SAVED BY THE BELL
“A personal conversation-ending device”
by
Mike Kukovec
Dave Nichols
ECE 345
T.A.: Lee Rumsey
May 5, 1999
Project Number 21
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ABSTRACT
This report documents the design and construction of a small, self-contained electronic
device that will emulate an incoming telephone call. The device can be connected between a
standard telephone and the incoming utility service without causing unwanted interference to
that service, and can be activated by a remote switch or wireless device. Power is supplied
from a standard wall outlet, and converted to required voltage levels internally. Device
characteristics allow the user to adjust the length of time between activation and initial ring,
the number of rings before deactivation, and the length of time before the telephone resumes
normal operation. This device will provide a convenient yet discreet way for the user to get
out of annoying conversations in an office (or similar) setting.
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TABLE OF CONTENTS
PAGE
1. INTRODUCTION…………………………………………………………….. 1
2. DESIGN PROCEDURES……………………………………………………. 2
2.1 Power Supply (MK, DN)…………….………………………………. 2
2.2 Ringer Signal Circuit (MK)…………..………………………………. 3
2.3 Transforming Circuit (DN)…………..………………………………. 4
2.4 Time Delay/Relay Circuit (MK)…..…………………………………. 4
3. DESIGN DETAILS…………………………………………………………… 7
3.1 Ringer Signal Circuit (MK)..…………………………………………. 7
3.2 Transforming Circuit (DN)..…………………………………………. 7
3.3 Time Delay/Relay Circuit (MK)..……………………………………. 8
3.4 Actuation Switch (MK)……………………………………………….. 10
4. DESIGN VERIFICATION……………………………………………………. 11
4.1 Power Supply (DN)…..………………………………………………. 11
4.2 Ringer Signal Circuit (MK)……………………………………………11
4.3 Transforming Circuit (DN)…………………………………………… 12
4.4 Time Delay/Relay Circuit(MK)………………………………………. 13
4.5 Actuation Switch (MK)……………………………………………….. 13
5. COST………………………………………………………………………….. 14
5.1 Labor (DN)……………………………………………………………. 14
5.2 Materials (DN)………………………………………………………… 14
6. CONCLUSIONS(MK)…………………………………………………………16
7. FIGURES (DN)……………………………………………………………….. 17
Figure 1 Block Diagram……………………………………………..17
Figure 2 Ringer Signal Circuit………………………………………17
Figure 3 Transforming Circuit………………………………………18
Figure 4 Time Delay/Relay Circuit…………………………………18
REFERENCES………………………..……………………………………… 19
APPENDIX ABBREVIATIONS……..…………………………………. 20
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1. INTRODUCTION
This report documents the design and construction of a small, self-contained
electronic device (hereafter referred to as the “ringer”) that will emulate an incoming
telephone call. Such a device would allow a person to get out of a lengthy or annoying
conversation, under the pretense of having to answer an “important” telephone call.
According to Consolidated Communications employee Tom Meyer [2], the signal
of an incoming call, which causes a telephone to ring, is 120Vac, 20 Hz. He further
described the ring signal as 3 seconds on, 6 seconds off. In order to prevent unwanted
interference, device operation must be completely isolated from the existing telephone
service. To accomplish these tasks, the project design was split into three modular
blocks: a ringer signal circuit; a transforming circuit; and a time delay/relay circuit.
The benefits of modular design are two-fold. First, it allows individual component
development and construction to proceed in parallel, allowing several goals to be
accomplished simultaneously. Second, circuit troubleshooting is simplified, as block
functions are tested individually before being integrated into the circuit as a whole.
Figure 1 shows the general flow of circuit operation (All figures appear at the end
of this report.). The power supply has outputs for both the transforming circuit and the
integrated circuit devices within the ringer signal and time delay/relay circuits. The ringer
signal circuit generates the ring “on/off” signal and the 20Hz square wave. The
transforming circuit provides the voltage level and rectification required for ring
operation, as well as isolation from the power supply ground. Each delay stage is part of
the time delay/relay circuit, and performs specific functions within the circuit, which will
be described in more detail later in this report. The ringer switching circuit, using the
ringer signal circuit as a trigger, switches the output of the transforming circuit on and
off, delivering power to the telephone bell coil.
