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DC Motors
Electric Circuits / System Design
Overview:
Rube Goldberg (1883-1970) was a Pulitzer Prizewinning artist, famous for his cartoons portraying
fictional inventions. Each “invention” consisted of a
long, complex sequence of steps to carryout a simple
task. While his inventions regularly violated some
engineering principles, there was always a great deal of
real engineering to be found in his humorous designs.
Although he made his career as an artist, Rube Goldberg
was educated as an engineer. His legacy continues today through the annual National Rube
Goldberg Machine Contest held at Purdue University. The event is organized by the local chapter
of Phi Chapter of Theta Tau, the National Student Engineering Organization.
In this activity, you will learn the basics of operating a small DC electric motor to achieve one
step in a sequence of tasks, and you will work with other students to assemble a Rube Goldberg
machine.
CAUTION:
This equipment described here is only appropriate for controlling low-voltage, low-current
DC devices, such as small battery-powered motors. NEVER CONNECT THE CIRCUIT
TO AC DEVICES OR TO ANY VOLTAGE SOURCE THAT EXCEEDS 12 VOLTS OR
300 mA.
PART 1 Some Basics of DC Motors and Relays
DC motors come in a large variety of sizes and shapes, but they all depend upon the interaction
between electric currents and magnetic fields to produce a rotation. Most (but not all) can rotate
either forward or backward depending upon the direction of the electric current.
Electromechanical relays are similar to motors in their dependence on electric currents and
magnetic fields, but they are designed simply to control the back and force movement of a
mechanical switch. Like motors, relays are available in a wide variety of shapes and sizes,
including the 2 types shown in the photo at the top of this page. A relatively small current is
needed to activate a relay, enabling the relay to control a much larger current. In many
applications, relays have been replaced by transistors or other purely electronic switches which
can be less expensive and which are not subject to the possible failure of a relay’s moving parts.
On the other hand, relays remain important in many applications because they can be much more
efficient in conducting relatively large currents with virtually no drop in voltage.
Relays are often used to control DC motors, both to turn them on and off and to control their
direction of motion. The schematic circuit diagram on the next page shows an “H-Bridge,” which
you will use to control a DC motor. There are also photos that show the inner workings of two
types of relays. Although the relays are quite different electrically and physically, both use a
current flowing through an electromagnetic to move a mechanical switch.
DC Motors
Participant Handout
Jan. 22, 2006
page 1
Circuit Schematic Diagram
SPST Relay
DC Motors
Participant Handout
Jan. 22, 2006
page 2
DPDT Relay
Question 1 on the Report Form shows black-and-white versions of the motor control circuit, with
the relays shown in all of their possible states. The top relay is a single-pole, single-throw (SPSP)
relay, which completes an electrical pathway when the relay’s magnet is energized. The other
relay is a double-pole, double-throw (DPDT) relay, which two pathways (2 poles) and which
directs the electric current in either of two different directions (2 throws). In your circuit, the
SPSP relay will be magnetized when your calculator sends the signal “1.” The DPDT relay will
be magnetized when your calcualtor sends the signal, “2”. Both relays will be energized by the
signal “3,” since 3 = 1 + 2.

Directly on the diagrams on the Report Form, draw lines and arrows to show how current will
flow from the battery positive for each of the 4 possible signals. Also indicated how the
motor will respond by checking either “forward,” “reverse” or “off.”

Use a different color of pen, pencil or marker on the diagrams above to show the path of
electric current which will activate the electromagnet.for each magnetized relay.

Answer the other question in Part 1 of the Report Form.
DC Motors
Participant Handout
Jan. 22, 2006
page 3
DC Circuits
REPORT FORM (Part 1)
NAME(S) ____________________________________________________________________
1) Directly on the diagrams on the Report Form, draw lines and arrows to show how current will
flow from the battery positive for each of the 4 possible signals. Also indicated how the
motor will respond by checking either “forward,” “reverse” or “off.”
Signal = 0
SPST relay: not magnetized
DPDT relay: not magnetized
Signal = 1
SPST relay: magnetized
DPDT relay: not magnetized
Motor: Forward ___ Reverse ___ Off ____
Motor: Forward ___ Reverse ___ Off ____
Signal = 2
SPST relay: not magnetized
DPDT relay: magnetized
Signal = 3
SPST relay: magnetized
DPDT relay: magnetized
Motor: Forward ___ Reverse ___ Off ____
Motor: Forward ___ Reverse ___ Off ____
2) Use a different color of pen, pencil or marker on the diagrams above to show the path of
electric current which will activate the electromagnet.for each magnetized relay.
DC Motors
Participant Handout
Jan. 22, 2006
page 4
3) The SPST relay shown has a coil resistance of 250 Ω and is rated for a control voltage of 6 V.
Use Ohm’s law to find the control current which will flow through the electromagnet’s coil
when the relay is activated.
4) The SPST relay is rated for a “maximum switched current” of 500 mA. Why is this value not
the same as the current calculated in question 3?
5) The 2 relays used here are SPST and DPDT. Describe with words or a diagram how a SPDT
relay would operate.
DC Motors
Participant Handout
Jan. 22, 2006
page 5
PART 2 Building and Testing the Control Circuit
CAUTION: The motor here must use a DC voltage of 12 V or less and it must use a current
of 500 mA or less. Be extremely alert for short circuits, which can cause fires and can cause
the batteries to over heat and spill harmful chemicals. DO NOT INSERT BATTERIES
INTO THE BATTERY PACK UNTIL AFTER THE CIRCUIT HAS BEEN CHECKED
AND APPROVED BY YOUR INSTRUCTOR.
The pictures on the next page show a recommended board layout for using a SPST relayt and a
DPDT relay to control a DC motor. The green lines represent metal clips inside a solderless
breadboard, such as Radio Shack’s #276-175. Solid 22AWG wire should fit easily into any of the
holes on the breadboard to complete other connections. If your motor or battery pack uses
stranded wire, it may be necessary either to make the attachment to the board using a short
jumper wire with alligator clips.

