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Science project idea

Forward voltage of LEDs of different colours and the energy of light

An LED only begins to conduct above a threshold voltage that is higher for colours of shorter wavelength: red about 1.8 V, green about 2.1 V, blue about 2.8 V. The threshold energy e V roughly equals the photon energy h c / lambda, so this experiment gives an estimate of Planck’s constant.

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What it shows

An LED only begins to conduct above a threshold voltage that is higher for colours of shorter wavelength: red about 1.8 V, green about 2.1 V, blue about 2.8 V. The threshold energy e V roughly equals the photon energy h c / lambda, so this experiment gives an estimate of Planck’s constant.

The science

An LED is a p-n junction in which electrons falling across the band gap release their energy as a photon. The band gap energy sets both the colour of the light and the voltage needed before current flows: a bigger gap means a bluer photon and a higher turn-on voltage. Measuring the voltage at which each LED just starts to glow and multiplying by the electron charge gives the photon energy e V, and setting this equal to h c / lambda with the wavelength from the LED packet gives h to within about 20 percent. The V-I curve of an LED is not a straight line, which also shows it is a non-ohmic device.

Materials

Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • Red, yellow, green, blue and white 5 mm LEDs (about Rs 2 each)
  • A 10 kilo-ohm potentiometer (variable resistor)
  • Three AA cells in a holder (4.5 V)
  • A 220 ohm resistor
  • A digital multimeter
  • A breadboard or crocodile clips, and graph paper

Safety

For this project: Keep the 220 ohm resistor in series always, or the LED burns out. Never look straight into the blue or white LED at full brightness. Cells only.

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. Wire the cells, the potentiometer as a voltage divider, the 220 ohm resistor and the red LED in series, with the meter across the LED on DC volts.

  2. Turn the potentiometer from zero and raise the voltage slowly in a darkened room until the LED just visibly glows; record this turn-on voltage.

  3. Continue raising the voltage in 0.1 V steps and record the current (meter in series on milliamps) at each step up to about 15 mA.

  4. Repeat the turn-on voltage and the V-I table for the yellow, green, blue and white LEDs.

  5. Record a results table with columns for colour, wavelength from the packet, turn-on voltage, and e V in electron volts.

  6. Plot the V-I curves of all LEDs on one sheet to show the sharp knee and the different thresholds.

  7. Plot turn-on voltage against 1 / lambda; the slope multiplied by e / c gives an estimate of Planck’s constant.

  8. Compare with the accepted value 6.63 x 10^-34 J s and explain the sources of error, such as judging when the glow starts.

What you should see

Turn-on voltages rise from red to blue, V-I curves show a sharp knee, and the Planck constant estimate comes out within roughly 20 percent of the textbook value.

Viva questions

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

1Why does a blue LED need more voltage than a red one?

Blue photons have more energy, so the semiconductor band gap and the turn-on voltage are larger.

2Is an LED an ohmic device?

No; its V-I graph is a curve with a knee, not a straight line through the origin.

3Why is the series resistor needed?

Above the knee the current rises very steeply with voltage, so the resistor limits it to a safe value.

4What relation links the photon energy and wavelength?

E = h c / lambda.

Free ebook: Viva Questions for Science Projects

Take it further

Measure the LED voltage in the dark with a bright torch shining on it to show the small photovoltaic effect in reverse.

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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