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Items relevant to "Phone/Fax Missed Call Alert":
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PHONE/FAX
MISSED CALL
ALERT
By JIM ROWE
This device detects an incoming call on your phone or fax and lights
a LED and latches on a relay, which you can use to switch on a lamp
or horn in another room or your garage.
S
o what’s wrong with listening for the phone ringer or
looking at the fax machine’s “out” tray? If you work
at home you need to be able to respond to phone calls
or faxes as soon as they come in but you cannot be hovering
over the phone all day; there are times when you have to
leave your office or home and if you do not physically lift
the phone handset and listen (if you have Messagebank) or
check the fax as soon as you arrive back, calls or faxes could
be without a response for some time.
Or maybe you have an office but spend some time in
the workshop. Same problem – or even more so if you are
waiting on an important call or fax.
So this is the remedy. Build this device and it will let
you know that a phone call has been made or a fax has
been received, without you needing to actually check the
phone or fax.
What’s more, you can use it with a remote light or siren
to alert you over the noise of machinery!
The Phone/Fax Missed Call Alert hooks up easily to the
fax machine’s phone line. It connects in very easily, using
a standard low-cost modular cable. The kit of parts should
set you back no more than about $30 and you should be
able to build it up in a couple of hours at most.
How it works
When someone dials your phone or fax number, the
exchange sends out short (200ms) bursts of a 75V 17Hz
AC signal – the ‘ring signal’.
It’s this signal that causes your phone to ring or triggers
the fax machine into answering and receiving the message.
Our Missed Call Alert is designed to monitor the phone
line and respond to the same ring signal, using it to trigger
a flipflop and hence latch a relay.
84 Silicon Chip
As you can see from the circuit diagram, the Missed
Call Alert has two standard RJ12 modular phone sockets,
interconnected so that it can easily be connected in ‘daisy
chain parallel’ (ie, it doesn’t break the circuit) with your
phone or fax machine.
It connects across the phone line with a simple ‘bridging’
circuit, consisting of a 470nF capacitor and 22kW resistor
connected in series, which passes only AC and limits the
current, plus a bridge rectifier formed from diodes D1-D4
to convert the 17Hz AC signal into pulsating DC.
This small pulsing DC current (about 2mA) is passed
through the input diode of the 4N25/28 optocoupler. With
each pulse the diode emits IR light, which in turn causes
the phototransistor inside the opto to conduct.
Zener diode ZD1 is connected across the opto’s diode
to protect it from damage in the event of noise ‘spikes’
on the phone line. These can occur, for example, during
thunderstorms.
Now until this happens, the phototransistor is not conducting and as a result the input of gate IC1a (pin 1) is
pulled up to +12V by the 100kW resistor. The 100nF capacitor also charges up to +12V. But when the phototransistor
conducts as a result of the ring signal, the 100nF capacitor
is discharged and pin 1 of IC1a is pulled down to ground
potential. As a result the output of the gate (pin 3) switches
high, pulling input pin 13 of gate IC1d high as well.
At the same time the 100nF capacitor connected between input pin 12 and ground also begins charging, via
the 1MW resistor.
If the sensing circuit has detected a true ring signal, the
output of IC1a will remain high for about 200ms – the
length of the first burst of 17Hz AC. This gives more than
enough time for the 100nF capacitor to charge up to logic
siliconchip.com.au
The circuit monitors the phone line for ring voltage and turns on a LED and the relay.
high level via the 1MW resistor and as soon as this happens both inputs of IC1d will be at this level. As a result
the output of IC1d (pin 11) will switch low – producing a
negative-going pulse to trigger the R-S flipflop formed by
interconnected gates IC1b and IC1c.
If the sensing circuit has not detected a true ring signal,
the output of IC1a will fall low again before the voltage
on pin 12 of IC1d has reached logic high level and when
it drops it will remove the charge on the 100nF capacitor
via diode D7. As a result both inputs of IC1d will return
to ground potential and its output will never switch low.
So the purpose of D7, the 1M resistor and 100nF capacitor
is to ensure that the flipflop is only triggered by a valid
burst of ring signal.
The flipflop based on IC1b and IC1c is normally in its
reset state (output pin 4 high, output pin 10 low), because
when 12V power is first applied pin 5 of IC1b is held at
logic low level for a couple of milliseconds until the 100nF
capacitor connected from this pin to ground has time to
charge up via the 100kW resistor.
