This is only a preview of the October 2022 issue of Practical Electronics. You can view 0 of the 72 pages in the full issue. Articles in this series:
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Tele-com
Intercom using analogue phones
by Greig Sheridan and Ross Herbert
Put your old analogue telephones to use and build an intercom! Perhaps you
have a classic telephone like our red ‘batphone’, or one of the other Bakelite
phones with a real bell that generates a fantastic ring sound. Now you can
not only hear it again but actually speak to someone at the other end!
T
echnically, the Tele-com
is a ‘private line automatic
ringdown unit’, known in the
industry as a PLAR. That means that
it allows two PSTN telephones to be
automatically connected by simply
lifting one handset. Colloquially,
though, most people would just call
it an intercom.
Because of this, the device which
allows the Tele-com to operate is
referred to as the OzPLAR.
If you need two-way communication between two nearby locations such as a house and a shed, or
a granny flat, or just two rooms in a
home, it doesn’t get much more convenient than this. Pick up the phone
and the other end rings, then when
the other person picks up, you can
have a conversation.
While much of the telephone network still supports analogue telephones, we suspect that many people (like us) simply haven’t bothered
plugging them in, and now have a box
of spare phones. Rather than throw
them away, now you can put them
to good use.
The central OzPLAR unit to which
both telephones are connected
(described in this article) performs
the following functions;
22
Provides power to the phones
(‘transmission battery feed’).
Detects when a phone is picked up
(‘off-hook detection’).
Automatic ringing of an electromagnetic or electronic AC bell.
Ringing uses standard PSTN
cadence – Australia/NZ/UK/EU/
USA (long and short) selectable.
The caller hears a ringtone while
the called telephone is ringing.
Upon answer, ringing ceases and a
speech path is established between
the two telephones.
Both telephones must be replaced
on-hook after a call before a new
connection can be established.
Ring-trip (stopping the ring signal)
occurs during either the silent or
ringing period, when the called
telephone is taken off-hook.
The design is based entirely on discrete components and logic ICs, and
has been designed with flexibility in
mind. The PCB accommodates various alternative parts for the battery
feed and the ringing generator. See the
features panel for more information.
Circuit details
The complete circuit of the Tele-com
is shown in Fig.1 and Fig.2, with
Fig.2 having the ring-related circuitry
(including cadence generation), and
Fig.1 the rest. The overall circuit has
a few basic jobs:
1. Power the telephones
2. Detect when one is picked up
3. When a call is initiated, cause the
called phone to ring and send a
ringtone to the calling phone
4. When the other phone is picked
up, stop the ringtone and establish
voice communications
5. Reset the system when both phones
are restored on-hook
To achieve this, it consists of multiple interconnected circuit blocks. The
left-hand section in Fig.1 is the battery
feed and loop detect/ring trip circuit,
while the middle section is the logic
engine which detects line status (offhook/on-hook) and ensures that ringing output occurs only when the first
telephone goes off-hook.
The far-right section in Fig.1
includes the components required
to add an optional polarity reversal
on answer (‘ROA’) to the calling telephone. Public telephones (PT) connected to Step-by-Step and ARF crossbar switching systems in the now discontinued PSTN used the reversal of
the line polarity as the signal to deposit
Practical Electronics | October | 2022
Features of the Tele-com
Can be run from 2 x 12V batteries for an off-grid,
portable or temporary setup
Support for 48V DC power
input (optional)
Ring tone is provided
to the calling party
20Hz ringing supply
for improved ringing
of mechanical bells
Support for
optional
bespoke
cadence
Superior audio
performance
over longer/
mismatched lines
(using an IC-based
battery feed)
Onboard jumpers (or an
external switch) to select
AU/NZ/UK, EU or two
variations of the USA cadence
Choice of inductor-based
or solid-state battery feed
Crystal-locked source for the
cadence generator and ringing
inverter requires no adjustments
the caller’s money in the coin tin. This
option requires 48V operation to work.
Off-hook detection and ring trip
When a telephone is taken off-hook,
current passes through the optocoupler LED associated with the calling telephone (OPTO1 for the one
plugged into CON3/4 or OPTO2 for
CON5/6). Its output transistor therefore conducts and initiates a series
of events to ring the other telephone.
The voltage across each optocoupler LED is limited by zener diodes
ZD1 and ZD2. At the same time, a lowpass filter (470W/220μF) bypasses
20Hz ringing signals around the optocoupler LED in the called telephone
circuit, to prevent it from conducting
during ringing.
