Simplifying the Interphone
telephone exchange
The Interphone 10-station telephone exchange
has created a lot of interest since it was
published in the August & September 1992
issues. Since then though, one of our readers has
pointed out that the circuit can be simplified
with a saving of two ICs on each remote station
board.
When the Interphone was first designed by the author, Geoff Monegal,
it was intended that each remote station board would be mounted in a
small case near its associated extension phone. Subsequently, when it
was presented ln SILICON CHIP, all the
circuitry was housed in the one case.
This means that there is now an opportunity to simply the circuit without prejudicing any of its functions.
To explain the modification, we reproduce the remote station circuit on
this page. A remote station board is
required for each extension phone.
The key components are IC9 (an
MC14536 DTMF decoder chip) and
IC10 (a 4028 1-of-10 decoder). IC9
and IC10 are there to decode the
specific DTMF (dual-tone multi-frequency) code for each extension. As
originally presented, the design specified that IC9 and IC10 were required
on each extension board.
However, one of our readers, Anthony Fouracre, of Tahmoor, NSW has
Fig.1 (below): the remote station
circuitry. IC9 & IC10 need only be
installed on one board, since the same
ICs can perform the DTMF decoding
for all _extensions.
+5VO-----------------------<I---------------6
4
16
3
10
1M
IC9
MC1 45436
+50Vo-----~
0.1
10 A
13 B
14
10k
12 C
11 D
IC10
4028
2.2k
10k
1W
220pfJ
.,.
D20
+
-:-
PHONE
..,.
..,.
STATION
SET
022
1N914
-:-
Off
HOOKO-----------------------'
120k
+
10 .,._
r-------•50V
-:!:.
22k
+5V
RING
.,.
01+
INTERPHONE REMOTE STATION
74
SILICON CHIP
-:
010
-[ill]-
I
---1u(;)~
Fig.2: this diagram
shows the revised parts
layout for a 3-station
board. Note that IC9 &
IC10 are only installed
in one section. Flying
links are then run from
the decoded outputs of
IC10 to the other
stations. Two of these
links are shown here,
designated as A & B.
o--CD-.....Jil.023
22k
• • 06
•B
•C 07
E•
Il
pointed out that only one MC14536
and one 4028 can do the DTMF decoding for all 10 extensions. All that
needs to be done is to install IC9 and
IC10 on one remote station board and
then take the 10 decoded outputs from
IC10 and wire each one to its corresponding D22 cathode on each remote station board. This is blindingly
obvious once it is pointed out and
comes under heading of "Why didn't
we think of that?". Well, we didn't
and we thank Anthony Fouracre for
his suggestion.
Wiring layout
We have also reproduced the wiring layout of the remote station boards,
i q I tf"'
in Fig.2. This shows three remote stations on the one board. We have modified it to show just one MC14536 and
one 4028 (ie, in the centre of the
board). IC9, IC10 and all the associated components - ie, the 3.58MHz
crystal, diodes D20 and D21, and various capacitors and resistors - have
been omitted from the other two remote stations on the board.
Flying links are then run from the
10 decoded outputs of IC10 to the
other remote station circuits. Two of
these are shown in Fig.2.
If you are building the Interphone
with 10 extensions, this simple modification means that you can save nine
MC14536s, nine 4028s, nine 3.58MHz
crystals, 18 diodes, 18 capacitors & 18
resistors.
Some readers may be tempted to
take this modification further and this
possibility does exist. Consider that
the base station board also features a
4028 and an MC14536 to decode the#
and* buttons. This raises the possibility of running the four decoded
data lines from that DTMF decoder
chip to another 4028 IC to decode the
10 extensions. That could be done but
there is the possibility that noise on
the necessary ribbon cable might
prejudice circuit operation.
Our recommendation is to simply
follow the modification to the remote
station boards described above. SC
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1992
75