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2. DESIGN PROCEDURE
2.1 Power Supply
A local 120V outlet will power the ringer. Power supply design could have taken
many forms. One method consists of an iron core ac-ac transformer with a 120V
primary and several ac secondaries. Depending on the primary/secondary turns ratio of
a particular tap, each secondary could then be rectified to an appropriate dc voltage
level. Potentially useful dc voltages include:
50V telephone “standby” mode
7.8V telephone “handset pickup” mode
12V integrated circuit power supply
5V various logic signals
A second option for voltage source requirements would be to have one rectifier
and several stages of dc-dc conversion. While components of the resulting dc-dc
converters would be very inexpensive compared to a complex iron core transformer, the
efficiency of the circuit would be very low and construction would become increasingly
complex. Another option would utilize a different power supply for each required
voltage, whether ac or dc. While this would eliminate grounding problems, this solution
would be both cumbersome and expensive. The most difficult signal to obtain would be
the 20Hz ac output. An ac to ac inverter using pulse width modulation was a potential
solution, but such a complex circuit would later prove unnecessary.
Simplifying one aspect of the circuit tends to complicate another. While dc-dc
converters are straightforward to build, each stage requires its own inductor and
becomes increasingly inefficient. However, after some investigation it was found that
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most of the voltage levels above were not needed for the circuit’s intended operation.
While normal telephone operation requires several dc voltage levels, the ringer circuit
itself has minimal requirements. A 12Vdc supply would be needed for the majority of
integrated circuit components, and a moderately high (> 50V) ac voltage level would be
needed for bell operation.
Since 12Vdc power supplies are readily available, one was chosen “off the shelf”
for this project. This 12Vdc would be used to power integrated circuits in the ringer
signal circuit and time delay/relay circuit (Refer to Figures 2 and 4.). One component in
the circuit, a wireless transmitter/receiver switch used for device actuation, required a
5Vdc source. Rather than add another stage to the power supply, this voltage was
generated by means of a simple voltage divider from the output of the 12Vdc source.
The high ac voltage needed for bell operation was obtained directly from the wall
outlet by tapping the primary side of the existing transformer within the 12Vdc power
supply. The transforming circuit (Figure 3) provided isolation from earth ground and
was used to produce the 20Hz signal required to cause the phone to ring. Testing
would later show that the 120Vac, 20 Hz signal could be varied to some degree. While
the 20 Hz aspect was critical, the voltage was not. The output voltage merely had to be
high enough to produce a “strong” ring.
2.2 Ringer Signal Circuit
Mechanical bell ringers such as those used in telephones have a resonant
clapper and should be driven with a frequency near 20 Hz [3]. It is therefore required to
have a circuit which generates a 20 Hz signal at all times, and is either applied (“on”) or
not applied (“off”) to the telephone by means of relay contacts or other switches. If the
signal were strong enough, it would emulate the natural ringing pattern of a telephone.
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The primary function of the project revolves around the operation of the ringer
signal circuit. This circuit is responsible for two tasks – generating both the “ring on/ring
off” signal and the 20 Hz aspect of the “ring on” signal. Several reference sources with
information on related telephone projects are available. One such source, Wenzel
Associates, Inc. [3] offers several communication circuits based on telephones,
including a telephone ringer. The circuit is comprised of several RC circuits integrated
with a multi-stage inverter, and will be described in greater detail later in this report.
2.3 Transforming circuit
The original design did not have a transforming circuit. Rather, the output of the
original ringer signal circuit was transformed directly. However, this method proved
ineffective, as a transformer that can handle low frequency, square wave signals was
not readily available. In order to utilize the materials available, an alternative method
was designed using the ringer signal as a trigger for a separate switching device. This
device would switch a rectified, transformed voltage on and off, powering the telephone
bell coil.
