Ramsey Electronics SP1 User manual

HANDS-FREE
SPEAKERPHONE-
INTERCOM KIT
Ramsey Electronics Model No. SP1
How many times have you played acrobat juggling the phone
on your shoulder while looking up something or while working at
the bench? And, wouldn’t it be nice to be able to answer a call
from anywhere in your shop or garage? This handy
Speakerphone will do the trick and doubles as a hands-free
intercom too!
• Completely Hands-free; talk and listen at will
• Needs no battery - runs off the phone line
• Sensitive pick up throughout the room
• Excellent speaker volume
• Doubles as a hands-free intercom too! Connect as many units as
desired across 2 conductor wire
• Informative manual answers questions on theory, hook-ups and
uses - enhances resale value too!
• Includes our case for a finished ‘Pro’ look.
• Clear, concise assembly instructions carefully guide you to a
finished kit that works FIRST time!

SP-1 •2
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM25B FM Stereo Transmitter
• AM1 AM Transmitter
• TV6 Television Transmitter
• FM100B Professional FM Stereo Transmitter
• STC1 Stereo Transmitter Companion
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Shortwave Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70A Speech Scrambler
• MX5, MX-10 Mixers
• MD3 Microwave Motion Detector
• ECG1 Heart Monitor
• LABC1 Lead Acid Battery Charger
• PG13 Plasma Generator
• SHA1 Stereo Headphone Amplifier
RAMSEY AMATEUR RADIO KITS
• DDF1 Doppler Direction Finder
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• VLF1 Low Frequency SWL Converter
• CPO3 Code Practice Oscillator
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
SP1 SPEAKERPHONE KIT INSTRUCTION MANUAL
Ramsey Electronics publication No. SP1 Revision E
First printing: January 1995
COPYRIGHT 1994 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.

SP-1 •3
SP1 HANDS-FREE
SPEAKERPHONE-
INTERCOM KIT
Ramsey Publication No. SP1
Price $5.00
TABLE OF CONTENTS
Introduction to the SP1...................4
Parts list for the SP1 ......................5
Parts layout diagram ......................6
Assembly instructions.....................7
Schematic diagram.......................12
Initial testing..................................15
Optimizing your set-up .................16
Understanding Speakerphones....17
Optional battery operation............18
Intercom operation .......................21
Warranty.......................................23
KITASSEMBLY
AND INSTRUCTION MANUAL FOR

SP-1 •4
INTRODUCTION TO THE RAMSEY SP1 SPEAKERPHONE:
There are, of course, all kinds of telephone accessories. Also just as various
"telephone amplifier" schemes have been around for years, the name
"speakerphone" is now a generic part of today's telephone vocabulary,
referring to hands-off talk-and-listen communicating. If we look at catalogs or
in phone stores, we also see that true hands-free speakerphones command
a handsome price or else may be designed only as accessories requiring
connection with a separate telephone.
The Ramsey SP1 Speakerphone gives you full, state-of-the-art
speakerphone technology...with a difference. First, it is a complete stand-
alone telephone for incoming calls. Use some other phone or dialer right
along with it only if you have a frequent need to initiate those unique
speakerphone-style calls. Otherwise, you can use your SP1 to answer all
incoming calls, handling them while you work or play. The SP1 is designed to
be practical and understandable, fun to build and to customize for your own
needs and decor.
The Ramsey Speakerphone is designed to let you stay in charge of your
time...on a budget. Think about the difference between that call you wish or
plan to place...and all those other calls that you ANSWER, whether you
wanted or expected them or not.
A pair of SP1's also makes a great hands-free INTERCOM system for which
you’ll imagine dozens of practical applications. You’ll figure out how to set
them up so they can serve as intercoms AND convenient speakerphones.
The kit style of the Ramsey SP1 lets you choose the type of enclosure and
the size of speaker for this very personalized call-answering or intercom
system. You will build a quality system around a high-tech Motorola IC, and
you'll never have to discard a plastic mystery box that has quit working for
you. The Ramsey SP1 Speakerphone PC board and circuit design are solid,
rugged applications of today's IC-based telephoning technology.
You may wish to read ahead and learn how to use the SP1 and how it works
before beginning actual kit construction. Build and learn about the Ramsey
SP1, and no other electronic telephone will ever again seem so intimidating
or mysterious!