When the called telephone is taken
off-hook to answer, current will flow
through the LED in the optocoupler
associated with the called telephone,
thereby initiating ring trip. Ring trip
can take place during the ringing
period or the silent period.
Initiating a call
The following description refers to a
call initiated by a telephone connected
to CON4 (or CON3) when the board
is constructed with the inductorPractical Electronics | October | 2022
Powered from a 24V DC inline power
supply; no mains wiring is involved
Easy to build using readily available parts
based battery feed (see below). Note
that in this case, the 1μF capacitors in
the feed bridge are replaced by links
(LK3 and LK4).
When the telephone is taken offhook, 24V DC flows through transformer L1 (wired as an inductor) and
the 68W resistor, the normally-closed
This ‘batphone’ is an example of an old analogue telephone that could be used
with the Tele-com. It’s important to note that not all analogue telephones have
rotary dials, some have push-button keypads instead; both types will work.
23
OzPLAR Telephone Intercom – Battery Feed, Logic and Power
contact of relay RLY1b, the LED in
OPTO1, the telephone and back to
ground via the normally-closed contact of relay RLY1a, the 68W resistor,
LK3, and transformer L2 (also wired
as an inductor).
The off-hook condition detected
by OPTO1 results in a high level at
the input of schmitt-trigger inverter
24
IC1a. The resulting low output on pin
2 starts the calling process through
the combined action of AND gate IC2c
and NOR gate IC3a.
The Q1 output on pin 1 of J-K master/slave flip-flop IC4a is preset high
in the idle state. With both inputs of
IC2d now high, its output at pin 11
also goes high. This feeds into both
IC2a and IC2b; however, the low level
on IC2b pin 5 prevents RLY1 from
operating. Since both inputs of IC2a
are high, the output will also be high,
which results in RLY2 operating.
The RLY2 contacts disconnect the
battery feed from the telephone at
CON6 (CON5), and instead apply
+24V to one leg of the line and the
Practical Electronics | October | 2022
Fig.1: the Tele-com circuit, minus the ring and cadence-generating circuitry, shown separately in Fig.2. The telephones
plug into the sockets at the top and bottom of the left-hand side. The circuitry between them mainly involves supplying
current to the phones and ensuring that voice signals pass between them. To the right, we have logic to detect when a
phone is picked up and either ring the other phone or ‘answer the call’ if the other has already been picked up.
ringing (VRING) signal to the other,
causing this telephone to ring.
At the same time, the high level
at the output of IC2d (pin 11) is
inverted by IC1e, sending the
Cadence Start line low to enable the
crystal oscillator and the logic controlling the ringing inverter, shown
in Fig.2.
‘Cadence’ refers to the timing of the
ring bursts and silent periods.
4060 counter IC5 is held in reset at
idle, but now commences oscillating.
The reset signal is also removed from
decade counter IC6, flip-flop IC4b and
Practical Electronics | October | 2022
the cadence generator decade counters IC7 and IC8.
Cadence Start is also presented to
pin 8 of NOR gate IC10c, which in
conjunction with IC7 and IC8, controls the cadence of the AC ringing
signal (for example, when set for Australia, producing the traditional ring
ring...ring ring... sound).
The 3.2768MHz crystal oscillator
based on X1 has its frequency divided
by IC5 to produce 200Hz at its O13
output. This is divided by IC6 to
produce the 20Hz alternating signal
required for the efficient operation
of electromagnetic telephone bells.
This signal is also fed to the input of
IC1b and IC10a, and in conjunction
with the cadence signal at the output
of IC1f, enables the ringing inverter.
The 20Hz signal at IC6 pin 12 is
halved by IC4b to produce the 10Hz
clock signal for IC7. The outputs of
IC7 go high sequentially, producing
a one-second clock signal to feed IC8.
The outputs of the 4017 decade
counters, IC7 and IC8, are encoded in
a manner that determines the on-off
cadence pattern sent to the ringing
inverter – see Fig.3 for details.
25
OzPLAR Telephone Intercom – Cadence Logic and Ring Voltage Generator
Regardless of the cadence selection,
the instant Cadence Start goes low, the
ringing inverter is enabled, and the
called telephone commences ringing.
When the inputs to NOR gate IC10c
are both low, its output is high. This
is inverted by IC1f and fed to one
input of gates IC10a and IC10b. The
second input of these two gates alternates high or low following the 20Hz
drive signal, while IC1b ensures that
both MOSFET drive signals are complementary (ie, alternately phased).