As alluded to earlier, a 1:1 ratio was not essential for operation. The final output
voltage of the transformer simply had to be high enough to produce a “strong” ring. The
higher the voltage, the stronger the ring. More important was the isolation from earth
ground provided by the transformer.
2.4 Time Delay/Relay Circuit
The ringer must be able to be connected between a standard telephone and the
incoming utility service without causing unwanted interference to that service. In order
to adhere to this need, ringer circuit operation must be completely isolated from the
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standard utility phone jack. Additionally, circuit operation should not take place
immediately upon activation, lest the victim realize they are being “shooed away.”
Finally, upon activation, the circuit should not just continue to ring. Circumstances may
not allow the user to tactfully answer the phone immediately, and numerous rings would
also be suspect to the victim, as a typical caller would hang up after several
unanswered rings.
Such needs are met be means of a multi-stage timer delay circuit. The outputs
of such a circuit would be used to trigger operation of several sets of relay contacts,
each associated with a different aspect of the overall circuit operation. Timer delay RC
circuit characteristics would allow the user to adjust the length of time between
activation and initial operation, the number of rings before operation ceases, and the
length of time before the telephone resumes normal service.
The first consideration for the time delay circuit is the amount of delay between
circuit actuation and initial operation. The length of the first delay stage must be long
enough to allow the user to avoid the embarrassment of being “discovered,” yet must be
short enough to accomplish the intent of the device in the first place, to end the
conversation.
An alternate design method of the first delay stage would provide several choices
to the user, so that they may be the judge of how much time they can afford to spend
before terminating the conversation. This is an attractive feature, both for product
marketability and convenience to the user. However, such flexibility of design could
also prove overwhelming for some users. In addition, the operation of the second and
third stages of delay are essentially referenced to the end of the first delay stage, not
the beginning. Changing the length of delay of the first stage would require changing
the lengths of the second and third stages as well. This obstacle could be overcome by
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means of a cascaded timer circuit, which would trigger operation of each successive
stage as the previous stage timed out.
One way to accomplish the intent of this alternate, while still maintaining the
simplicity and low cost of the basic circuit, was to leave the lengths of each delay time
adjustable, so that the user could determine what seemed to work best in most
situations. This is the method that was selected for this design.
The second stage of the timer delay circuit dictates how long the phone will “ring”
once circuit operation has been initiated. In this case, the length of delay determines
when the circuit will stop working instead of when it will start. The length of this delay
can be measured practically by multiplying the desired number of rings by nine seconds
and adding the length of time delay #1.
The third and final delay stage determines how long the circuit will remain
disconnected from the phone line once ringing operation has ceased. This length of
time is also user-adjustable, and should be set with enough time to get rid of the
unwanted conversationalist while not tying up the phone line and missing potentially
important, “real” telephone calls.
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3. DESIGN DETAILS
3.1 Ringer Signal Circuit
The ringer signal circuit, as mentioned earlier, is accomplished using a multi-
stage inverter circuit coupled with several RC circuits, as shown in Figure 2. The
integrated circuit used is a 4069 CMOS hex-inverter. A 74C04 hex-inverter is a direct
substitute. The first two inverter gates produce the “on/off” pulse train required to
emulate an incoming telephone call. The output of this stage is a square wave that is
“high” (12V) for 3 seconds, “low” (0V) for 6 seconds. Resistor values in this stage are
each 22M, with a .22F capacitor and a 1N4003 diode on the inverter outputs.
The second inverter stage turns the 3 second “high” into a pulse train with a
frequency of 20 Hz, while leaving the 6 second “low” unchanged. Resistor values for
this stage are 1M, with a .033F capacitor. The output of the triggering signal is sent
to the switching device, an IRF 640 MOSFET. This signal, connected to the drain of the
MOSFET, provides the telephone with a 0-170V, 20 Hz square wave for 3 seconds, and
0V for 6 seconds.
Only four of the six inverters on the chip are used. Pins 7 and 14 are connected
to ground and 12Vdc, respectively.