With the flipflop in this reset state, the gate of MOSFET
transistor Q1 is held down at ground potential via pin 10
of IC1 and the 100W suppressor resistor. Q1 is therefore
held off. But when pin 11 of IC1d drops low as a result
of the circuit sensing a valid burst of ring signal and the
flipflop is triggered into switching into its set state, this
results in pin 10 of IC1c switching high. This switches on
Q1, allowing it in turn to activate the relay and LED2, to
indicate that a call has been received.
It stays this way until you physically reset the Missed
Call Alert by pressing pushbutton switch S1 briefly. This
discharges the 100nF capacitor, applying a negative-going
pulse to the pin 5 input of IC1b until the capacitor recharges
siliconchip.com.au
NOTE: The Miss
ed Call Alert
is not ACA App
roved
All devices desig
ned to connect dir
ectly to the PSTN
(public switched
telephone networ
k) require official
Approval before
ACA
they may be leg
ally connected to
Telstra network.
the
Strictly speaking
this requirement
also applies to ou
Phone/Fax Missed
r
Call Aler t. Howeve
r in designing the
we have taken gr
unit
eat care to ensure
that if built up exac
as described, the
tly
Missed Call Aler t
may be connected
PSTN line with co
to a
mplete safety.
via the 100kW resistor.
This brief negative-going pulse switches the flipflop back
to its reset state, with pin 10 held at ground potential and
Q1 turned off again. Thus the relay is de-activated and
LED2 also turns off.
Most of the circuit operates from a nominal 12V DC
supply, which can come from either a 12V plugpack or a
12V battery. Diode D5 provides protection against reverse
polarity damage, while ZD2 ensures that the supply voltage for IC1 can never exceed about 12.5V. LED1 provides
power indication.
By the way, you can use either a standard 5mm red LED
for LED2 or alternatively, one of the types featuring its own
built-in flasher. The latter cost about 75 cents more than a
standard LED but the flashing feature makes them better at
attracting your attention. Whichever type of LED you use,
March 2006 85
the relay simply turns on when a call occurs and remains
on until you press the reset button.
should be hassle-free if you use the overlay diagram and
photos as a guide.
Begin by fitting the two RJ12 sockets at the left-hand end.
No electrical connection
Make sure all six of their fine connection wires are passNote that because we’re using an optocoupler to sense
ing down freely through the matching board holes before
the presence of a ring signal on the phone line and couple
you push their plastic mounting spigots through the larger
this information to the rest of the circuit, there is no ohmic
holes, as the latter tend to ‘snap’ into place and this can
connection between the two – or between the phone line
buckle any of the fine wires that isn’t free to move. Then
and anything connected to the Missed Call Alert via the
when both sockets are snapped into position, turn the board
plug pack supply or the relay contacts.
over and solder all wire leads to their pads.
In fact the optocoupler provides a very high degree of
Next fit DC connector CON3 to the board, along with
isolation: it can withstand voltages of more than 5000V
the 3-way terminal block. Then fit the resistors, including
peak for over a minute, with an isolation resistance of
the larger 22kW 1W unit which fits in the lower left-hand
something like 100GW (yes, Gigohms!).
corner.
So the Missed Call Alert itself is well protected against
Follow these with the non-polarised capacitors, noting
damage from ‘spikes’ on the phone line, and the phone
that the 100nF MKT cap goes just above the vertically
line is also protected against anything nasty which might
orientated 100W resistor, while the three smaller 100nF
somehow find its way into the circuit.
multilayer monolithics go in the other positions. Note
also that the board has four holes to fit the 470nF capaciConstruction
tor in the upper left-hand corner, so you should have no
Building the Missed Call Alert is particularly easy, betrouble fitting either a 630V metallised polyester cap or a
cause virtually all of the elec250V AC X2 class metallised
tronic parts are mounted on a
polypropylene.
small PC board. This measures
Next fit the two electrolytic
122 x 58mm, carries the code
caps, taking care to fit both
Please note that the Phone/Fax Missed Call Alert is only
12103061 and has rounded
with their positive leads
suitable for detecting the arrival of calls for a standard
cutouts in each corner so that
downwards. After these you
(‘POTS’) phone or fax machine.
it fits easily inside a standard
can fit the 1N4004 power
It is not suitable for use with fax machines connected to
UB3 utility box. It actually
diodes D1-D6, which are all
Telstra’s FaxStream system, as this uses carrier signals
mounts behind the lid of the
orientated with their cathode
higher in frequency than speech signals and does not use
box, via four 25mm long M3
band ends upwards. Then fit
a 17Hz ring signal – rather like ADSL data communication.
tapped spacers.