MOSFETs Q6 and Q7 alternately
switch the 12V DC supply through
each primary winding of transformer T1. Due to the step-up
ratio, an alternating voltage in
the order of 120V peak-to-peak is
produced in the secondary. PTC
thermistor PTC1 provides overcurrent protection, while the 2.2kW
26
resistor provides a degree of clamping
of the output voltage, should there be
no load connected.
While the ringing inverter is operating, the 6.8nF capacitor, normally
bypassed by RLY2a, feeds a tiny amount
of the ringing voltage back to the calling telephone, serving as the ringtone.
Cadence generation and selection
Jumpers JP1, JP2 and JP3 allow easy
selection of the ‘ring-ring-pause’
(400ms on, 200ms off, 400ms on, two
seconds off) cadence familiar to Aussies, our Kiwi neighbours and the UK.
Other options are for the European
cadence (one second on, four seconds
off) and the two common versions of
the US cadence (two seconds on, four
seconds off and one second on, two
seconds off), commonly referred to as
‘US Long’ and ‘US Short’ respectively.
There are many cadences globally,
and they’re documented in the ITU
PDF at: https://bit.ly/pe-oct22-itu
Let’s assume the board is set up for
AU cadence.
When Cadence Start goes low
(t=0.0s), the counter in IC6 is released
from its reset state and commences
counting. At that same instant, the
reset signal is removed from IC4b,
IC7 and IC8 in readiness for clock
ticks to arrive.
Having just been released from
reset, output O0 of IC7 is high. Pin
12 of NOR gate IC9 is thus high, so
its output is low.
O0 of IC8 is also high. This feeds to
pins 12 and 13 of IC10d via JP2 pins 2
and 3, and thus pin 11 of IC10d is low.
For a brief period, the inputs of
NOR gate IC3d are both low, so its
output is high. IC1d again inverts this
Practical Electronics | October | 2022
Fig.2: the rest of the circuitry which wouldn’t fit on Fig.1. At left is the cadencegenerating circuitry; the outputs of IC7 go high in sequence at 100ms intervals,
while those of IC8 go high at one-second intervals. These signals are fed into
a series of logic gates depending on the position of jumpers on JP1-JP3 and
possibly LK5, resulting in a signal at output pin 10 of IC10c that indicates
whether the phone should be ringing or not at any given moment. This is then
converted into an AC voltage sufficient to ring a telephone by MOSFETs Q6 and
Q7 and transformer T1.
►
O2 and O3. The ringing generation is
maintained by linking these outputs
to IC9’s inputs, resulting in a continuous on-period of 400ms.
Outputs O4 and O5 of IC7 are not
connected, so for those two 100ms
ticks, IC9 has all low levels on its
►
to a low signal and this is fed via JP1
pins 1 and 2 to pin 9 of IC10c. The
ringing inverter is enabled and it generates the 20Hz alternating voltage to
ring the telephone.
100ms later, counter IC7 increments, sending O1 high, then on to
Fig.3: this logic analyser screengrab demonstrates how the cadence generation
circuitry works. Ch0 is the Cadence Start line (active-low), Ch1 is the 200Hz
square wave at the O13 output of IC5, Ch2 is the 20Hz signal from pin 12 of IC6,
Ch3 is the 10Hz signal at TP5 feeding into pin 14 of IC7, and Ch4 is the resulting
cadence signal at pin 10 of IC10c (inverted so it is active-high). This shows the
AU cadence.
Practical Electronics | October | 2022
inputs, its NOR output goes high, so the
ringing inverter is disabled for 200ms.
For the final 400ms of the first one
second of cadence, IC7 outputs O6-O9
are clocked sequentially high, and
the ringing inverter is enabled again.
At t=1.0s, IC7 resets and IC8 increments, sending its O0 output low.
IC10d now prevents further signals
from IC7 and IC9 from enabling the
ringing inverter for the remaining
period of the selected cadence pattern up until the instant output O3 of
IC8 goes high, at t=3.0s. This signal,
via JP3 pins 2 and 3 and diode D5,
resets IC7 and IC8 and the cadence
pattern repeats.
The US and EU cadences are simpler, as IC9 and its related logic are
no longer in play. JP2 instead directs
either O0 or O1 of IC8 via IC10d and
JP1 to the ringing inverter’s drive logic,
thereby enabling the inverter which
produces ringing for either one second (EU, US-S), or two seconds (US-L).