3.2 Transforming Circuit
The transforming circuit as shown in Figure 3, outputs 170Vdc. For packaging
and weight considerations, a transformer of a small physical size is desired. 120Vac
from the wall outlet was applied to a 1:1, 115V 35VA isolation transformer. This
transformer was sufficient in size due to the low current requirements of the circuit, and
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isolated the earth ground of the wall outlet from the common ground used throughout
the rest of the circuit.
The output of the transformer is applied to the full-wave bridge rectifier, made
from four MUR440 diodes. These diodes were chosen for their high voltage rating
(400V). The maximum expected voltage across the diodes at any time is 340V (+170V
to –170V). The output of the rectifier should be roughly 168Vdc, due to the 1V drop
across each diode.
A large, 820F capacitor was placed in parallel across the output of the rectifier
to filter the rectified voltage. No specific ripple requirements were necessary, but a
smooth output is desired. This voltage was then applied to the drain of the MOSFET,
which was used as the switching device.
3.3 Time Delay/Relay Circuit
The time delay/relay circuit (refer to Figure 4), powered by 12Vdc, utilizes a
standard 555 timer IC, specifically the Motorola MC1455 chip. A momentary contact to
ground, provided by the actuation switch, begins the timing cycle for all three stages of
the time delay circuit. Each stage of the circuit provides a 12Vdc signal to its associated
relay, keeping the relay energized throughout the entire timing cycle. The length of time
delay is adjusted my changing the RC circuit time constant value. “C” remains constant
at 100F, while “R” varies in value from 200k-800k. A brief table of time delays and
their corresponding R and C values are shown below in Table 1 [1]. For the purpose of
this device, the delay lengths associated with a 10F capacitor were too short to be
practical. A 100F capacitor was used in all three time delay stages. Actual resistance
values and associated delay lengths are shown in Table 2.
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Each delay stage is powered by the 12Vdc supply. A .01F capacitor was used
in the circuit to prevent false triggering, while a diode in parallel with the relay absorbs
any voltage generated by the relay coil when switched off.
TABLE 1: TYPICAL TIME DELAYS (seconds)
R (Ohms) C = 10F C = 100F
100K 2 16
220K 3 33
470K 6 70
1M 15 175
TABLE 2: ACTUAL TIME DELAYS (seconds)
Delay Stage R (Ohms) Time
1 165K 24
2 463K 65
3 800K 95
Delay stage #1 opens the normally closed contacts between the output of the
rectifier and the rest of the circuit. After the 24 second delay, this relay de-energizes
and the contacts re-close, allowing power to be sent to delay stage #2 contacts and
hence to the MOSFET switching device. Delay stage #2 closes the normally open
contacts between the output of delay stage #1 and the MOSFET. These contacts will
remain closed for 65 seconds. The first 24 seconds of this stage are insignificant, since
the contacts of delay stage #1 are still open. Once they close, the rectifier output is
directly connected to the MOSFET, and the bell coil is energized; the telephone rings.
During the 40 seconds in which both sets of contacts are closed, the phone is allowed
to ring 5-6 times.
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After delay stage #2 times out, the contacts resume their normally open position,
and power to the telephone bell coil is disconnected. Normal telephone operation has
not yet resumed, as delay stage #3 is still active. Stage #3 has three sets of contacts.
The first set is responsible for isolating ringer operation from the telephone utility
service. These contacts essentially act as a selector switch for the telephone input.
The normally open contacts are connected to the ringer output, while the normally
closed contacts are connected to the telephone utility. This ensures that, even in the
event of circuit power loss, normal telephone operation will be available to the user.
The other set of delay stage #3 contacts are normally open. When the relay is
energized, these contacts close, and the 12Vdc power supply output is sent to the
ringer signal circuit. The ringer signal is therefore generated for the entire duration of
circuit operation, though due to delay stages #1 and #2 high voltage is not necessarily
present at the MOSFET switching device.
3.4 Actuation Switch
Delay stage actuation is accomplished by means of a momentary contact
pushbutton switch, which connects the 555 chip to ground. Other possible triggers
include photoelectric sensors, voice or sound recognition circuits, even clock settings
(for victims who visit daily at a consistent time).