Assemble the components and connection on the breadboard as shown. Note that the coil
connections on the DPDT relay are furthest from the other pins and that these go to the
left in the suggested layout. This means that the lettering on the DPDT relay will face
towards the back.

After the breadboard has been approved by your instructor, use the calculator, CBL2 and
Binary Basic Trainer to test the operation of your motor, verifying that it turns both
directions and goes off with the appropriate calculator instructions. If the system does not
function as it should, use the troubleshooting guide which follows the pictures to find and
correct the problem. Describe the results of your tests on Part 2 of the Report Form.
DC Motors
Participant Handout
Jan. 22, 2006
page 6
Board Layout
DC Motors
Participant Handout
Jan. 22, 2006
page 7
DC Circuits
REPORT FORM (Part 2)
NAME(S) ____________________________________________________________________
1) Describe below the steps you took to determine whether your control circuit is functioning as
as it should.
2) Describe any problems you encountered with the system and explain how you attempted to
identify and correct the problems. Also desribe the final result of your troubleshooting.
3) In your system, what is the source of energy which powers the motor?
4) In your system, what is the source of energy which powers the relays?
DC Motors
Participant Handout
Jan. 22, 2006
page 8
PART 3 A Rube Goldberg Megasystem
CAUTION: The motors here must use a DC voltage of 12 V or less and must use a current
of 500 mA or less. Be extremely alert for short circuits, which can cause fires or cause the
batteries to over heat and spill harmful chemicals.
In the spirit of Rube Goldberg, the goal of this section is to move a ping pong ball from one end
of the room to the other in the most complex and entertaining way possible. Work with others in
the class to design a megasystem which satisfies these requirements:

There must be at least 5 distinct and different steps to the ball’s motion. (The more steps, the
better.)

The entire motion must be planned in advance, with control of each step assigned specifically
to one team. Control may only be exercised by entering appropriate signal codes into a
graphing calculator.

Once the trial begins, no one except the instructor may touch the ping pong ball directly.
Complete Part 3 of the Report Form to describe the Megasystem.
DC Motors
Participant Handout
Jan. 22, 2006
page 9
DC Circuits
REPORT FORM (Part 3)
NAME(S) ____________________________________________________________________
1. In the table below, describe each step in moving the ping pong ball. Add more pages as
needed.
Step # People Responsible
Description
1.
2.
3.
4.
DC Motors
Participant Handout
Jan. 22, 2006
page 10
5.
6.
7.
8.
9.
DC Motors
Participant Handout
Jan. 22, 2006
page 11
Trouble Shooting
Sooner or later, you will encounter problems with the control circuit. Although it can certainly be
frustrating to find that an experiment or demonstration is not working as expected, most
problems (like those in most other electrical systems) will be bad connections that can be quickly
corrected. Real-world systems don't always work perfectly, and it is extremely valuable to know
how to deal with the inevitable problems.
The key strategy for diagnosing and repairing
problems with the control circuit depends upon
understanding that the circuit merges a flow of
information from the calculator and the CBL
with one or more flows of energy from the
batteries. If you don't see the desired action, it
is possible that a faulty or damaged component
is causing the problem, but it is far more likely
that the flow of either information or energy is
blocked at some point. You need to find the
blockage and eliminate it.
Check the information path:
Measurement
To check the calculatorCBL2 connection, exit
the SIGNAL program if
it is running and restart
the program. The
program checks to
verify that the CBL2 (or
LabPro) is responding
to the calculator and
gives an error message
if the two devices are
not communicating.
Check to see if the
Binary Basic Trainer is
responding to the CBL2
or LabPro by using the
SIGNAL program’s
“Count 0 to 15” option.
Expected result
If there is no error, the
SIGNAL program goes
quickly to the welcome
screen. When you actually
send a signal, the yellow
and green LEDs on the
CBL2 or LabPro should
flash.



If there is no error, all 4

LEDs on the Binary Basic
Trainer should light and in
turn and go out again

according to the binary
sequence.

