Elenco Electronics EP-130, Manual

Page: 53 / 160

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One of the nice things about the quad two-input
NAND IC is that we can combine the four NAND
gates to make up other logic circuits. Our last two
projects have shown how we can use NANDs to
make up some other logic circuits. This project will
show how to make up an OR gate from the NAND
gates.
Take a look at the schematic for this project – can
you trace what happens from each input to the
eventual output? (Sure you can, just give it a try.)
As you work on this project, keep the switch set to B.
When you’ve finished, connect terminals 13 and 14.
Now press the key. What happens to LED 1? Release
the key and set the switch to A. When happens to
LED 1 now? Keep the switch at A and press the key
again. Is there are any change in LED 1?
You see that this circuit indeed behaves like other
OR gates you’ve played with. If at least one or the
other of the inputs is 1, the output to the LED is 1.
Have you traced what happens from input to output
yet? The explanation is in the next paragraph, but no
fair peeking.
Let’s say you press the key with the switch set to B.
This enters 1 as both inputs of the NAND, causing
the NAND’s output to become 0. This 0 output is one
of the inputs to the NAND gate controlling the LED.
Since a NAND’s output is 0 only if all inputs are 1,
the 0 input causes the NAND’s output to go to 1, and
LED 1 lights!
We can make up AND, NOR, XOR, and NAND gates
using the quad two-input IC. Can you figure out how
we would connect the NANDs in the IC to make
these other logic circuits? Give your best try and
take notes – because we’re soon going to find out.
Notes:
EXPERIMENT #37: TTL “OR” GATE
Wiring Sequence:
o
13-49-131-137
o
14-119
o
31-58
o
72-59-60-62-33-133-121
o
50-51-71-138
o
52-56
o
53-54-132
o
55-57
o
13-14 (POWER)
Schematic