A wireless circuit was added as an alternate to the project, as a means of
actuating the ringer from a remote location. The remote transmitter requires a 12V
L1028 battery for operation, while the receiver is powered by the 5Vdc output of the
voltage divider circuit. The entire unit was purchased “off the shelf” as a wireless
doorbell from a local department store for under $15. No aspect of the doorbell circuit
(other than the initiating signal) was used for this project.
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4. DESIGN VERIFICATION
4.1 Power Supply
Testing for the power supply circuit was limited, as the circuit was of the pre-
packaged “off the shelf” variety. Simple readings were taken to ensure that the 12Vdc
and 120Vac outputs were accurate. More important was to ensure that the common
ground point of the ringer circuit was completely isolated from the earth ground of the
power supply input. This test was performed with a simple ohmmeter, to ensure there
was infinite resistance between the third prong of the wall plug and the common ground
of the ringer circuit.
The 12Vdc output was connected to the ringer signal circuit and time delay/relay
circuit to ensure each component had the required 12V. These measurements were
made with a voltmeter. Likewise, the voltmeter was used to verify the output of the
voltage divider circuit, which was to produce 5Vdc to power the wireless receiver. 150
and 100 resistors were used for this circuit to produce a 4.8Vdc output, which was
sufficient for wireless circuit operation.
4.2 Ringer Signal Circuit
Verification of the ringer signal circuit design was accomplished with an
oscilloscope. A tabulation of measurements is shown below in Table 3.
The output to the ringer switching circuit was just under the desired 20 Hz, but
close enough not to impede circuit operation. The output frequency could have been
fine tuned using a potentiometer instead of fixed resistance values. While this would be
acceptable in the design phase, construction grade components would have a similar
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margin of error. It would not be cost effective to replace fixed resistors with
potentiometers in a mass produced device.
TABLE 3. RINGER SIGNAL CIRCUIT OUTPUTS
Inverter Pin Output Description
1 1 5Vdc
1;2 2;1 0V for 6 sec, 12V for 3 sec
2 2 0V for 3 sec, 12V for 6 sec
3 1 12V for 6 sec, 0-12V, 18.72 Hz for 3 sec
3,4 2,1 0V for 6 sec, 0-12V, 18.72 Hz for 6 sec
4;5;6 2;1;1 12V for 6 sec, 0-12V, 18.72 Hz for 3 sec
5,6 2;2 0V for 6 sec, 0-12V, 18.72 Hz for 6 sec
4.3 Transforming Circuit
Transforming circuit testing was relatively straightforward. The first test
investigated what voltage level was required to produce the desired volume of the ring
and therefore, the required turns ratio of the transformer. A variac connected to a
120Vac input was used to vary the voltage sent to the bell coil of the telephone. As
output voltage from the variac was increased, the bell ring strength increased. The
maximum voltage level of the rectified variac output was 168V. The minimum rectified
output that would produce an acceptable ring was 50V.
Tests were then performed on an actual telephone, to determine the maximum
voltage that could be used without causing damage. The tested unit showed no
resulting damage even at the maximum voltage available to the circuit, 170V. Results
of this test allowed a standard 115V isolation transformer to be used as the ringer power
supply.
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4.4 Time Delay/Relay Circuit
Time delay stages are actuated by a momentary contact switch, which connects
the 555 chip to ground. This operation was verified using both the wireless
transmitter/receiver circuit and the hardwired switch using an ohmmeter. Once the
circuit was actuated, voltage measurements at all three relays should and did read 12V
for the duration of each stage. That is, relay stage #1 read 12V for 24 seconds, stage
#2 read 12V for 64 seconds, and stage #3 read 12V for 95 seconds. Operation of each
relay was also verified by an audible “click” as contacts changed positions.
Potentiometers in each delay stage were adjusted until the above delay times were met.
To ensure contacts were being made or broken, further readings were taken on the
output side of each relay. This was vital in the case of relay stage #3, to ensure the
ringer circuit was in no way connected to the telephone utility service.
