5L30.20 Bi-Coloured LED


Profile of an AC signal, cutoff voltage of an LED


When connected to an AC power source, the LED alternates between red and green as the AC source sweeps through positive and negative voltages.



  • [1] Bi-coloured LED (red and green)
  • [1] Battery
  • [1] Transformer
  • [1] Function generator

Classroom Assembly

  1. Plug in the function generator and the transformer.

Important Notes

  • Disconnect the LED when not in use.


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


Additional Resources


  • PIRA 5L30.20


  • Don't attempt this at home!

Last revised

  • 2020


  • Original construction: the bi-coloured LED was purchased. The battery is a 6 V lantern battery. The transformer is 120 V to 12.6 V. The function generator is an Interstate Electronics F34. The bi-coloured LED was soldered in series to a 1 kΩ current-limiting resistor. The LED and resistor were then soldered onto a 2' long cord and fitted with banana plugs.

Related AV

Related demos


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.