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Science project idea

Knob dimmer for an LED using a 10 kΩ potentiometer on a 9 V battery

A potentiometer wired as a variable resistor changes how much current an LED gets, so turning the knob dims or brightens the light. Less resistance gives more current and more light.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Components and Spares.Browse Components and SparesAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

A potentiometer wired as a variable resistor changes how much current an LED gets, so turning the knob dims or brightens the light. Less resistance gives more current and more light.

The science

The LED current is about (9 V minus 2 V) divided by the total resistance. With the knob at zero, only the 470 Ω resistor limits it: about 15 mA, fully bright. At full turn the total is about 10.5 kΩ and the current is about 0.7 mA, so the LED glows faintly. Brightness does not fall in a straight line with the knob because of the LED’s nonlinear behaviour.

Materials

Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • One 10 kΩ potentiometer (rotary or small slide type)
  • One 470 Ω resistor
  • One LED (any colour)
  • A 9 V battery with snap connector
  • A small cardboard box for a front panel
  • Jumper wires, tape and a marker

Safety

For this project: Battery power only. The 470 Ω resistor must stay in the circuit, or turning the knob to zero would overdrive the LED.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Look at the three pins of the potentiometer; the outer two are the ends of the track and the middle is the wiper.

  2. Connect the battery red wire to one outer pin of the potentiometer.

  3. Join the middle pin and the other outer pin together with a short wire so only part of the track is used.

  4. Connect the middle pin to the 470 Ω resistor and the resistor to the long leg of the LED.

  5. Connect the short leg of the LED to the battery black wire.

  6. Snap on the battery and turn the knob slowly from one end to the other.

  7. If the LED gets brighter in the wrong direction, just swap the two outer pins.

  8. Mark five knob positions on the card panel and write the brightness at each.

What you should see

The LED smoothly changes from nearly off to fully bright as the knob turns, showing that more resistance means less current.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What does a potentiometer do here?

It works as a variable resistor, changing the circuit resistance and so the current.

2Why is the fixed 470 Ω resistor still needed?

At zero knob resistance it limits the LED current to a safe level.

3Which pin is the wiper?

The middle pin, which slides along the resistive track.

4Where is the same idea used in daily life?

In volume knobs and in small lamp dimmers.

Free ebook: Viva Questions for Science Projects

Take it further

Use a multimeter to measure LED current at five knob positions and plot current against knob angle.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

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