4.5 Actuation Switch
Actuation switch operation was tested using both the hardwired switch and the
wireless circuit. In both cases, the switching action provided the required momentary
ground to each of the 555 chips. Momentary contact was essential. If the switch were
to remain closed, circuit operation would not perform as required.
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5. COSTS
5.1 Labor
Design costs for labor are calculated based on the total recorded hours spent on
the project (132), using an average electrical engineer’s starting salary of $46,000/year.
Using a standard work-year of 2080 hours based on (50) 40-hour weeks and (2) weeks
paid vacation, the equivalent of a $46,000 salary is $22.12/hr. Considering a capital and
overhead factor of 1.5, total labor cost for the design and construction of this project is:
$22.12/hr * 132 hr * (1 + 1.5) = $7299.60
5.2 Materials
The parts as listed in Table 4, below, include only those required to construct one
operating device. Burned out or otherwise expended components are not included in
this list. Additionally, prices as shown are considered retail. Mass production costs
would be significantly lower.
Total costs for design, development, construction and testing amount to:
Labor: $7299.60
Parts: $103.08
Total: $7402.68
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TABLE 4. INDIVIDUAL COMPONENT COSTS
Quantity Part Description Cost (each)
1 Transformer 1:1 isolation $12.11
6 Diode 1N914 $0.10
2 Diode 1N4003 $0.10
4 Diode MUR440 $0.85
3 Capacitor .01F $0.10
1 Capacitor .033F $0.10
1 Capacitor .22F $0.18
3 Capacitor 100F $0.15
1 Capacitor 820F,200V $0.31
3 Resistor 10K $0.13
2 Resistor 1M $0.13
3 Resistor 22M $0.13
3 Potentiometer 1M $0.13
2 Relay 12V, 1P 220V $7.44
1 Relay 12V, 1P 220V $12.76
1 MOSFET IRF 640 $0.47
1 Hex inverter 74C04 or 4069 $0.47
3 Timer MC1455 $0.50
1 Push button Momentary $1.50
switch
1 Transmitter/ $13.75
receiver
1 Power supply 12Vdc $34.80
2 Receptacles Telephone $1.91
Parts total = $103.08
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6. CONCLUSIONS
The functionality of this device was tested and proven, and several inquiries have
been made regarding the availability of additional prototypes. This alone shows that
such a product is extremely marketable. Actual production costs would be far below the
estimates included in this report. Further consideration to alternative design methods
could reduce costs and improve functionality even further.
Packaging issues have not yet been addressed, but even in its prototype stages
the device is non-obtrusive. The entire circuit could easily be located under a desk or in
some other inconspicuous location. The already compact circuit could be reduced in
size and weight even further with a smaller power supply, and by using MOSFET
switches in place of relays. This would also significantly reduce the cost of the final
product, as relays were typically twelve times more expensive than the cost of a
MOSFET substitute.
Design alternates such as wireless operation and multiple-length time delays are
easily implemented. In addition, actual output voltage can be reduced and still provide
a strong ring to the bell coil. This would greatly reduce the size, weight and cost of the
isolation transformer.
In all, the project met or exceeded the goals defined in the original proposal.
Further development is necessary to bring the prototype to a product that is both
marketable and cost effective to manufacture.
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7. FIGURES
Fig. 1. Block diagram.
Fig. 2 Ringer signal circuit.
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Fig. 3. Transforming circuit.
Fig. 4. Time delay/relay circuit.
19
REFERENCES
[1] M. M. Forrest III, Engineering Mini-Notebook. Fort Worth, TX: Tandy Corporation,
1997, pp.9.
[2] T. Myers, Illinois Consolidated Communications. Mattoon, IL:
[3] Wenzel Associates, Inc.; http://204.251.59.186/Notebook/telephone.htm
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APPENDIX 1. ABBREVIATIONS
Unit or term Abbreviation
Alternating current ac
Direct current dc
Hertz Hz
Integrated circuit IC
Volt V
Volt Amp VA