Bi-Coloured LED

Equipment: A bi-coloured (red and green) LED powered by a battery, a transformer and a function generator.

What it does: When connected to an AC power source, the LED switches between red and green as the AC source sweeps through positive and negative voltages.

Concepts Demonstrated: The profile of an AC signal. The cutoff voltage of an LED.

Classroom assembly:

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.

Cautions: Disconnect the LED when not in use.

Setup Time: Short.

Difficulty: Straightforward.

Visibility: High. This demonstration is highly visible when the lights are dimmed and is appropriate for large lecture halls.

Related demonstrations:

References: PIRA 5L30.20

Original Construction: The bi-coloured LED was purchased. The battery is 6V lantern cell. The transformer is 120V to 12.6V. The function generator is an Interstate Electronics, F34.