The silent period for both AU and
US-S cadence is terminated after three
seconds, when output O3 of IC8 goes
high, as explained earlier. The silent
period for the EU cadence is terminated after five seconds, via JP3 pins 1
and 2 and diode D5. The silent period
for US-L cadence is terminated after
six seconds, when output O6 of IC8
goes high, via diode D4.
Bespoke cadence creation is beyond
the scope of this article, but any combination of 100ms on/off times can
be created by mating the required O
outputs of IC7 with up to eight inputs
of IC9. This is via the pins of JP1-JP3,
CON7, CON8 and LK5 as described at:
https://bit.ly/pe-oct22-bcc
Called party answers (ring trip)
The 20Hz ringing voltage is superimposed upon the 24V DC supply. This
ever-present DC allows the LED in the
optocoupler associated with CON6 (or
CON5) to conduct when the handset is
lifted to answer a call. That’s regardless of whether it happens during a
ringing or silent period.
When ringing is present, the LED
is prevented from conducting by the
low-frequency filter formed by the
two 470W resistors and the 220μF
NP capacitor.
The 10MW resistor provides a slight
‘off’ bias to the base of the optocoupler
transistor, while the 56pF capacitor
minimises noise pickup in the base
connection. The 330kW resistor acts
as the emitter load for the optocoupler output transistor.
When answered, the optocoupler
transistor turns on, and the resulting
low at the output of inverter IC1c pin
6 causes NOR gate IC3a pin 3 to go
27
high, thereby resetting flip-flop IC4a,
causing its Q1 output to go low and
RLY2 to release. The low on IC4a Q1
also causes the Cadence Start line to
go high, holding all the counters reset
and disabling the ringing inverter.
The release of RLY2 restores the
change-over contacts to normal, thus
connecting the called telephone to the
battery feed and establishing a speech
path between the two telephones.
One party clears
If we assume the telephone at CON4
(or CON3) hangs up first, the output
of OPTO1 goes low and pin 2 of IC1a
goes high. IC3a’s output goes low,
removing the reset on IC4a, but the
flip-flop’s outputs remain unchanged
in the absence of any other stimulus.
If this telephone again goes offhook before the other telephone hangs
up, the reset on IC4a is once more
asserted, but again there is no change
of state in its outputs, so the speech
path remains connected.
Both parties clear
If the telephone at CON6 (or CON5)
hangs up after the other telephone
goes on-hook, both of the inputs to
IC2c become high, causing its output
to go high, setting the flip-flop in IC4a
and restoring all circuitry to the idle
state in readiness for the next call.
Indicator LED
The bi-colour LED (LED1) displays
the various phases of a call. At idle,
driver transistors Q3 (red) and Q4
(green) are both off, preventing both
LEDs from illuminating, despite Q5
being on at this time.
When a telephone is being called,
both Q3 and Q4 are fully on while
Q5 switches alternately on and off in
response to the 20Hz LED drive signal, resulting in both red and green
LEDs following the ring cadence.
When a call is in progress, both telephones are off-hook. The green LED
is illuminated due to the high on the
output of IC3c forcing Q4 to conduct,
while the low on the output of IC3b
holds the red LED off. These two gates
toggle when only one party has hung
up, resulting in a steady red LED to
indicate a possible fault condition –
see the troubleshooting section below.
Feeding power to the phones
The Tele-com can be configured to
use an inductor-based battery feed,
as shown in Fig.4, where a 24V DC
supply is fed to both legs of the line
via inductors L1 and L2. Since the
total available current must be shared
between both telephones, the current to each telephone is dependent
mainly upon line length, ie, the shortest line gets the most current.
The two 1μF capacitors shown on
the circuit diagram are omitted and
replaced by links in this case. Tests
show that good speech is possible
with line lengths up to 500m or more
in this configuration – quite adequate
for most situations.
Provision has also been made to
replace the inductor-based battery
feed with an electronic battery feed
using special 8-pin ICs – see Fig.5.
The use of two such devices allows
the implementation of what’s known
as a Stone Bridge, such that the transmitter current supply to the two telephones is separate and determined
only by individual line lengths.
In Fig.5, the electronic battery feed
ICs are depicted as individual inductors designated IC13 and IC14.
The electronic battery feed device
was designed by AT&T with the part
number LB1011. It is now obsolete
and available only from electronics
surplus component suppliers (eg,
via eBay). It simulates two separate
inductors having very high impedances at voice frequencies.