SP-1 •5
PARTS SUPPLIED WITH SP1 KIT
CAPACITORS

1 100 pF disc [marked 100 or 101] (C1)
 4 .01 uF disc [marked .01, 103 or 10nf ] (C9,10,13,14)
 2 .05 or .047 uF disc [marked .047 or 473 or .05 or 503] (C3, C18)
 1 .1 uF disc [marked .1 or 104] (C28)
 8 .47 uF electrolytic (C2,C4,C6,C7,C8,C17,C22,C23)
 7 2.2 uF electrolytic (C5,C11,C15,C19,C20,C21,C29)
 2 47 uF electrolytic (C16,C26)
 1 220 uF electrolytic (C24)
 3 470 uF or 330 uF electrolytic (C12,C25,C27)
RESISTORS
 1 100 ohm [brown-black-brown] (R10)
 2 270 ohm [red-violet-brown] (R4 alternate R10)
 1 820 ohm [gray-red-brown] (R8)
 1 1K ohm [brown-black-red] (R2)
 4 4.7K ohm [yellow-violet-red] (R3,R9,R13,R19)
 4 10K ohm [brown-black-orange] (R1,R6,R14,R20)
 2 47K ohm [yellow-violet-orange] (R7,R12)
 4 100K ohm [brown-black-yellow] (R11,R15,R16,R17)
 1 68K ohm [blue-gray-orange] (R5)
SEMICONDUCTORS
 1 MC65118 28-pin DIP speakerphone IC (U1)
 1 LM386 8-pin DIP audio amplifier IC (U2)
 1 6.2 volt zener diode [gray body with black band] (D5)
 6 1N4002 black epoxy rectifier diode (D1,D2,D3,D4,D6,D7)
OTHER COMPONENTS
 1 miniature microphone element (MK1)
 2 1000-to-8 ohm audio transformer (T1,T2)
 1 2K,5K or 10K potentiometer (R18)
 2 PC mount DPDT push-switch (S1,S2)
HARDWARE AND MISC.
 1 SP1 printed circuit board
 1 28-pin DIP socket for U1
 1 subminiature phone/speaker jack (J1)
 1 modular telephone line jack (J2)
 1 9 volt battery snap connector (battery use is optional)
 1 5" hookup wire (optional connection "X" to "X")
REQUIRED, NOT SUPPLIED
 1 Modular telephone cord (your choice of length)
 1 8-ohm speaker
OPTIONAL: Ramsey CSP1 case and knob kit

SP-1 •6
SP-1 PARTS LAYOUT DIAGRAM

SP-1 •7
KIT BUILDING TIPS:
Use a good soldering technique - let your soldering iron tip gently heat the
traces to which you are soldering, heating both wires and pads simultaneously.
Apply the solder on the iron and the pad when the pad is hot enough to melt the
solder. The finished joint should look like a drop of water on paper, somewhat
soaked in.
Mount all electrical parts on the top side of the board provided, unless
otherwise specified. This is the side that has no traces or pads on it.
Electrical part installation - when parts are installed, the part is placed flat to the
board, and the leads are bent on the backside of the board to prevent the part
from falling out before soldering. The part is then soldered securely to the
board, and the remaining lead length is clipped off.
SP1 ASSEMBLY INSTRUCTIONS:
We have a twofold "strategy" for the order of the following kit assembly
steps. First, we install parts in physical relationship to each other, so there's
minimal chance of inserting wires into wrong holes. Second, whenever
possible, we install in an order that fits our "Learn As You Build" Kit-building
philosophy.
FOR EACH PART OUR WORD "INSTALL" ALWAYS MEANS THESE STEPS:
1. Pick the correct part value to start with.
2. Insert it into the correct PC board location.
3. Orient it correctly, which means: Please follow the PC board and
the written directions for all parts where there's a right way and a
wrong way to solder it in. (Diode bands, electrolytic capacitor
polarity, transistor shapes, dotted or notched ends of IC's, and so
forth).

