Fundamental Circuits Analysis Lab
LAB

Electromechanical Devices
It is important for you to understand basic electromechanical devices such as switches, relays, and
electronic switches. These devices are used in many mechatronics applications and you as a technician
need a solid understanding of these devices both for design, maintenance, and troubleshooting.
Objectives
After completing this lab, you should be able to:
•
•
•
•
•
Construct basic switching circuits
Design switching circuits
Explain basic switching circuits
Test basic switching circuits
Use basic laboratory instruments
Materials Required
The following are quantities and descriptions of materials need in this lab experiment.
1
1
X
X
1
1
2
1
Power Supply 0 – 12V DC at 1 Ampere
DMM or VOM
Miscellaneous resistors, (values to be determined)
Assorted switches
DPDT 12V DC relay
2N3904 or 2N2222 NPN Bipolar Transistor
12V DC incandescent Lamps
Protoboard and connecting wire
Discussion
Fundamental Circuits Analysis Lab
T
he purpose of this laboratory experiment is to give you an introduction to some of the
basic electromechanical devices used in industry. In this experiment we will investigate
basic mechanical switches, electrical relays, and a transistor used as a solid-state switch.
A switch is a device for making or breaking an electric circuit or they maybe used for selecting
between multiple circuits. Switch types are mechanical, electrical, or electronic. In this experiment
we will be looking at the mechanical, electrical, and electronic switches. Each switch type can come
in many different switching arrangements which are defined by their contacts. Some contacts are
normally open until closed by the operation of the switch, while others are normally closed and
then opened by the operation of the switch. There are also two important terms pole and throw
which are used to describe switches. A pole refers to the movable arm located inside the switch. A
throw is one or more positions/contacts that the switch can select. There terms are usually
abbreviated along with additional letters and numbers to define the switching modes. Let’s take a
look at a few examples to get a better understanding of switch labeling. The simplest switch
configuration is the single-pole-single-throw switch and that would be abbreviated as SPST. This
switch type is used to simply open or close a circuit and its schematic symbol is show in Figure 4-1
(a) along with a picture of a SPST toggle switch shown in Figure 4-2. A single-pole-double-throw is
shown in Figure 4-1 (b). We can see that this switch has one moving arm, (single pole) and two
contact positions, (double throw). The switch shown in Figure 4-1 (c) is a DPST and the dashed
line between the two arms indicate that they are mechanically but not electrically linked. This
indicates that the actuator or lever on the switch moves both arms at the same time. It is common
to use a slightly different labeling scheme for rotary switches and they also add another term known
as timing. The switch shown in Figure 4-1 (g) is a rotary switch and would be labeled as a SP4P
switch. The SP4P abbreviation means single-pole-four-position. The timing terms are nonshorting – break before make or shorting – make before break. What this means is as the movable
arm is going to the next position it will either make an electrical connection to the new contact
before disconnecting from the old contact, (make before break) or it will first disconnect from the
old contact before making an electrical connection to the new contact, (break before make).
Different applications require different timing switches and in instrumentation and control this can
be a very important factor. The rotary switches used in this experiment are non-shorting to control
the intensity of the lamp.
Fundamental Circuits Analysis Lab
(a)
(e)
(b)
(f)
(d)
(c)
(g)
Figure 4-1 Switch Symbols
Toggle
Slide
Rotary
Push-button
DIP
Figure 4-2 Sample Switches
Fundamental Circuits Analysis Lab
Once we have made a particular switch type selection we should consider at a minimum, the
voltage and current rating of that switch. As an example the following miniature toggle switch has
the following characteristics: SPDT, ON-OFF, 5A-120V AC / 28V DC, 2A-250 AC, Initial
Contact resistance 10 mΩ [Note: That is milli lowercase m not Mega uppercase M.], Insulation
Resistance 1000 MΩ, Silver Plated Contacts. Notice that the current rating for the switch is
different depending on whether the switch will be operating in an AC or DC circuit. We will be
discussing this difference in a later experiment and for now we are only interested in the DC rating
because we will be using a DC power source. Another factor that affects the current rating of the
switch is what type of load the switch is controlling. If it is purely resistive we can use the specified
value. However if it is an inductive load (motor) or an incandescent lamp we need to de-rate the
value by 25% or 75% respectively. Most damage to switches is due to overheating which is caused
by too high a contact resistance and/or too large of a current following through the switch. Also
too high a voltage that causes arcing when the switch breaks the circuit can damage the switch
contacts. We can check a switch by measuring its on and off resistance using an ohmmeter. The
contact resistance in the on position should be very low… much less than one ohm while the off