When IC13 and IC14 are installed
in place of inductors L1 and L2, the
two 1μF capacitors need to be fitted
to the board. These capacitors provide speech coupling between the two
telephones connected to CON4 (or
CON3) and CON6 (or CON5). In this
configuration, the maximum current
in each telephone circuit is approximately 36mA, so line lengths of several kilometres are possible.
Optional reversal on answer
To allow this Tele-com to work with
with public (coin) telephones that
require a line reversal on answer,
the polarity of the line to CON6 (or
CON5) can be made to reverse when
This is the finished Tele-com PCB without the optional IC-based battery feed, 48V power input components or ‘polarity
reversal on answer’ feature.
28
Practical Electronics | October | 2022
the telephone at CON4 (or CON3)
answers a call. This means that the
public telephone must be connected
to CON6 (or CON5).
Two flip-flops (IC12a and IC12b)
are interconnected to provide this
function. With both telephones onhook, both flip-flops are held reset.
When either phone goes off-hook, the
reset signal is removed.
If the telephone connected to CON6
(CON5) is the caller, the output of
IC2b presents a high to pin 7 of IC12a,
setting this flip-flop. The high on the
Q1 output is tied to the J2 input of
IC12b, and with J2 high and K2 low,
an answer signal from IC4a pin2 will
toggle IC12b and set output Q2 high.
NPN transistor Q8 then switches on
and RLY3 operates, reversing the line
polarity of CON6 (CON5).
Should the telephone connected to
CON4 (CON3) initiate a call, pin 7 of
IC12a will not be set, and the J2 input
to IC12b will remain low; therefore,
the outputs of this flip-flop will not
change state when the answer signal
from IC4a pin 2 is applied to pin 13 of
IC12b. RLY3 will remain in the unoperated condition and the line polarity
will not be reversed.
Flip-flops IC12a and IC12b will reset
only when both telephones are restored
on-hook, causing RLY3 to release.
Power supply
The power supply takes an incoming +24V DC through reverse-polarity protection diode D1, and REG3
supplies +12VDC to power the logic,
relays and the ringing inverter. A linear 7812 regulator was tried during
the design phase, and replaced with a
switchmode equivalent due to excessive heat dissipation, particularly
when ringing.
For an application where a higher
ringing duty cycle is anticipated, or
the Tele-com is to be powered from
batteries, a switch-mode equivalent should be used instead (eg, our
August 2021 design, see the Switchmode Replacement for 78xx Regulators project).
If a 48V DC supply is to be used,
REG3 is omitted and instead, a
MeanWell (REG2) or Traco (REG1)
DC-DC converter is fitted to accept
the higher input voltage and step it
down to +12V.
Construction
The Tele-com project is built on a
double-sided PCB coded 12110121,
which measures 200.5 x 143mm and
is available from the PE PCB Service.
Start by giving the PCB a quick visual
inspection for any obvious damage
(although that is quite unusual). Use
Practical Electronics | October | 2022
Fig.4: this shows the simplest way to power two telephones. Two high
impedance inductors allow DC current to supply the transmitter while blocking
AC signals through the low resistance of the battery. However, the proportion
of the available current to each telephone is dependent upon the length of both
lines and a very long line may reduce the current to an unworkable level.
Fig.5: this is a more complicated battery feed scheme known as a Stone
Bridge which uses virtual inductors to feed DC current to each telephone
independently, with capacitors coupling speech signals between them. It can
handle very long lines over 1km in length. The virtual inductors are contained
in a special IC available via eBay or suppliers of obsolete components
the PCB overlay diagram (Fig.6) as a
reference during construction, but
note that there are a few different
options that affect which components
are fitted.
If you are planning to build the
Tele-com with a custom cadence, you
will need to cut some tracks on the
underside of the board below LK5,
separating the rows of pads on either
side. Take care when cutting these
tracks, as there is very little separation between the two rows of pads.
If you plan to add the Reversal on
Answer relay RLY3, there are two
tracks noted with the word ‘cut’ on
the underside of the board – they are
also indicated on the component overlay as two short lines joining two of
the centre pads below RLY3.
In both cases, if cutting, check with
a continuity tester to ensure that the
tracks have been completely separated before continuing.
The six mounting holes in the board
fit mounting posts in the PacTec LH96200 enclosure. If you’re using that case,
you can jump to the board assembly.
If you’re building into the Altronics H0476 instead, there are two holes
near the rear (connector) edge that
align with two mounting posts under
the board. They’re marked on the component overlay (Fig.6) with ‘#’ marks.
