COCO
LABORATORY PRACTICES
Please read this document carefully and bring it
with you to each studio session.
Step 1: Obtaining the equipment
One team member should go to the north rear door of the studio and check out an electronic
prototyping board, a protoboard. Another team member should go to the south rear door of
the studio and check out a toolbox. You need to deposit your Rensselaer ID card for this with
the TA. The toolboxes will be shared among students in other sections as well as students in
other courses, and should be handled very carefully.
Step 2: Getting to know the equipment
A. The toolbox
The toolbox consists of the following items:
Multi-meter
Use this to measure voltages, resistances, currents, etc.
Logic probe
Use this to probe the logic levels (0 volts and 5 volts). Connect the red wire
to +5volts, and the black wire to ground (0 volts). Touch the tip of the probe
to a part of your circuit to determine its logic level. The small lights on the
probe indicate the logic levels.
Wire stripper Use this to strip the plastic insulation off wires. Make sure that the
stripped plastic is properly disposed of in the trash can.
Wire cutter
Use this to cut wires. Make sure that the small wire pieces are properly
disposed of in the trash can.
Long nose
Use this for bending wires.
pliers
Chip puller
Use this carefully to pull chips off the protoboard.
Screw driver Use this to adjust device such as trimmed potentiometers.
Power leads
Use these to connect the power supply to the universal protoboards. Use the
following color convention: Red: +5 volts; Green/Black: Ground (0 volts);
Purple: -12 volts; Yellow: +12 volts.
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B. The prototyping board
You will do all your wiring on Universal Protoboards (prototyping boards). These boards
1
2
(see Figure 1) were custom fabricated at RPI for this course. The parts labeled
and
will be needed for the second half of this course and we’ll not worry about them now.
3
and 4 . This area consists of an
You will build all your circuits in the areas labeled
array of holes separated 0.1 inch from each other. The pins on integrated circuits (ICs),
commonly referred to as “chips” are also separated 0.1 inch. The holes on the protoboard are
connected internally in rows and columns as explained below.
3
The areas labeled
in Figure 1 have two columns of holes each. These holes are
connected together lengthwise, as indicated in Figure 2. These buses are usually used for
power and ground connections. Do not place components such as integrated circuits (ICs) on
this bus.
1
2
3
4
5
6
7
8
9
BA
COM
A
COM
A
B
-12V
B
GND
+12V
+5V
1
60
UNIV ERSAL PROTOBOARD
4
3
2 1
Figure 1: The universal protoboard that you will use in the laboratory.
The areas labeled 4 in Figure 1 have holes that are connected in the other direction. This
part of the protoboard is where you will place ICs and discrete components such as resistors,
capacitors, etc. Refer to Figures 2 and 3 for proper placement and connection of ICs. In
these figures, there is a +5V "bus" and a GND "bus". Any wire connected to this bus will be
connected to either +5V or GND (provided these power signals have been connected from
the power supply). It is a good practice to always use red colored wires for +5V lines and
black wires for GND lines. The black object in these diagrams represents an IC (Integrated
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Circuit) which you will use in this course. All ICs have a notch or dot at one end that helps
us figure out their orientation. The pins are usually numbered counter-clockwise starting
from the pin below the notch (when viewed from above). For instance, Figure 3 shows pin
number 11 of this IC connected to GND, and pin number 22 connected to
+5V.
Use these for
+5V and GND
Insert ICs in the
middle like this,
noting the position
of the notch
Use these
holes to
connect pins
of ICS to
multiple
places
Figure 2: Showing how the protoboards are wired internally (shown by the thick arrows),
and also the procedure for placing chips.
+5V
Notch
pin 1
GND
Figure 3: Showing the connection of ICs. In particular, this diagram shows pin number 11 of
this IC connected to GND, and pin number 22 connected to +5V.
Some guidelines for wiring circuits
•
•
Plan your layout in advance: Look at the way the pins are arranged on each chip and
try to place your chips on the protoboard so that they can be connected easily.
Modular layout: When you have to build a large circuit, it makes sense to build it in
small pieces (modules) that can be tested independently, and subsequently connected
together. Later, when you are trying to debug your circuit, it makes sense to disconnect
the major modules, and test them independently. Try to plan your layout with such
modularity in mind.
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•
•
•
Test each part beforehand: It is often possible for you to encounter chips that are
defective. It is a good idea to test each chip before it goes into a complicated circuit. It is
a lot harder to locate a defective chip later!
