Elenco Basic Electronic Experiments - Manual (Page 26)

26
EXPERIMENT #13: ONE-WAY LIGHT BULBS
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 circuit below (note that the two LEDs will be referred to as “left” and “right”). Touch the
loose wire to the battery and watch the left LED. 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. The right LED will not light since it is reverse-biased.
Then touch the loose wire to the negative side of the battery (“ground”) and watch the right LED. 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. The left LED
will not light since now it is reverse-biased.
As in Experiment #11, you may try different resistor values in this circuit if you like.
+9V
3.3k Ω
100
μ
F
(LOOSE WIRE)
(UNCONNECTED)
(LOOSE WIRE)
“RIGHT” LED
(flat side is
on right)
“LEFT” LED
(flat side is
on right)
+
-