Temporarily screw the board to
these, as this will align the board
correctly within the box, then use
the mounting holes in the four corners as a template to drill holes that
will support the board. Remove the
temporary screws and continue with
the assembly.
Breaking with tradition, mount
the connectors first and ensure these
all align and project through the rear
panel. The pads for the power and
screw connectors have been drilled
oversize to provide a little extra wriggle room.
Continue with the resistors and
other low-profile components like the
axial diodes and the crystal. If you’re
building it with the inductor-based
battery feed, don’t forget to replace
the 1μF capacitors to the right-hand
side of the transformers with links.
Also, if you’re building for a 48V
supply, note that the resistors marked
on the overlay with an asterisk have
different values for 48V. See the parts
list for details.
You can then install the SIL resistor array if you will be using the custom cadence feature, with its dot at
the end shown in Fig.6 and on the
PCB silkscreen.
Now add the capacitors, starting
with the smallest ceramic types and
29
Parts List – Tele-com
1 double-sided PCB coded 12110211, 200.5 x 143mm,
available from the PE PCB Service
1 PacTec LH96-200 ABS instrument case or equivalent,
260x180x65mm [Altronics H0476, RS 291-4169,
Mouser 616-74213-510-039]
1 set of front and rear 3D-printed panels (size to suit
case, see: www.thingiverse.com/thing:4922521)
1 24V DC 2A power supply [Altronics M8970D, WES
SMP2500-24RLP + ACL104-075]
1 3VA 12+12V PCB-mount mains transformer (T1)
[Altronics M7024A ➊]
2 600W:600W isolation transformers ➋ (L1, L2)
[Altronics M1000 or Triad TY-305P/306P/400P]
2 Omron G5V-2-H1DC12 12V DC coil relays or equivalent
(RLY1, RLY2) [Altronics S4150]
1 3.2768MHz crystal resonator (X1)
1 RXEF030 300mA hold current PTC thermistor (PTC1)
[element14 1175861, Mouser 650-RXEF030, Digi-Key
RXEF030-ND]
1 10kW 9-pin, 8-element SIL resistor network (RN1; only
needed for bespoke cadence) [element14 9356819,
Digi-Key 4609X-101-103LF-ND]
1 PCB-mount barrel socket, 2.1/2.5mm inner diameter
(CON1) [element14 1854512, RS 805-1699]
3 right-angle two-way pluggable headers (CON2, CON3,
CON5) [Jaycar HM3102 + HM3122, Altronics P2592 +
P2512, element14 2527811 + 2527762]
2 PCB-mounting 6P6C RJ12 sockets (CON4, CON6)
[Altronics P1425, Jaycar PS1474, Wurth 615006138421]
2 1-pin headers (can be snapped from a longer strip)
(CON7, CON8; only needed for bespoke cadence)
3 3-pin headers with shorting blocks (JP1-JP3)
1 2 10 pin header or header socket
on needed or
bespoke cadence)
5 PCB pins (optional; for test points TP1-TP5)
12 M3 x 6mm panhead machine screws
6 6mm-long M3-tapped spacers
6 6mm-long 6G self-tapping screws (PacTec case only)
3 300mm-long 4mm-wide cable ties
5 14-pin DIL IC sockets (optional)
5 16-pin DIL IC sockets (optional)
1 12-pin snappable IC socket strip (optional, for OPTO1-2)
➊ alternatives include RS 504-464, element14 1712727
(Vigortronix VTX-120-003-612), Mouser 553-FS24-100
(Triad FS24-100) and 838-3FD-324 (Tamura 3FD-324),
RapidOnline 88-3883 (Vigortronix VTX-120-3803-412)
Semiconductors
1 40106B or 74C14 hex inverter IC, DIP-14 (IC1)
1 4081B quad 2-input AND gate IC, DIP-14 (IC2)
2 4001B quad 2-input NOR gate ICs, DIP-14 (IC3, IC10)
1 02 d a
ip op ,
1
1 4060B 14-stage ripple-carry binary counter IC, DIP-16 (IC5)
3 4017B decade counter/divider ICs, DIP-16 (IC6-IC8)
1 4078B 8-input OR/NOR gate IC, DIP-14 (IC9)
2 4N35 optocouplers, DIP-6 (OPTO1, OPTO2)
then working your way up to the
bigger ones.