SP-1 •8
4. Solder all connections unless directed otherwise. Use enough
heat and solder flow for clean, shiny, completed connections.
Don't be afraid of any pen-style soldering iron having enough heat
to damage components.
5. Trim or "nip" excess wire lengths after soldering.
 1. Install S1, the on-off switch, simply press it into position and solder all
six points. This switch is the equivalent of the telephone "hook"or handset
button. Incoming calls are answered by pressing S1.
 2. Similarly, install S2, the Mute switch. This switch can turn off the
microphone circuit without breaking the phone connection, providing a
privacy or "hold" function.
 3. Install R18, the volume control. It may have three of five soldering
points, with the optional outer two tabs offering additional mechanical
rigidity. Solder all points.
 4. Examine J2, the modular telephone jack. Press its pins into their
correct PC board location. Notice that the outer support posts are plastic
and are not to be soldered. If you anticipate heavy use of this jack
(frequent cord changing, moving, etc.) use a dab of glue to secure J2
before soldering. Solder all 4 connections for modular jack J2. If you look
closely, you'll see that only two pins are actually used in the circuit.
 5. Examine the subminiature jack (J1) supplied for speaker connection. It
fits into the PC board position easily. Notice that the tab nearest to the
edge of the board is connected to the common ground of the circuit. The
other two tabs are connected together to the SP1's speaker audio output.
You have a choice of installing J1 supplied or your own speaker wire,
depending on your intended application. Install J1 or a suitable length of
your own speaker wire.
 6. Examine the audio transformers, T1 and T2. Notice the red marking on
one side of the top insulator on each unit. These marked sides must be
pointed toward the center of the board, as shown by the dark sides for T1
and T2 on the PC board drawing. For each transformer (they are identical
units), a total of six solder connections should be made. The outer 2 tabs
are soldered to common ground for mechanical rigidity. Install
transformers T1 and T2.
Completion of your SP1 requires installing over 20 "jumper wires," designed
JMP1, JMP2, etc. These simple wire bridges are formed from scrap bare wire
snipped from resistors, capacitors or diodes. Unless you have a handy supply
of bare, tinned wire, be sure to save all cuttings for these jumper wires.
Jumper wires allow a single-sided PC board to have the benefits of a double-
sided PC board without the extra cost.
 7. A choice to make:

SP-1 •9
 A) If you plan to use your SP1 as a telephone speakerphone, install zener
diode D5 (small gray body with black band). The banded end marks the
cathode and must be oriented as illustrated.
 B) If you plan to use your SP1 only as a duplex intercom, do not install D5.
Place it in a safe place for possible future use.
 8. The six 1N4002 style black epoxy diodes are installed in upright or
vertical position. Correct orientation of the banded cathode ends is
important, so always install the diode body in the hole marked by the circle
on the PC board illustration. The banded or cathode end should face up.
The diodes may come with straight leads or pre-bent, ready-to-insert leads.
Install diode D1.
 9. Install D2, another black epoxy diode.
 10. Install D3, another black epoxy diode.
 11. Install D4, another black epoxy diode.
 12. Install D6, another black epoxy diode.
 13. Install D7, the last black epoxy diode.
 14a. The method for installing the microphone to face the
front panel and therefore requires soldering the MK1
element's terminals to three short bare wires (nipped from a
previously installed component), so that the three wires are
at right angles to the MK1 lugs. Notice that one MK1 lug is
common to its metal case and must be oriented toward the side of the
board so that it is connected to the common ground plane. You may wish to
do both ends of this connection first to hold MK1 in place to do the other
connections.
 14b. If the mike element has only two pins, install a bare wire jumper from
"D" to "E" and omit C5 (Step 16). Study the schematic diagram for any
clarification needed.
 14c. Install mike element MK1.
 14d. Solder a wire loop to secure the mike, using the 2 PC board holes on
each side of the microphone.
 15. Near MK1, install C9, .01 uF (marked .01 or 103 or 10 nf).
 16. Install C5, a 2.2 uF electrolytic. This is the first of 21 electrolytic used
in your SP1. Be sure to orient the (+) side as shown on the PC board
drawing. Note on electrolytic capacitors: Ordinarily, the negative (-) side is
clearly marked by a heavy stripe and the "-" sign. However, the positive (+)
side may be marked instead, especially on low values such as the eight .47