position should show an extremely high resistance usually beyond the measurement capabilities of
the ohmmeter. Many digital ohmmeter will display this by either a flashing display, the letter OL, or
an ∞ sign.
Electromechanical relays are devices that complete or interrupt a circuit by physically moving
electrical contacts into contact with each other. A relay involves two circuits: the energizing circuit
and the contact circuit. The coil is on the energizing side and the relay contacts are on the contact
side. When a relay coil is energized, current flow through the coil creates a magnetic field. Whether
in a DC unit, where the polarity is fixed, or in an AC unit where the polarity changes 120 times per
second, the basic function remains the same: the magnetic coil attracts a ferrous plate, which is part
of the armature. One end of the armature is attached to the metal frame that is formed so that the
armature can pivot, while the other end opens and closes the contacts. Relay contacts are designed,
built and specified for the type of application for the relay. No single voltage and current rating
applies to a given set of contacts under all circumstances. In this laboratory experiment we will be
using a miniature 12V DC, DPDT relay. This relay can be plug directly into your protoboard.
Transistors have many uses, and one is the ability for bipolar or field effect transistors to turn
devices on and off. While there are limitations as to what the transistor can switch on and off,
transistor switches offer generally lower cost and substantial reliability over conventional
mechanical relays. Single transistor by themselves do not have a latching characteristic and when
the transistor input control signal is removed they will stop conducting. Latching is the
characteristic of memory and in this instance means that once the device is triggered or turn on it
will normally stay on even if the control signal is removed. A complete discussion of transistors
operation is beyond the scope of this lab manual and you will need to take an additional class in
solid-state devices. For this lab experiment we just need to know that when sufficient voltage and
current are applied to the Base (B-termial) of the transistor it will conduct current like a closed
switch through its Collector (C-terminal) and Emitter (E-terminal). When the voltage and current
are removed from the Base circuit the transistor will stop conducting between the C-E terminals.
Fundamental Circuits Analysis Lab
Pre-Lab
1. Review Ohms Law and the power formulas.
2. Using the Internet go to Mouser Electronics at: http://www.mouser.com/ or your
instructor may request that you visit another site. At the Mouser home page select
electromechanical for the products links area. Locate an Alpha Rotary switch that is
shorting, ¼-inches round shaft w/solder terminals, and has 4 poles and 3 positions.
Recorded the Mouser Stock Number of this switch: ______________
3. Using the Internet or a transistor manual located in the laboratory determine the three lead
designations for a 2N3904 or a 2N2222 NPN bipolar transistor.
4. Visit GlobalSpec at:: http://relays.globalspec.com and read their learn more information on
electromechanical relays.
Procedure
Section I
Switches
1. Reference the textbook or the Internet and complete Table 4-1.
2. Use an ohmmeter and measure the resistance of the miniature SPDT toggle switch labeled 1
in its position. Note: Position  is with the lever of the switch in the up position view
from the back. If the resistance reading indicates no continuity record the word “open” in
row 1 of Table 4-2.
3. Repeat step 2 for the switch in row 2 of Table 4-2.
4. Repeat step 2 for the switch in row 3 of Table 4-2. For this switch you will rotate the
selector shaft clockwise looking from the back and verify the resistance readings supports
good continuity for lugs A through D with respect to E.
5. Repeat step 2 for the switches in rows 4 and 5 of Table 4-2. For these switches position 
is with the button released or not pressed and position  is with the button pressed in.
6. Using the switches provided and with the B1 power supply off and disconnected, construct
the first circuit shown in Figure 4-3.
7. With the power supply still disconnected turn on the supply and adjust it for 12V DC and
set the power supply’s current limiter control to its 12:00 o’clock position. Turn the power
supply off and then connect it to the circuit. Set the SPST switch S1 in the circuit to
position 1, open and then turn the power supply on.
8. Measure the voltage across the switch terminal lugs and then across the L1 lamp. Record
your voltage measurements and the lamp’s state on or off in Table 4-3.
9. Close switch S1 by setting it to position 2 and repeat the above step.
10. Turn the power supply off and then replace SPST switch S1 with a PBNO switch as shown
in Figure 4-4. Turn the power supply on and record your results in Table 4-3 for the PBNO
switch. Position 1 is with the switch not depressed and position 2 is with the switch
depressed.
Fundamental Circuits Analysis Lab
11. Turn the power off and replace the PBNO switch with a PBNC switch then turn the power
supply on and record your results in Table 4-3 for the PBNC switch. Position 1 is with the
switch not depressed and position 2 is with the switch depressed.
12. Disassemble the circuit in Figure 4-4 and construct the circuit in Figure 4-5. Given that L1