Confirm the polarity of the two
electrolytics at the top right of the
board and double-check that you have
non-polarised electros adjacent to the
telephone connectors. Now is also a
good time to fit the PTC thermistor.
The LED should be soldered at
full extension onto the board if it’s
to go into the PacTec case; however,
30
1 Switchmode 12V 1A regulator ➌ (Pololu D24V10F12 or
the PE August 2021 design: Switchmode Replacement
for 78xx Regulators) (REG3)
3 BC547 100mA NPN transistors (Q1-Q3)
2 BC557 100mA PNP transistors (Q4, Q5)
2 IRFZ44N 55V, 49A N-channel MOSFETs (Q6, Q7)
1 3-pin bicolour/tricolour (red/green) common cathode
5mm LED (LED1) [Jaycar ZD0252]
2 3.3V ±5% 1W zener diodes (eg, 1N4728A) (ZD1, ZD2)
1 MBR10100 100V 10A schottky diode, TO-220 (note: not
dual [CT] version) (D1)
2 1N4004 400V 1A diodes (D2, D3)
3 1N4148 or equivalent small signal diodes (D4-D6)
Capacitors
2 220 10V non po arised
e ectro tic
[Altronics R6600A or Mouser 667-ECE-A1AN221U]
2 100
V e ectro tic
1 1 100V
T
3 100nF X7R ceramic
2 8n
V
T
2 56pF 50V NP0/C0G ceramic disc
2 18pF 50V NP0/C0G ceramic disc
Resistors (all ¼W 5% metal film unless otherwise stated)
3 10MW
1 2.2kW 3W 5% 2 330W
2 330kW
2 1.5kW
4 68W ➌
6 10kW
4 470W
2 15W
Additional parts for IC-based battery feed
(exclude parts marked ➋ above)
2 AT&T/Lucent LB1011 battery feed ICs, DIP-8 (IC13,
IC14) (eBay or one of the suppliers listed at: www.
digipart.com/part/LB1011AB)
2 8-pin DIL IC sockets (optional)
2 1 2 0V
T capacitors
2 0n
V
T capacitors
2 1kW ¼W 5% resistors
4 180W ¼W 5% resistors ➌
Additional parts for reversal on answer
1 Omron G5V-2-H1 12V DC coil telecom relay or
equivalent (RLY3) [Altronics S4150]
1 16-pin DIL IC socket
1 02 d a
ip op ,
1
12
1 BC547 100mA NPN transistor (Q8)
1 1N4004 400V 1A diode (D7)
1 10kW ¼W 5% resistor
Additional parts for 48V DC supply
(exclude parts marked ➌ above)
1 Traco TMR 6-4812 48V DC to 12V DC converter (REG1)
[Mouser 495-TMR-6-4812] OR
1 ean e
0
12 8V
to 12V
con erter
(REG2) [Mouser 709-SKMW06G-12]
4 390W
meta fi m resistors
4 150W ¼W 5% resistors
you’ll need to add some short flying
leads for it to reach the panel in the
Altronics case.
Add the remaining active components (ICs, regulators, optos and
transistors), plus the TO-220 package diode, ensuring all the ICs have
pin 1 on the right-hand side, and the
TO-220 devices all face left (with
their metal tabs to the right). The
use of IC sockets is recommended
(including the optos), but check that
+12V and GND (0V) are present on
the correct pins before inserting ICs
in their sockets.
The optional test point PCB stakes
and jumpers can be fitted next, then
the relays, which must be oriented as
shown in Fig.6.
If you need LK5 and haven’t already
fitted it, do so now, along with the
headers for jumpers JP1-JP3. Follow
Practical Electronics | October | 2022
The Tele-com is recommended to be built into the PacTec
LH96-200 enclosure as shown (which can be purchased from
RS Components or Mouser). However, mounting holes for the
larger Altronics H0476 case are also provided on the PCB.
with the switchmode DC-DC converter (REG1 or REG2) if you will be
using a 48V supply.
Finally, fit the transformers one by
one. Place them, then wrap a cable
tie around them firmly before soldering their pins. Take extra care if
you’re using Tamura or Triad transformers for T1, as these can go into
the board either way, but only one
way is correct.
Their ‘mains’ winding faces the
rear panel connectors. The formers of
both have pin numbers moulded into
them, with the ‘1-2-3-4’ side being the
mains side.
Troubleshooting
There isn’t much to testing it. Plug in
a couple of known-good telephones,
apply the appropriate DC voltage and
check that it works as expected.