Problem solution
The most common problems involve
the link cable between the calculator
and CBL2 or LabPro. Be sure it is
inserted fully into both devices. You
may also need to try a difference
cable, particularly if the tip is bent.
Be sure the CBL2 or LabPro has good
batteries or that it is connected to the
proper AC adapter.
Reload the SIGNAL program to verify
that it has not been changed by
accident.
Verify that the cable from the CBL2
to the Binary Basic Trainer is snapped
completely into place at both ends.
Verify that the Binary Basic Trainer’s
batteries are good and in place.
If using the LabPro, verify that the
cable is attached to DIG/SONIC1, not
DIG/SONIC2.
DC Motors
Participant Handout
Jan. 22, 2006
page 12
Check to see if
information is reaching
the SPST power relay,
by using the SIGNAL
program’s “Key Press”
option to press
alternately “0” and “1.”
By listening closely or

putting your finger on the
tube-shaped SPST power
relay, you should be able
to detect the faint sound of 
the relay clicking off when
you switch from “1” to
“0.” (It is very difficult to 
hear the relay switch “on”
when “1” is sent.)
Trace the wires which lead from one
of the Binary Basic Trainer’s “+”
terminals to the relay’s input and
reseat as needed.
Verify that the SPST relay’s 4 pins
are all in the correct holes of the
breadboard.
If necessary, use a voltmeter to find
the break in the connection. Send a
continuous “1” from the calculator
and place the voltmeter’s negative
probe where the wire from the Binary
Basic Trainer’s “1” attaches to the
breadboard. Move the voltmeter’s
positive probe along the current path,
starting where the wire from the
Binary Basic Trainer’s “+” terminal
attaches to the breadboard. The
voltmeter should read about 5 volts
until you reach the relay and it should
read 0 volts after the relay. When you
find an exception, reattach the wire.
Check to see if
information is reaching
the DPDT direction
relay, by using the
SIGNAL program’s
“Key Press” option to
press alternately “0”
and “2.”
By listening closely or

putting your finger on the
rectangular DPDT
direction relay, you should
detect the sound of the

relay coming on when “2”
is sent and going off when
“0” is sent.
Trace the wires which lead from one
of the Binary Basic Trainer’s “+”
terminals to the DPDT relay’s input
and reseat as needed.
Verify that the DPDT relay’s 8 pins
are all in the correct holes of the
breadboard.
If necessary, use a voltmeter to find
the break in the connection. Send a
continuous “2” from the calculator
and place the voltmeter’s negative
probe where the wire from the Binary
Basic Trainer’s “2” attaches to the
breadboard. Move the voltmeter’s
positive probe along the current path,
starting where the wire from the
Binary Basic Trainer’s “+” terminal
attaches to the breadboard. The
voltmeter should read about 5 volts
until you reach the relay and it should
read 0 volts after the relay. When you
find an exception, reattach the wire.
DC Motors
Participant Handout
Jan. 22, 2006
page 13
Check the energy path:
Measurement
Verify that energy is
reaching the breadboard
by using a voltmeter to
measure the voltage
where the 2 wires from
the battery pack attach
to the breadboard.
Expected result
The voltage at
the breadboard
should be close
to the nominal
value expected
from the
batteries (about
1.5 V time the
number of
batteries).
Verify that the main
The motor
batteries can operate the should one
motor by connecting
continuously in
the motor leads directly one direction
at the point where the
regardless of any
battery leads attach to
signal from the
the breadboard.
calculator.
Verify that the SPST
The motor
power relay can switch should run in one
the motor on and off, by direction when
moving the motor’s
you send “1” and
positive lead connect
go off when you
with the opposite side
send “0.”
of the SPST relay. Use
the SIGNAL program’s
“Key Press” option to
send alternate signals of
“0” and “1.”
Verify that the DPDT
The motor
direction relay can
should run in one
switch the motor’s
direction when
direction, by
you send “1” and
reconnecting the
in the opposite
motor’s positive lead to direction when
the correct output of the you send “3.”
DPDT relay. Use the
SIGNAL program’s
“Key Press” option to
send alternate signals of
“1” and “3.”









Problem solution
Remove the batteries and reinstall them,
making sure none are backwards
Verify the batteries are not dead. Replace them
if needed.
Check for a short circuit on the breadboard. If
the battery pack leads show 6 volts when they
are not connected to the breadboard but the
voltage drops when the are connected to the
board, there may be an undesired connection
from positive to ground on the board.
Verify that the voltage and current
requirements of the motor match the capability
of the batteries.
Use an ohmmeter to check for a short circuit,
and open circuit in the motor and replace the
motor if needed.
Check that all 4 pins of the SPST relay are
seated correctly and that the connecting wire
are in the correct location
Replace the SPST relay, if needed.
Check that all 8 pins of the DPDT relay are
seated correctly and that the connecting wires
are in the correct location
Replace the DPDT relay, if needed.
DC Motors
Participant Handout
Jan. 22, 2006
page 14