SP-1 •10
uF units used in this circuit. Simply make sure that correct polarity is
followed for all electrolytic capacitors.
 17. Install C7, a .47 uF electrolytic.
 18. Near the volume control, install R19, 4.7K (yellow-violet-red).
 19. Install R1, 10K (brown-black-orange).
 20. Near S1, install electrolytic C26, 47 uF.
 21. Install R16, 100K (brown-black-yellow)
 22. Install R3, 4.7K (yellow-violet-red).
At this point, we will begin installing jumper wires near parts already installed, if
you run short of useful wire lengths during assembly, simply skip that jumper
step and go back to it after you've built up your scrap wire supply. Jumpers act
as electronic "bridges" carrying signals over PC board traces underneath.
 23. Near R3, install JMP11.
 24. Install JMP5.
 25. Install JMP7 (between R16 and S2).
 26. Install C2, .47 uF electrolytic. Watch polarity.
 27. Install C3, .05 or .047 uF (marked .047 or 473 or .05 or 503).
 28. Install JMP6.
 29. Install JMP12.
 30. Install C13, .01 uF (marked .01 or 103).
 31. Install R8, 820 ohms (gray-red-brown).
 32. Install R4, 270 ohms (red-violet-brown).
 33. Install R12, 47K (yellow-violet-orange).
 34. Install C14, .01 uF (marked .01 or 103 or 10 nf).
 35. Install JMP15 (near T1).
 36. Install JMP18.
 37. Install C10, .01 uF (marked .01 or 10 nf or 103).
 38. Install R2, 1K (brown-black-red). Install the following jumpers.
 39. JMP14.

SP-1 •11
 40. JMP13.
 41. JMP16.
 42. JMP10.
 43. JMP9.
 44. JMP8.
 45. Install C12, 470 uF electrolytic, watch polarity.
 46. Install R6, 10K (brown-black-orange).
 47. Install R7, 47K (yellow-violet-orange).
 48. Install C4, .47 uF, watch polarity.
 49. Install C8, .47 uF, watch polarity.
 50. Install C1, 100 pF. (marked 100 or 101).
 51. Install R5, 68K (blue,gray,orange).
 52. Install JMP4.
 53. Install JMP17.
 54. Install R10, 100 ohms (brown-black-brown). Note: if a battery will be
used, for intercom or because of low phone line voltage, R10 should be 270
ohms (red-violet-brown).
 55. Install JMP19.
PROGRESS SUMMARY: The schematic diagram makes it clear that building
the kit consists largely of providing the supporting components for the many
functions of the MC65118 IC. Much of that has been done already. This would
be a good time to double-check the correctness of parts selection and
orientation. Next, we'll build up the LM386 speaker amplifier circuit, install U1
and a few more components, and we'll be ready for testing.
 56. Install R13, 4.7K (yellow-violet-red).
 57. Install C27, 470 uF, with correct polarity. C27 couples the output of U2
the speaker.
 58. Install C17, .47 uF with correct polarity.
 59. Install C29, 2.2 uF, with correct polarity.
 60. Install C28, .1 uF (marked .1 or 104).
 61. Audio amp U2, IC LM386 is soldered directly to the PC board with the

SP-1 •12
SP1 SCHEMATIC DIAGRAM

SP-1 •13
notched/ banded end facing the rear of the board as illustrated. There is
no need to use a socket, but if you do, use care in soldering the socket
and inserting the chip.
 62. Install U2, 8-pin audio amplifier IC LM386.
 63. Install C25, 470 uF, with correct polarity.
 64. Install C24, 220 uF, with correct polarity.
 65. Install C22, .47 uF, with correct polarity.
 66. Install R9, 4.7K (yellow-violet-red).
 67. Install R17, 100K (brown-black-yellow).
 68. Install R15, 100K (brown-black-yellow).
 69. Install JMP1.
 70. Install JMP3.
 71. Insert the 28-pin DIP IC socket into its PC board position. Check and
make sure that all 28 pins are visible on the solder-side of the board.
Slightly bend the 4 outermost pins to lock the socket in place. Make sure
the socket is flush against the top of the board. Solder all 28 pins with
clean connections and no solder bridges.
 72. Install R11, 100K (brown-black-yellow).
 73. Install C16, 47 uF, observe polarity.
 74. Install C11, 2.2 uF, observe correct polarity.
 75. Install C15, 2.2 uF, observe correct polarity.
 76. Install C23, .47 uF, observe correct polarity.
 77. Install C19, 2.2 uF, observe correct polarity.
 78. Install C20, 2.2 uF, observe correct polarity.
 79. Install C21, 2.2 uF, observe correct polarity.
 80. Install C6, .47 uF, observe correct polarity.
 81. Install jumper wire JMP2.
 82. Install another jumper wire JMP20.
 83. Install C18, .05 or .047 uF (marked .05 or 503 or .047 or 473).