If you encounter problems, the
nature of the fault should tell you
which part of the circuit requires
attention, but always start by confirming that the ‘Vin’ voltage (24/48V) and
12V rails are present.
You can sometimes isolate faults
by touching the top of each IC, where
any heat detected indicates a faulty
device (CMOS ICs generally don’t
produce significant heat unless they
are faulty). If you’ve done this before,
Practical Electronics | October | 2022
you probably know to apply a little
saliva to your fingertip first to prevent
burning yourself.
No sidetone
You should only connect knowngood telephones to the Tele-com.
You should hear ‘sidetone’ if they
are working correctly – some amount
of your own voice is audible in the
receiver. The easiest way to check
for sidetone is to gently blow into
the transmitter – you should hear the
resulting hiss in the receiver.
If sidetone is absent in either telephone, start by checking that power
is switched on and 24V (48V) is present on the board test pins. If the fault
is not in the telephones, then check
the wiring.
If one is working and the other not,
follow the circuit with your multimeter and compare between the two
channels until the fault reveals itself.
Don’t forget to swap the phones as a
first check!
No ringing
First, check that jumpers JP1-JP3
are correctly set for one of the ring
cadence patterns – follow the silkscreen legend on the board adjacent
to these jumpers to select the cadence
that you want.
If there’s no ringing when the first
telephone goes off-hook, then check
the LED.
If the LED is not lit at all, first make
sure that it is a common-cathode device
and driver transistors Q3, Q4 and Q5
are fitted in their correct positions.
Briefly short pins 4 and 5 of OPTO1
or OPTO2. If that brings it to life,
there’s most likely a problem with
the optocoupler or the components
on the LED side of this device. Check
that the 3.3V zener cathodes are both
facing ‘up’, towards the rear panel.
If one of the relays operates when
a telephone goes off-hook, that confirms that the main logic engine is
functioning correctly. If neither relay
operates, this narrows your focus to
IC2-IC4 or the 12V rail.
If the LED is flashing, this confirms
the oscillator and cadence components are working OK, suggesting
you should check the MOSFETs and
transformer. TP4 should have a pulsing 120V (approximately) alternating
voltage on it, according to the selected
cadence. Check also that the centre
tap on the secondary of the transformer has +12V applied.
If the LED is lit but not flashing,
check with an oscilloscope, logic
probe, or the frequency range on your
multimeter that TP5 (near the LED) is
31
Fig.6: assembly
of the Tele-com is
straightforward,
but there are quite
a few different
options, some of
which involve
fitting different
parts. So you won’t
necessarily install
everything shown
here. It’s best to
work out what
you will or won’t
be mounting, and
the components
that might change
in value, before
you start. As you
build the board,
be careful to
ensure that all the
ICs, diodes, LED,
optocouplers,
transformers,
transistors
and polarised
electrolytic
capacitors are
oriented correctly,
as shown here. If
using a 48V DC
supply the four
180W resistors in
the centre red box,
and marked with
an asterisk, are
replaced with 390W
resistors, while
the 68W resistors
marked with an
asterisk become
150W.
fluctuating at 10Hz. If 10Hz is present,
focus on IC7, IC8, the jumpers LK5,
JP1 and JP2, diodes D4, D5 and D6,
and the 10kW resistor immediately
adjacent to these diodes.
If TP5 is not fluctuating at 10Hz,
focus on the 3.2768MHz crystal, its
loading caps, IC5, IC6 and IC4b.
Cadence problems
An unexpected cadence indicates
an incorrect placement or missing
jumper on LK5 or JP1-JP3. Try changing the jumpers to select an alternative cadence. If correct operation
can be achieved when set to the EU
32
or US cadences but not the AU/NZ/
UK ones, check that IC7 and IC9 are
correctly seated. Check also that RN1
is not reversed and has the correct
internal configuration, and one end
pin is common.
If you’ve cut the tracks under
LK5 in anticipation of using a custom cadence, make sure you have
inserted links to replace the track
segments which have been cut.
If problems remain, confirm that
TP5 is pulsing at exactly 10Hz,
re-check the board for any solder
shorting adjacent IC pins and repeat
the ‘touch test’ on the tops of the ICs.
Red LED on idle
If both telephones are on-hook and the
LED is solid red, there’s most probably a fault on the line or with one of
the telephones, causing one not to
be correctly seen as on-hook. Unplug
each phone in turn to see if the LED
extinguishes. If it does, the fault is in
the wiring or telephone itself.
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
Practical Electronics | October | 2022
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