Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 25)

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Diodes made of Gallium Arsenide need a higher voltage
across them to turn on, usually about 1.5V This turn-on
energy is so high that light is generated when current
flows through the diode. These diodes are the light
emitting diodes that you have been using.
To demonstrate this, connect the wires according to the
Wiring Checklist. Touch the loose wire to the battery and
watch LED1. It will be bright initially as a current flows to
charge up the 100
μF capacitor and then will dim as the
capacitor voltage reaches the battery voltage. LED2 will
not light since it is reverse-biased. Then touch the loose
wire to the negative side of the battery (“ground”) and
watch LED2. It will be bright initially as a current flows to
discharge the 100
μF capacitor and then will dim as the
capacitor voltage drops to zero. LED1 will not light since
now it is reverse-biased.
As in Experiment #10, you may try different resistor
values in this circuit if you like.
EXPERIMENT #12: One-way Lightbulbs
Wiring Checklist:
o 37-to-42
o 43-to-1-to-4
o 2-to-3-to-26
o 36-to-unconnected
(use a long wire)
Schematic
Loose Wire