is a 12 V DC, 40mA Lamp solve for the values of R1 and R2 such that R1 limits the series
circuit current to 10 mA and R2 limits the series circuit current to 20 mA. S1 is a four
position non-shorting rotary switch. When you have solved for the resistor values
determine the close %5 tolerance resistor values and then build and apply 12v DC to the
circuit. In S1’s position 1 the circuit is open and no current flows. In S1’s position 2 the
current is limited and the lamp glows dimly. In S1’s position 3 the lamps brightness
increases and finally in position 4 the lamp is at normal brilliance. Confirm the proper
operation of the circuit and then record voltage measurements across the resistors and
lamps in positions 2 and 3 in Table 4-3. In the Lamp state column comment on the
brightness of the L1 lamp. When you are finished with this step disassemble the circuit and
proceed to the Section I design tasks.
Section I Design Tasks
1. Using a DPDT ON-ON switch design a circuit that will reverse the direction of rotation of
a 12V DC miniature motor. Devise a method for determine motor shaft rotation. When
you have completed the design build and verify the proper operation of the circuit.
Demonstrate the circuit operation to your instructor. On a separate sheet of paper draw the
schematic circuit diagram and be sure and include reference designators and values for each
component. Briefly describe in written on the paper the basic operation of the circuit.
Instructor: ________________
2. Using as a minimum, two SPDT switches design and build a three-way switched circuit to
turn on a 12 V DC lamp. When you have completed the design build and verify the proper
operation of the circuit. Demonstrate the circuit operation to your instructor. On a
separate sheet of paper draw the schematic circuit diagram and be sure and include
reference designators and values for each component. Briefly describe in written on the
paper the basic operation of the circuit.
Instructor: ________________
Section II
Relays
1. Construct the circuit in Figure 4-6 and then apply 12V DC and 24 volts DC to the circuit
from the laboratory tri-power supply. Make sure that both current limit controls are at 12
o’clock. Operate S1 and verify that the relay closes and supplies power to the two series
wired lamps L1 and L2 .
Fundamental Circuits Analysis Lab
2. Do not disassemble the circuit and proceed to the Section II design task.
Section II Design Tasks
1. Using the circuit in Figure 4-6 redesign it using two momentary action pushbutton switches
so that one switch latches the lamps on while the other switch latches them off. You are
encouraged to work in teams and solicit help from the instructor. [Hint: Use the additional
contacts on the relay.] On a separate sheet of paper draw the schematic circuit diagram and be
sure and include reference designators and values for each component. Briefly describe in
written on the paper the basic operation of the circuit.
Instructor: ________________
Section III
Solid State Switch
1. Construct the circuit in Figure 4-7 and then apply 5V DC and 24 volts DC to the circuit
from the laboratory tri-power supply. Make sure that both current limit controls are at 12
o’clock. Operate S1 and verify that the transistor conducts and supplies power to the two series
wired lamps L1 and L2 .
Fundamental Circuits Analysis Lab
Post-Lab 4
Name: _____________________________________
Score: _______________
Date: _______________
Team: _______________
 Questions:
1. In your own words and using sketches explain what a shorting rotary switch is.
2. What does SP12P mean for a switch?
3. A ¼ HP, 48V DC motor is controlled by a SPST toggle switch. What should the minimum
current rating of the switch be for safe operation? (Hint: 1 HP = 746 watts)
4. List several advantages of using a switch/s and relay to control a circuit load.
5. Why would a non-shorting switch be the best choice in Figure 4-5?
6. For a complete circuit how many wires are needed between two 3-way switches?
7. Name a disadvantage of using a solid-state switch.
8. What is a latching relay or switch?
9. Why is silver or a silver alloy used on many switch contacts?
10. What part of a switch is sometimes referred to as the common?
11. In section 1 - design task 1 how could the circuit be change to allow for a motor off state as
well as also selection rotational direction of the shaft?
Fundamental Circuits Analysis Lab
12. What is an advantage of a transistor or solid-state switch over an electromechanical relay?
13. Refer to Figure 4-7. Is the transistor Q1 acting as a latching device? Briefly explain your
answer.
14. In terms of push-button switches what does “NO” and “NC” represent?
15. List several disadvantages of using incandescent lamps over Light Emitting Diodes in
control panel displays?
 Conclusion:
Table 4-1
Component Identification
Switch
Label
Type
Number
Style
1
SPDT
Toggle
2
DPDT
Toggle
3
SP4P
Rotary
4
NOPB
Push-button
5
NCPB
Push-button
Schematic
Symbol
Fundamental Circuits Analysis Lab
Table 4-2
Switch Resistance Testing
Switch
Resistance
Label
Position 
Number
Resistance
Position 
Pictorial
Symbol
Position
1
Lug A to B = __________Ω
Lug A to B = __________ Ω
Lug B to C = __________Ω
Lug B to C = __________ Ω
Lug A to C = __________Ω
Lug A to C = __________ Ω
Lug A to B = __________ Ω
Lug A to B = __________ Ω
Lug B to C = __________Ω
Lug B to C = __________ Ω

