Build the Interphone:
a 10-station exchange for
your home; Pt.2
In this final article on the Interphone, we give
the assembly details and the installation
procedure. Depending one how many remote
station boards you build, you can connect up
to 10 tone dialling phones to the system.
By JEFF MONEGAL
The construction of the PC boards
for this project requires no special
care or attention , just a fine soldering
iron tip and some patience. Start by
going over the PC boards, looking for
problems such as shorted tracks or
fine breaks in the copper pattern. Any
faults fixed now will save a lot of time
later.
How many phones do you want to
be able to connect? The standard remote station board will accommodate
up to three phones so if you make two
remote station boards, you can have
six phones; if you build three remote
station boards you can have nine
phones. If you want the full 10 phones
that the system is capable of, you will
need to build a single station board in
addition to three standard remote station boards.
When it comes to building the remote station boards, we suggest you
build just one standard 3-station board
first. That way, if you do find problems, they will have been corrected
by the time you come to build the
other boards.
ERPHClNE
A large plastic case accommodates virtually all of the circuitry, including the
base station board & the remote station boards. A buzzer & a Busy LED are also
installed at each extension & wired to a standard telephone socket.
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STLICON CHTP
We also suggest that you complete
each board before starting on the next.
Our prototype used IC sockets but
these can be regarded as optional.
They do provide an advantage if you
have to troubleshoot the circuitry later.
Let's start with the base station
board. The wiring diagram is shown
in Fig.3. Make and insert all the wire
links first and then install the smaller
components such as resistors, diodes
and small capacitors. Next, install the
electrolytic capacitors and integrated
circuits, paying particular attention
to the orientation of the ICs which
will be damaged if they are incorrectly installed. The same comment
applies to the installation of the 3terminal regulator and the transistors.
Complete the base station PC board
assembly by inserting all the remaining components such as the isolation
transformer, 12V relays and the crystal. Our prototype boards also used
PC-mount screw terminal blocks
which are handy for making all the
off-board terminations . Note that three
insulated wire links must be installed
on the copper side of the board - ie,
from A to A, B to B and C to C.
With the base station board complete, go over your work carefully,
comparing it to the wiring and circuit
diagrams. This done, you can turn
your attention to the remote station
board. The wiring diagram for the 3station board is shown in Fig.4.
Don't forget that each remote station unit has to be programmed with
an identification number before final
installation. Each station should use
a different output link option from
IC10 so that when any extension is
paged from another, only one extension will respond. Remote station 1
(ie, extension 1) should have a link
from the cathode of diode DZZ to pin
14 ofICl0. Station 2 should have the
Fig.3: make sure that
all parts are correctly
oriented when
building the base
station board. Note
that three wiring
points marked A, B &
C must be linked to
three similarly
marked wiring points
using insulated wire
on the track side of the
board (ie, A-A, B-B &
C-C).
Fig.4 (right): this is the
parts overlay for the
remote station board.
Each remote board
accommodates up to
three stations & you can
build a maximum of 10
stations. Note that each
station must use a
different output link
from IClO - see Table 1.
link from D22 to pin 2 ofIClO. Station
3's link would go to pin 15 of IClO
and so on up to a maximum of 10
station units.
The full list of remote station links
(from IClO) is shown in Table 1.
Each remote station board should
be labelled to show which extension
number it is coded for. This will simplify the installation procedure.
Case wiring
When all the PC boards are fully
assembled, the next stage is to install
them in the case and complete all the
wiring. Our prototype used a large
instrument case which accommodates
all the PC boards with space to spare.
Our prototype also had the boards
Table 1: Remote Station Links
Extension
IC10 Pin No.
0
Pin 3
1
Pin 14
2
Pin 2
3
Pin 15
4
Pin 1
5
Pin 6
6
7
Pin 7
!
Pin 4
8
Pin 9
9
Pin 5
mounted on a baseplate made of PC
board material as can be seen in the
photos. However, this was a carryover from an earlier version and is not
really necessary; you can mount the
boards directly into the case.
The base station is mounted by itself on the lefthand side of the case
while the remote station boards are
stacked on each other, on the righthand side of the case.
Our prototype was wired using conventional 4-core telephone cable and
with standard Telecom-style phone
sockets on the rear of the case. However, the wiring itself is not standard.
The wiring of the various pins in
the sockets is as shown in Table 2.