SP-1 •14
 84. Install R14, 10K (brown-black-orange).
 85. Install R20, 10K (brown-black-orange).
 86. If you have not done so already, install U1, MC65118 in its previously
installed socket . Do this with care and make sure that all 28 IC pins make it
into their socket holes. (It's quite easy to get a pin bent back under the IC
body as well as just missing a socket hole.) Also be sure to place the IC so
that the dotted or notched end lines up with the dot or notch on the PC
board silkscreen. If necessary consult the Parts Layout Diagram for correct
placement.
Take a break, clean up the workspace a bit, and then give your work a good
double check, looking especially for the following:
• Correct orientation of parts?
• Any missed solder connections?
• Untrimmed wires bent back on another connection?
INITIAL TEST: SPEAKERPHONE MODE
1. Solder a Jumper Wire from point "A" to point "C" (near D3).
2. Set both S1 and S2 to their "out" or off position. Set the volume control to a
mid-range point.
3. Next, test the modular phone cord you intend to use with the SP1, even if it is
brand new. Do this simply by trying it on another telephone. Then, connect the
cord between the SP1's J2 and a working, correctly-wired phone jack.
(Defective modular plugs, cords or jacks are a major cause of telephone
malfunction.)
4. Connect a speaker in accordance with Assembly Step 5. For decent audio
quality, use a speaker at least 3" in diameter. A properly-enclosed speaker will
sound better and louder than a speaker lying naked on a workbench.
5. Press S1 to the "in" (on) position. You should hear a normal dial tone. Adjust
volume control as desired. If you do not get a dial tone, recheck all your work,
starting with the phone line and speaker wires.
6. With S-1 still on, tap the microphone, and you should hear the sound in the
speaker.
7. Speak in a normal tone of voice near the microphone, and the dial tone

SP-1 •15
should drop out when you speak and return when you are quiet.
8. With S2 (Mute/Hold) pressed IN, you should still hear the dial tone, but the
microphone should not respond to even a loud yell.
9. Turn S1 OFF; this is the equivalent of putting a phone handset back "on the
hook." If everything above checks out, you're ready for your first Speakerphone
conversation. Ask someone to call you. When you hear the ring (on any other
phone), you can answer the call by pressing S1. You should be able to carry on
a normal chat while you are anywhere in an average-sized room. Be aware that
since the speakerphone is switching between "receive" and "transmit" there will
be a short delay or "pumping" effect as you move from talking to listening. Don't
forget to "hang up" when you are finished.

