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PROBE
150n
Fig. 6. Logic probe using NOR gates.
Fig. 7. Two input comparator.
24
150 n
The Permanent IC Breadboard
When experimenting with ICs, we can
fi nd ourselves in a wire jungle. The
innocuou s 14 pin chip can strangle us in its
spi dery tentacles. The permanent breadboard
will save us. It is si mple to build and
convenient to use. No excuses for non-IC
experimenting will be valid after this article
is digested.
Start with a perfboard. Allow about 4" x
3" for each socket that you want. Since
most ICs are 16 pins or less, I decided to
design the system around these holders.
Punch a ho le in the board and mount the
holder. Since the holders will be subject to
lots of wear and tear. I spread a th in film of
epoxy around each one to hold it fast. Then,
on each side mount the typc of soldcrless
termi nals you prefcr. I used the spring type
termi nals. They are amaz ingly sturdy and
the spring can keep a largc number of wires
vcrv tight. Soldcr each terminal to its respective
socket lug and yo u are in business.
I also mounted two LEOs to use as testers
and logic tracers. If you plan to work with
chi ps of more than 16 pins, mount them .
Keep in mind that a 16 pin socket can also
be used for two 8 pin chips. Mount as many
sockets as you wa nt. A TO holder may prove
helpful. Feel free to customize.
For a deluxe model, mount the power
supply and clock in a chassis, but instead of
a metal cover, use the bread board . Mount
terminals for v ee, ground and clock output,
and another for cloc k speeds, and voila! A
self contained IC experime nte r. All you will
ever need in one un it.
As your chip collection grows, a safe way
of storage is needed. Try putting the chips in
a soft piece of styrofoam. The pins remain
undamaged and notations can be made on
the styrofoa m with a pen. Sometimes chips
'1 ou
}-J
NO.1 HIGH come with "house numbers" instead of the
usual 7400, 7402, etc. The notations keep
track of these.
TPUT
Don't Do Much
Readers have asked for simple sc hematics
that teach logic even if they don't do too
much. Now that the permanent breadboard
is built, we can get star ted.
The 7400 chips are good ones to play
with. Buy a few. Especially get the 7400
NAND gates, 7402 NOR gates, 7408 AND
gates and the 7432 OR gate chi ps. This will
give yo u the four basic gates to experiment
with .
In Fi g. 5 we have a circuit for a blinker.
Wh cn the clock output is high the #1 LED
lights. LEO #2 sees low logi c because the
NAND gate makes its 1,1 input into a 0
output. When the clock emits a low, LED #1
stays off but LED #2 sees a high and lights.
Cute, heh ?
Fig. 6 is a logic probe . It's a long time
favorite circui t and has been around for
quite a wh il e. (Dec. 74, 73 has another one.)
It's a useful tool and does do much. You
may wan t to make a permanent one. I
include it because it's a good example of
logic. Whcn the probe contacts a high, LED
# 1 lights. For a low, LED #2 lights. The
logic is simple to trace. It is si milar to the
blinker but it will not load down the circuit
under test because the LEOs are not "seen"
directl y. Add this to the power supply, clock
and breadboard , and you are all set.
Fig. 7 is a comparator. (Did you really
think I would mention it before and not
explain it now?) Its job is to compare two or
more si gnals and if they are the same logic,
give us a logic 1 output. Its logi c is simple to
follow. Kcep in mind that it compares any
similar signal 0 or 1. Instead of the NAND
gates you can use the 7404 NOT gate chip.
Try yo ur hand at design ing a comparator to
handl e more than two inputs using the
two-input comparator as a building block.
If you have not already done so, try
building a few of the flip flops that you have
seen in the last articl e. They will work just as
casily as these si mple ones. From personal
experience, I have had better luck with ICs
than tu bes or transistors. They arc easier to
trace and troubleshoot and either "work or
don 't work." There is usually ve ry li ttle
opt imizing and fud ging as in tubes or biasing
problems that you encounter in transistor
circuits. And it's so much fun whcn the
majority of your projects need only a 5 volt
supply.
Please continue to write and tell us what
you would like to see "explained ," You arc
our input. _