As you can see, pins 2 and 6 are
SEPTEMBER1992
47
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This view shows how the boards are mounted inside the case. The base station
is on the left, while the remote station boards are stacked on each other to the
right. Note that the base station board was modified after this photo was taken.
used for the internal line connections
and so are compatible with any standard DTMF (tone) phone.
You can now complete the wiring
inside the case. This includes the wiring to the front panel LEDs, the AC
plugpack socket and the incoming
phone line plug.
System testing
The first step is to check the DC
supplies generated by the circuit. Connect a 12VAC plugpack and use a
multimeter to check the +50V and
+5V supplies on both the base station
and remote station PC boards. Check
for the presence of +5V at the supply
pin of each IC and also at the collectors of transistors Q4 and Q5. Check
for the presence of +50V at pin 6 of
each extension socket and also at the
collector of Q7, the buzzer driver on
each remote station board.
Now put the whole system aside
and begin modifying the extension
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SILICON CHIP
handsets you will be using. Each one
will require the addition of a buzzer
and a red LED (Busy LED). Depending
on the type of handsets you are going
to use, the buzzer and Busy LED may
be installed in the base, the cradle or,
if it is a one-piece phone, in the receiver itself.
One side of the buzzer and the negative connection (K) of the red Busy
LED connect to the GND line (pin 2 of
the extension socket). The other side
Table 2: Socket Connections
Pin 1
No Connection
Pin 2
GND
Pin 3
Busy LEDs
Pin 4
No Connection
Pin 5
Buzzer
Pin 6
Phone Line
I, ..
of the buzzer connects to the buzzer
line (pin 5 of the extension socket),
while the positive side of the Busy
LED connects to pin 3 of the socket.
When your tone phones are suitably modified, you are ready to connect and test the complete system.
Connect at least two extension phones
to the system and apply power.
Lift the handset and the Busy LEDs
should start flashing. Now press the#
button. The Line LED should light.
Hang up the handset and both the
Busy and Line LEDs should go out.
Pick up the phone again and press
the * button and the Hold LED should
light. Now press the# button and the
Hold LED should go out again while
the Line LED goes back on. Press the
* button and the Hold LED should
light and stay alight when the phone
is hung up. After about 30 seconds,
the buzzer will sound, warning you
that a call is still on hold. At this
stage, you have tested most of the
functions of the base station.
Now try calling another extension.
Pick up a phone and press the wanted
The rear panel of the case carries standard telephone sockets for the
connections to the extension telephones, plus a power socket for the plugpack
supply. The two sockets at lower left are not used in the final version.
number of another extension for at
least two seconds. The ringer in the
wanted extension should then sound
for as long as you hold down the
number button. If you now pick up
the other phone, you can have a 2way conversation on two phones between yourself and yourself - great if
you are a split personality! Better still,
get a friend to pick up the other phone
and talk to him or her instead.
You can now connect as many handsets as desired and repeat the tests to
make sure that all is well.
Finally, you are ready to connect
the incoming phone line. The Interphone is connected in parallel with
an existing tone phone. Now pick up
an extension phone and press the #
button. You should now hear a dial
tone in the earpiece. Dial a number to
confirm that all is correct.
To hang up, you simply replace the
handset. Pick up the phone again and
press the # button. Dial a friend and
wait until you hear them talking. Inform them as to what you are doing
and that they should not hang up the
phone. Press the * button, then wait a
few seconds then press the # button
again.
Your friend should still be on the
WARNING!
In order to comply with the Telecommunications Act, we dri3-w
the reader's attention to the following: "This customer equipment has no AUSTEL permit and
mat be dangerous or damage a
telecommunications network.
Connection to a telecommunications network is an offence
under section 253 of the Telecommunications Act 1991 and
may attract a maximum fine of
$12,000".
line and you should be able to talk as
normal.
Now check that you can transfer
you call to another extension phone
using the # button and the appropriate number button. If all is well, the
Interphone is working as it should.
You can now run 4-core wiring and
wire phone sockets to each point in
your home requiring an extension.
Once this has been completed,
power the whole system up again and
go through the functions again to ensure that all is working properly.
Kit availability
At the time of writing, the copyright for this project has been sold to
Altronics of Perth. They should have
kits available shortly after this issue
goes on sale. Readers should contact
Altronics for details of kit prices.
The author and designer of the Interphone project will be offering a
repair service for readers who need it.
He can be contacted at PO Box 33,
Condell Park, NSW 2200. Phone (02)
708 3763.
SC
SEPTEMBER1992
49
