5K40.80 Bi-Coloured LED

Concepts

Electromagnetic induction, conservation of energy, Faraday's law, profile of an AC signal, cutoff voltage of an LED

Overview

The bi-coloured LED glows red or green, depending on the direction of current going through it. It can be powered by a hand-cranked generator consisting of a coil of wire and permanent magnets that can induce current or EMF. The direction of generated current depends on the direction of crank rotation. When connected to an AC power source, the LED switches between red and green as the AC source sweeps through positive and negative voltages. Note: this demonstration is highly visible when the lights are dimmed and is appropriate for large lecture halls.

Details

Equipment

  • [1] Hand-cranked generator
  • [1] Wire connector
  • [1] Circuit with bi-coloured LED (red and green)
  • (Optional) [1] Battery
  • (Optional) [1] Transformer
  • (Optional) [1] Function generator

Script

  1. Connect the hand-cranked generator to the circuit. 
  2. Turn the crank of the generator to light up the LED. Demonstrate that either red or green light is emitted.
  3. Reverse the direction of rotation. The other colour is now emitted.

Optional:

  1. Connect the LED to the battery and demonstrate that either red or green light is emitted.
  2. Reverse the polarity of the LED and demonstrate that the other colour is now emitted.
  3. Connect the LED to the transformer and demonstrate that the light flickers and appears orange.
  4. Spin the LED in a circle to demonstrate that the LED is strobing between red and green. Point out that there is a gap between the red and green streaks. This gap represents the cutoff voltage of the LED.
  5. Connect the LED to a function generator.
  6. Lower the frequency until the colour switching can be seen when the LED is stationary.
  7. Increase the frequency to about 60 Hz and change the output to a square wave.
  8. Spin the LED in a circle to demonstrate that there is now no gap between the red and green streaks.
 

Additional Resources

References

  • PIRA 5K40.80, 5L30.20

Disclaimer

  • Don't attempt this at home!

Last revised

  • 2018

Technicals

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.