Using the wrong two legs

When you use it as an adjustable resistor, you must connect the middle leg and one end leg. If you connect the two end legs (1 and 3), the resistance stays fixed and turning the knob changes nothing.

Connecting the LED backwards

An LED gives light in only one direction. If you swap the long and short legs, the LED will not light at all. If it does not light, check the polarity first.

Forcing the potentiometer

The knob has a starting point and an ending point. If you keep forcing it once it reaches the end, the part inside can be damaged.

Safety note

• Use only low-voltage, educational sources in this circuit: AA batteries (2×AA, 3 V), USB, or boards such as the micro:bit or Arduino. Never work with wall sockets, mains electricity or exposed household wiring.
• Have an adult nearby while you work with batteries, LEDs and wires.
• Connect the poles (+ and −) correctly, and do not let the plus and minus wires touch each other, which would cause a short circuit. A short circuit can heat up the wires and the battery.
• Lithium batteries can heat up quickly if they are short-circuited or crushed. For this reason ordinary AA batteries are preferred in these experiments; keep lithium batteries away from heat and short circuits.
• If a part gets hot, gives off a strange smell or you see smoke, disconnect the circuit at once and tell an adult.

Lesson summary

• A potentiometer is an adjustable resistor whose value we change by turning a knob or sliding a lever.
• It has three legs: two ends and a middle sliding contact (wiper); the resistance between the middle leg and one end changes as you adjust it.
• It comes in two common types, rotary and slider, and both do the same job.
• When adjusting LED brightness, a fixed current-limiting resistor (for example 220 Ω) must always sit alongside the potentiometer.
• These experiments are done only with low-voltage sources and under adult supervision.

Review questions

1. In what way is a potentiometer different from an ordinary resistor?
2. How many legs does a potentiometer have, and what is the middle leg called?
3. Why do we add the 220 Ω fixed resistor in the LED circuit?
4. For a 3 V battery and an LED that needs 2 V, use Ohm's law to find the resistor needed at a current of 5 mA.
5. When you use a potentiometer as an adjustable resistor, which two legs should you connect?

Answers

1. An ordinary resistor has a fixed value, while a potentiometer's value can be changed by turning a knob or sliding a lever.
2. It has three legs. The middle leg is called the sliding contact, or the wiper.
3. At its lowest position the potentiometer can show almost zero resistance. The fixed resistor limits the current through the LED and keeps it safe in every case.
4. The voltage across the resistor is 3 V − 2 V = 1 V. R = 1 V ÷ 0.005 A = 200 Ω; the nearest standard value, 220 Ω, is used.
5. You connect the middle leg (the wiper) and one of the end legs. Connecting the two end legs would keep the resistance fixed.

Source and verification note

For “The Potentiometer”, verification focuses on whether the relationship between What is a potentiometer? and Rotary and slider types remains consistent across examples. Activities are designed only for low-voltage direct-current circuits; mains electricity is not used. Component values must be rechecked for the actual circuit, and connections should be reviewed with an adult before power is applied.

Next lesson

The Buzzer: You will meet a small part that makes sound from electricity and produce your first audible signal in your circuit.