Physics Dept. Demonstration Manual

DEM55 - Diode (LED) Demonstration


To illustrate the unidirectional flow of current in diodes, the blocking action of diodes when reverse biased and the polarity and approximate amplitude of waveforms.

  • oscilloscope (D61A)
  • signal generator (Wavetek)
  • light emitting diode, red
  • light emitting diode, green
  • resistor, 680 ohms
  • coax cables
  • coax "T"


The two LEDs are connected in parallel, "back to front." The "T" is connected to the output of the signal generator, which is set at "X1" Hz, dial at approximately 1.0, amplitude "max." Coax cables are run to the LED assembly and to the oscilloscope. When the 'scope shows a positive waveform, the red LED will conduct and when it shows a negative waveform the green LED will light. When showing rectangular or sinusoidal waveforms, the LEDs will be "brightest" at each maximum or minimum. When one LED is on, the other is off due to reverse biasing.