Color coding of wires: Be very neat, and follow a systematic color scheme for your
wires (see Figure 4 below). For instance, the color red is typically used for +5volts and
black is usually used for Ground (0 volts). Use other colors in a systematic manner. This
will help you debug your circuits more easily.
Avoid loops: Try to connect any pair of points in your circuit with a wire that runs close
to the protoboard, and is as short as possible. Whenever we have wires that stick out to
form loops on the board, we run the risk of this loop acting as an antenna, and
introducing unwanted electrical disturbances into our circuits.
Bad Wiring!
Better
Figure 4: An example of good wiring.
•
Use supply bypass capacitors: Ideally, the power supply is a rock-steady voltage
waveform at +5V and 0V. In practice, however, the supply waveform is noisy, i.e., it has
a small rapidly-fluctuating component. This fluctuating component is usually because of
surge currents drawn by each of the little transistors in our digital system, and because of
the inability, due to high inductance, of the power supply to meet these surge current
requirements. Also, every now and then, one may find large transient fluctuations. These
effects can sometimes become large enough to cause unwanted changes in our digital
circuit. Supply Bypass Capacitors help us in this regard. A capacitor has a high
impedance to DC and a low impedance to voltage fluctuations. Putting a large (e.g.,.
0.1μF) capacitor between the +5V and ground pins of your chips effectively short
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circuits the fluctuations without affecting the steady DC voltage. We can think of bypass
capacitors as storing charge that can be used to quickly supply surge current needs.
When wiring supply bypass capacitors, we need to be careful! Here’s why. When
you pick a large (0.1μF – 10μF ) capacitor, you are most probably using an electrolytic
capacitor. Electrolytic capacitors cannot handle both positive and negative voltages. They
are usually marked quite clearly with plus + and/or minus – symbols to indicate their
preferred polarity. You can damage them by wiring them with reverse polarity. Usually,
the damage is accompanied with overheating, and sometimes a small explosion!
C. The switches on the universal protoboard
The upper left part of the universal protoboard consists of an array of switches and two pushbuttons. They are described in detail below. It is suggested that you use the logic probe tool
in your toolbox to get to know the operation of these devices.
Switches 1,2,3 and 4: These are toggle switches that behave as follows.
B
A
COM
A
B
COM
When the switch is in the A position, the COM line is connected to A. When the switch is in
the B position, the COM line is connected to B. These switches can be made to stay in either
position for as long as we wish.
Switches 5,6, and 7 (momentary toggle switches): These switches are similar to the first
four switches except for the fact that they are spring loaded. They ordinarily stay in the A
position, as shown below.
A
B
COM
Normal Position
When operated, they do two things, as illustrated below:
• disconnect COM from A for as long as the switch is pressed;
• connect COM to B for as long as the switch is pressed.
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A
B
COM
When Pressed
Here are two ways to wire them up for use. The right-hand circuit is preferred.
+5V
+5V
A
(not connected)
1K
A
B
1K
B
OUTPUT
= +5V in position A
0V in position B
OUTPUT
= +5V in position A
0V in position B
Switches 8, 9: These are push-button switches, and have only two connections A and B, as
indicated below.
A
B
A
B
Button not pressed means no connection
between A and B.
Button pressed means A and B are
connected as long as the button is held
pressed.
B
A
How you wire up a push-button switch makes a difference. You can wire up a push-button as
either a high asserting or low asserting switch as shown below.
+5V
+5V
1K
Low-asserting output
(0V when pressed)
High-asserting output
(+5V when pressed)
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D. Connecting power to the prototyping board
Never apply power to your circuit until you have checked all connections carefully. In
particular, check for short-circuits. Shorts between power and ground will probably not
harm the power supply, but it could be disastrous to your circuit. Also check the polarity of
devices such as electrolytic capacitors. Many devices can be destroyed by improper
connection.
The upper right portion of the protoboard consists of terminals for attaching the power cable.
(It should be in your toolbox.) Use the following color convention:
• Red: +5 volts
• Green/Black: Ground (0 volts)
• Purple: -12 volts
• Yellow: +12 volts
Step 3: Signoff and Cleanup
Demonstrate your circuit to your TA and get it signed off.
Cleanup after yourself. Specifically, make sure that there are no pieces of wires or plastic on
the tables and floors. All pieces of wires are to be placed in the trash can or recycle box.
Keep in mind that your TAs are instructed to not sign off on your activity unless you clean
up properly.
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