SP-1 •16
INITIATING SPEAKERPHONE CALLS:
While the MC65118 IC makes provision for adding a DTMF tone dialer, the
simplest and least expensive approach rate an ordinary telephone set as part of
your Speakerphone installation. The average electronics buff is well aware that
suitable phones are abundant at negligible cost, often with memory and other
handy features. Simply connect the auxiliary phone and your SP1 to the phone
line with a commonly-available dual-modular adapter, or use a dual wall jack,
etc. Use the auxiliary phone for dialing, incoming rings and situations where
you prefer a handset. Enjoy the Speakerphone for efficient, hands-free
communication. Be sure to hang up the auxiliary phone after turning on your
SP1 Speakerphone.
RINGER CIRCUITS:
The SP1 does not include a ringer circuit for the simple reason that the average
home, shop or office already has plenty of ringer noises whenever there are
phone calls. If you are a telephone electronics enthusiast and really want a
built-in ringer circuit, it is up to you to understand and comply with telephone
company ringer circuit equivalency requirements.
OPTIMIZING YOUR SPEAKERPHONE SET-UP:
There are three variables to be considered in any line-powered Speakerphone
installation:
1. Positioning of speaker in relation to the microphone.
2. Your distance from the telephone switching complex which supplies line
voltage. (If the voltage is marginal because you are on a distant point of the
phone company "loop," supplementary battery power may be needed for this
or any other Speakerphone.
3. Basic "talk power" of the party on the other end. Let's consider the three
details. First, if your Speakerphone works perfectly from the moment you set
it up, then you are in great shape. Otherwise, consider the following:
A. In general, some degree of acoustical separation must be provided
between microphone and speaker. Put simply, they should not "point" at
each other, or there will be some form of oscillation or erratic operation.
Some experimenting may be needed. See also the section, Understanding
Speakerphone Operation.
B. Usually, telephone line voltage is sufficient for good SP1 performance.
However, the available voltage is determined by the distance of your
location from the central office and local factors. The easiest way to
determine if a supplementary battery is needed is just to try it out. Please
study the section Installing Optional Battery before making any type of

SP-1 •17
battery connection. If the battery improves performance, then continue
using it for that particular location. It is switched in and out of the circuit by
the "B" section of S1.
C. Finally, you may encounter situations where the person on the other
end just is not speaking loud enough for positive SP1 switching between
receive and transmit. This can be caused either by the other party's use of
a poor-quality phone or just because they are speaking too softly. You may
need to ask them to try a different phone, or to just speak up or to speak
more directly into their handset microphone. Or, redial for a better
connection.
UNDERSTANDING SPEAKERPHONE OPERATION:
Because we take telephones so much for granted, it is easy to not care or
appreciate the many functions and features of the SP1 and the MC65118 IC.
That's OK: if it's working fine, then just enjoy it and don't worry about how it
works. However, since its operation characteristics will seem different from a
conventional handset telephone, you may get more satisfying use from your
speakerphone if you have at least some grasp for what it is accomplishing.
The easiest way to appreciate the functions of the MC65118 is to go through
the exercise of designing a two station speakerphone system in your head or on
a sketch pad. Both stations must have speakers and sensitive microphones and
be interconnected by two wires only, and there can be no manual talk-listen
switches. You must be able to turn the speaker up to normal room volume, and
the microphone circuit must be sensitive enough for you to speak from
anywhere in a small room. How would you do it? Remember: no switches just
two wires, and no squealing "feedback"!
The more we ponder how we would "easily rig up " such a system using a $1
amplifier chip, the more impossible it will seem, and the more we start to
appreciate the actual functions of true speakerphone technology. To put it very
simply, the speakerphone is "smart": it knows when to be idle, when to transmit,
when to receive, when to mute, when to attenuate, what to do about
background noise. It even recognizes the difference between steady
background noise and the phone line's dial tone.
Motorola explains the MC65118 this way: The fundamental difference between
the operation of a speakerphone and a handset is that of half-duplex versus full-
duplex. The handset is full duplex since conversation can occur in both
directions (transmit and receive) simultaneously. A speakerphone has higher
gain levels in both paths and attempting to converse full duplex results in
oscillatory problems due to the loop that exists within the system. The loop is
formed by the receive and transmit paths, the hybrid, and the acoustic coupling
(speaker to microphone.)

SP-1 •18
"The only practical and economical solution used to date is to design the
speakerphone to function in a half-duplex mode: only one person speaks at a
time while the other listens. To achieve this requires a circuit which can detect
who is talking, switch on the appropriate path (transmit or receive) and switch
off (attenuate) the other path. In this way, the loop gain is maintained less than
unity. When the talkers exchange function, the circuit must quickly detect this,
and switch the circuit appropriately. By providing speech level detectors, the
circuit operates in a hands-free mode, eliminating the need for a push-to-talk
switch."
"The MC65118 provides the necessary level detectors, attenuators, and
switching control for a properly operating speakerphone... (and) provides
background noise monitors which make the circuit insensitive to room and line
noise, hybrid amplifiers for interfacing to Tip and Ring [i.e. the phone line], the
microphone amplifier and other functions....."
OPTIONAL BATTERY OPERATION:
The use of a battery is optional for speakerphone operation but is required for
the Intercom operation described in the next section.
The use of a battery in speakerphone operation can compensate for abnormally
low telephone line voltage which may occur when a phone connection is at the
far end of the local "loop" connecting your location to the phone company's
central office. The supplementary battery can reduce the "pumping" effect
caused by incoming voice peaks.
IMPORTANT: Use a battery for this purpose, not a DC power supply. It is
important to remember that the telephone line loop voltage is a minus 48 volts
DC with respect to earth ground. Introducing a common AC mains ground at the
phone line input may damage SP1 components.

