20
STAR*TECH JOURNAL/APRIL 1983
* LOGIC TRUTH TABLES* "SCOUTBALL" TROUBLESHOOTING TIPS
By Mark "Bear" Attebery, Vending International Corp., San Juan Capistrano, CA
Hello there techs and future techs. Yes .. . it is time for future tech lesson
# 2. All of you understand how a relay switch works or you wouldn't be
reading this journal, but many of those same people do not understand
exactly how a transistor works. A transistor is a switch, but for teaching
gate logic, it is easier to use switches and coils to show functions. First,
let's fully explore the AND gate. The symbol is:
=[)-
The only way to get a 1 out is if all inputs are 1. If you used relays, this is
how it would look:
~~-------➔OUT
~ :--='.5
l7~----,
----,
l
Your truth table is as follows:
A
0
0
1
1
B
0
1
0
1
OUT
0
0
0
1
If you were to inject +5 volts at either input A or B, you would get no
output, but if you were to inject +5 volts at both A and B at the same
time, you get a positive output. Simple, isn't it? Now, using your 2 input
AND gate IC chip (CMOS 4081 , TTL 7408), insert it in your proto-
board. Look at the IC diagram. Pin # 1 is designated by the circle
imprinted in the case and the pins are numbered counter clockwise as
shown.
1413 12 1110 9
8
~::: :: : : I
IC DIAGRAM
1 2 3 4 5
6
7
The imprinted circle is on the top of the IC as are the designation
numbers. Now, using your IC is easy, but think before you start! Hook up
pin 14 to VDD ( +) and pin 7 to GND. After choosing the one gate that
you are going to use, if you are using CMOS, connect all unused inputs to
GND. Now connect a +5v (or +12 volt for CMOS) LED from the
output pin #3 to GND. Run a wire from VDD (+) to #1 input pin(no
light will glow), then hook that wire to the other input pin #2 (no light will
glow).'fhen hook 2 wires from VDD (+) to both inputs and the LED
should now glow. Simple you say? OK ... let's look at an OR gate. The
symbol is:
The truth table is:
A
0
0
1
1
OUT
0
1
1
1
B
0
1
0
1
Since the logic gates are set up differently, you'll need to look up the chip
you are using in a data book. If using Radio Shack IC chips, you' ll find
the logic diagram on the back of the container.
Insert your TTL # 7 43 2 or CMOS #4071 into your proto board and
hook VDD pin 14 to your positive voltage and your VSS (pin 7) to GND.
(On CMOS hook pins 5, 6, 8, 9, 12, 13 toGND.) Hook pin 1 toGND,
pin 3 to the LED (which goes to GND) and pin 2 to VDD. The red LED
should glow. Now reverse the wires so that pin 1 goes to VDD and pin 2
goes to VSS. The LED should glow again. If you hook both wires to VSS
(GND) the LED will remain off. Your truth table is now justified.
Now we will look at an inverter gate which takes a low input and
inverts it to a high or a high input and inverts it to a low. The inverter
symbol is:
-f>-
The CMOS #4069 is an exact replica of the TTL 7405 so far as gate
placement and pin locations on the IC chip go.
Insert your IC chip into your bread board and hook up VDD and
VSS to pin 14 and 7 respectively. Hook pins 3, 5, 9, 11 , 13 to GND.
Hook pin 2 to an LED and hook the LED to GND. Hook pin 2 to VDD
and turn on the power. You should get no light. VCC is a 1 or positive
voltage, so the inverter lets nothing out. Now hook pin 1 to VSS (0 or
GND) and the LED should glow. It is like a normally closed switch in
that relay configuration. For example:
tvcc
1
~ OUT
o•~~
1 A
,a
OUT
o-----fYYYL::t_
Just to show what it takes to do this using integrated components, here is
the actual CMOS equivalent schematic.
In order to get a 1 out you must have a 1 at any input ( or both). Using
relays, your logic would look like this:
vcc
B e>-- - - ~
VIN
If you comprehend how this schematic works, then you shouldn't be
reading this part of this article!
If you think that you understand each of these gates completely, then
given the following inputs, tell me what the output will be. The answer is
at the end of this article.