Project 88
This AND That
This circuit is commonly called an AND gate. AND gates
are used in digital logic circuits to perform logical
multiplies. When one of the inputs is low (one of the
switches is off) the output is low (LED off). The output
will only be high (LED on) if both inputs are high (both
switches are on). Combinations of AND and OR circuits
are used to add and multiply numbers together in
modern computers. These circuits are made of tiny
transistors in massive integrated circuits.
Build the circuit shown. Notice that if you turn on the slide switch (S1)
AND press the press switch (S2) the color LED (D8) lights up. Once
again, there is no partially lit state here, the LED is either totally on or
totally off. Two switches like this may be used to turn on the same light
in your house, the room switch and the master switch in the electrical
box. You could also have more than two switches and the circuit would
function the same way.
Project 87
This OR That
This circuit is commonly called an OR gate.
OR gates are used in digital logic circuits to
perform logical additions. When one of the
inputs is high (one of the switches is on) the
output is high (LED on). The output will only
be low (LED off) if both inputs are low (both
switches are off).
Build the circuit shown. Notice that if you turn on the slide switch (S1)
OR press the press switch (S2) the color LED (D8) lights up. There is
no partially lit state here, the diode is either totally on or totally off. While
this may seem very simple and boring, it represents an important
concept in electronics. Two switches like this may be used to turn on a
light in your house, or they might be two sensors at a railroad crossing
used to start the ding-ding sound and lower the gate. You could also
have more than two switches and the circuit would function the same
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