Elenco Electronics EP-130, Manual, Page: 59 / 160

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NAND gates can act as electronic sentries. If you
don’t want a signal to be input into a part of the
circuit, a NAND gate can be sure it is not.
You probably recognized one circuit in the schematic
right away – the multivibrator. You can see the output
of the multivibrator by watching LED 1. You’ll also
notice that the multivibrator provides one of the
inputs to the NAND gate. Use the schematic to figure
out what happens when the switch is set to A? To B?
Can you figure out what LEDs 1 and 2 do with the
switch set to A and set to B? Be sure to make some
notes and compare them with what you learn.
Before you complete this circuit, set the switch to B.
When you finish the wiring, connect terminals 13
and 14 and look at LEDs 1 and 2. You’ll see LED 1
“blink” to indicate the output of the multivibrator. But
look at LED 2. You’ll see that it lights continuously,
indicating that something is preventing the signal at
LED 1 from reaching LED 2. Now set the switch to A
and observe LED 1. What is happening? Is the same
thing happening to both LED 1 and LED 2?
You can see that LED 1 and LED 2 take turns going
on and off. This is because we make one of the two
inputs to the NAND equal 1 when the switch is set to
A. The multivibrator sends 0 and signals to the other
NAND input. When the multivibrator’s output is 1,
LED 1 lights, but because both input signals to the
NAND are 1, the NAND’s output is 0, and LED 2
goes out. When the multivibrator’s output is 0, the
NAND’s output becomes 1, and LED 2 lights. Try to
figure out what happens when the switch is set to B
– why LED 2 always lights. Hint: switch B supplies an
input of 0.
Now, did you figure all that out before building the
circuit? We hope so.
Notes:
EXPERIMENT #43: TTL “NAND” ENABLE CIRCUIT
Wiring Sequence:
o
13-49-53-54-42-45-131
o
14-119
o
71-50-31-44-114
o
86-82-80-72-56-57-59-60-62-33-36-121-133
o
34-52
o
40-113-85
o
41-116-79
o
43-115-81
o
51-132
o
13-14 (POWER)
Schematic