Lug A
Lug B
Lug C

Back
View
Position
2
Lug A to C = __________ Ω
Lug A to C = __________ Ω
Lug D to E = __________ Ω
Lug D to E = __________ Ω
Lug E to F = __________ Ω
Lug E to F = __________ Ω
Lug D to F = __________ Ω
Lug D to F = __________ Ω
Lug D
Lug B
Lug E
Lug C
Lug F

Back
View
Lug B
Lug A to B = __________ Ω
3

Lug A
Lug C
Lug E
Lug B to C = __________ Ω
Lug A
Lug D
Lug A to C = __________ Ω
Back
View
Lug D to E = __________ Ω

4
Lug A to B = __________ Ω
Lug A to B = __________ Ω

Position
Lug A
Lug B
Back
View
Fundamental Circuits Analysis Lab
Switch
Label
Number
Resistance
Position 
Resistance
Position 
5
Pictorial
Symbol

Lug A to B = __________ Ω
Lug A to B = __________ Ω

Position
Lug A
Lug B
Bottom
View
Table 4-3
Circuit Measurements with 12V DC Power Supply
Circuit
Switch
Voltage Across
Voltage Across
Figure
Type
Switch S1
Lamp L1
Position 1
Fig. 4-3
SPDT
Position 2
Fig. 4-4
Position 1
PBNO
Position 2
Fig. 4-4
Position 1
PBNC
Position 2
Fig. 4-5
Position 2
Rotary
Position 3
Lamp
State
Fundamental Circuits Analysis Lab
S1
1
2
B1
12V
L1
12V
Figure 4-3
S1
1
2
B1
12V
Figure 4-4
L1
12V
Fundamental Circuits Analysis Lab
R1
OFF
1
2
S1
3
R2
B1
12V
COM
4
L1
12V
Figure 4-5
S1
B1
12V
4
6
8
13
11
9
1
L1
12V
L2
12V
Relay
16
Figure 4-6
B2
24V
Fundamental Circuits Analysis Lab
L1
12V
S1
L2
12V
R1
2.2k
C
B1
5V
B
E
Figure 4-7
Q1
2N2222 / 2N3904
B2
24V