Data Sheet

-59-
88. Forward & Reverse Bias
Build the circuit shown on the left and you will see the heart LED (69) is
on. If you want to turn on the star LED (70), you can turn on the switch
(62), or you can press the press switch (61) and move the magnet (7)
near the reed switch (83). Note that the LEDs must be in the circuit in
the direction shown in the diagram. If they are put in backwards they
will not light. This is because they have to be “Forward Biased” to allow
current to ow through them.
LEDs are typically made of two types of semiconductor materials side-
by-side. One material has an excess of electrons and the other material
has a depletion of electrons (excess of holes). By doing this, even a small
“forward bias” voltage can be applied in one direction and current will
ow making the LED light. However, even large “reverse bias” voltages
are not enough to enable current to ow in the other direction through
the LED.
89. More AND/OR Gate Logic
Build the circuit, turn on the switch (62) and you will see the heart LED
(69) is on. If you want to turn on the star LED (70), you can press the
press switch (61) and touch the reed switch (83) with the magnet (7)
at the same time. The logic diagram for this circuit is shown below.
+
–
+
–
+
–
+
–
1st level
2nd level
1st level