SP-1 •19
Procedure to hook-up external battery:
 1. Remove (or don't install) zener diode D5.
 2. Change R10 from 100 to 270 ohms (red-violet-brown).
 3. Install the 9V battery snap connector near S1. Red is positive, black is
negative or ground.
 4. Connect "X" near S1 to "X" near D5, using hookup wire.
 5. Install a fresh 9 volt battery. It will be in use only when S1 is "off the
hook."
SP1 CIRCUIT NOTES:
Schematic diagram also provides a sufficiently complete block diagram of the
MC65118's functions. How self- evident the circuit functions seem to you
depends on your level of electronics experimenting or learning. Here are the
highlights of what makes the SP1 tick.
S1: A and a jumper from B to A connect the circuit to the phone line's two wires.
There's nothing hi-tech about calling these two wires "tip" and "ring." That's our
inheritance from manual switchboard days. Tip and ring referred to parts of the
plug and patch cord sets handled by the operator. As mentioned, S1: A
performs the same function as a conventional telephone "hook" switch. As soon
as S1 is pressed in, the central office equipment is able to sense that the SP1
is connected to the line or "loop."
The phone line carries both audio energy (AC) and DC voltage. T1 and T2
couple the audio signals to U1's hybrid amplifiers (pins 5 and 6). In telephone
terminology, "hybrid" is a circuit that divides a single transmission channel into
two, one for each direction. Diodes D1-D4 are arranged to make the input
insensitive to DC polarity: the circuit gets correct DC polarity no matter which
way the phone line is connected!
U2 is a self-contained audio amplifier capable of outputs up to 400 miliwatts,
quite sufficient to drive quality speakers to room volume. This same LM386 is
also used in our receiver kits with great results.
Switch S2 simply activates the microphone amplifier muting feature built into
the speakerphone IC. Pin 12 is ordinarily low (0 voltage) for normal operation.
The microphone input circuitry is set up to accommodate both self-powered
microphones and those which require a small DC voltage for proper operation.
Hands-free intercom operation is the same as half-duplex speakerphone
operation except that a source of DC voltage is required. This is made easy by
S1B and a jumper from A to C.
Perhaps the easiest overview of the circuit would be the following pin by pin
notes on the MC65118. In these notes and on the schematic, BNM+
"background noise monitor." TX = transmit. RX = receive.

SP-1 •20
PIN DESCRIPTION
1. Filter output
2. Filter input
3. Chip disable (not used in SP1design).
4. Vcc (DC supply voltage)
5. Hybrid amplifier output #2
6. Hybrid amplifier output #1
7. Input to hybrid amplifier #1
8. TX attenuator output
9. TX attenuator input
10. Microphone amplifier output
11. Microphone amplifier input
12. Mute input (mike amplifier muted by + DC at pin 12 )
13. Volume control input
14. Response time to switch between transmit and
receive established by R15 and C21.
15. Supplies bias to volume control, filtered by C24.
16. Time constant for TX BNM set by R11 and C16.
17. Input to TX level detector on mike/speaker side.
18. Output of TX level detector and input to TX BNM
19. Output of RX level detector on mike/speaker side
20. Input to RX level detector on mike/speaker side
21. Input to RX attenuator and dial tone detector.
22. Output of RX attenuator
23. Input to transmit level detector on line side
24. Output of transmit level detector on line side
25. Output of RX level detector on line side,
also input to RX BNM
26. Input to RX level detector on line side
27. Time constant for RX BNM set by R16, C26.
28. Ground for entire IC