ZD1, which is orientated the
Wiring up the PC board
same way, and finally signal
It’s not compatible with FaxStream
86 Silicon Chip
siliconchip.com.au
Construction is really simple because everything mounts on the one PC board. Note the “extensions” to the pushbutton
(reset) switch – these are required to bring the switch up to front panel height. The LEDs just poke through the panel.
diode D7 and ZD2. The last two
are both mounted ‘east-west’,
with their cathode bands towards the west (or left).
Now fit the ‘active’ semiconductor parts, starting with
Opto1 and progressing through
IC1 and Q1. Take care when
you’re soldering the last two
in place, as they’re both CMOS
devices and therefore susceptible to static charge damage. Use
an earthed soldering iron and
ideally, earth yourself as well
using a wrist strap.
Don’t fit the two LEDs permanently at this stage but just pass
the leads of each one through
the board holes (longer anode
leads uppermost) and just
tack-solder the cathode leads
to their pads to hold each LED
in place.
Now fit the relay to the
board, soldering all five of its
connection pins to the pads
underneath. This will make
your board assembly ready for
fitting to the lid of your box.
If you’re building the project
from scratch, you’ll now need
to drill the various holes in the
box lid as shown in the cutting
siliconchip.com.au
Use the photo above in conjunction with the layout below to assemble the project.
March 2006 87
ALL DIMENSIONS IN MM
F
F
F
F
HOLE A:
HOLES B:
HOLES C:
HOLE D:
HOLE E:
HOLE F:
12mm diam
5.25mm diam
3.5mm diam, csk
8 X 15mm, rect.
8mm diam
Existing
You’ll need to drill
the UB3 case as
shown in these
diagrams. The four
spacer holes are
later hidden by the
front panel.
diagram. You might also want to cut
the holes in the ends of the box itself
as well, ready for the final assembly.
If you’re building from a kit, this may
all have been done for you.
Before you fit the PC board assem-
bly to the lid, first fit pushbutton S1
through the 12mm diameter hole in
the lower centre and gently tighten its
large plastic nut underneath using a
spanner. Don’t apply too much force,
as this will strip the plastic threads.
Now turn over the lid, and solder two
20mm lengths of tinned copper wire
(or resistor lead offcuts) to the switch
connection lugs, to extend them.
Next fit the four 25mm long M3
tapped spacers to the rear of the lid, us-
Here’s how to connect it to the phone line. At left are shots of the drilled case lid
and the completed assembly going into the case.
88 Silicon Chip
siliconchip.com.au
ing four countersink-head M3 screws
and with an M3 flat washer between
each spacer and the rear of the lid. The
washers are used to extend the lid-PC
board spacing to about 25mm, because
this is necessary to provide clearance
for the relay.
Bring the PC board up to the rear
of the lid, lining it up carefully so
the wires from the lugs of S1 pass
through their matching holes in the
board and the bodies of the two LEDs
pass through their matching holes in
the lid. Then with the board resting
on the outer ends for the four spacers,
fit the four round-head M3 screws to
fasten the assembly together.
You should now be able to solder
the wires from S1 to their surrounding
pads on the board. It will probably also
be necessary to untack the temporary
LED lead joints, to let you extend the
LED bodies through their lid holes as
far as they’ll go, while leaving enough
of the leads on the copper side of the
board to allow you to make the final
‘permanent’ solder joints. Your Missed
Call Alert will then be fully wired and
assembled, and ready to be fitted into
the box. It simply slips down into the
box vertically, after which you can fit
the four box assembly screws and the
plastic sealing bungs.
Putting it to work
There are no setting-up adjustments
to be made to the Missed Call Alert, so
you should be able to put it to work
as soon as it’s finished. Just connect
it into the phone line of your phone
or fax machine using readily available
RJ12-RJ12 modular cables, as shown
in the hookup diagram, then supply it
with 12V DC power from either a plug
pack or a suitable battery. You won’t
need a heavy-duty plug pack -- the
Call Alart only draws about 36mA in
standby mode, or about 80mA when
it has registered a call and activated
LED2 and the relay.
That’s it. You should find that as
soon as a call arrives, the Missed
Call Alert will light (or flash) LED2
and activate the relay. So if you use
Above is the same-size front panel artwork (also useful as a template!) while the
PC board pattern is shown below.
the relay contacts to control a lamp
or siren in another room, they’ll be
activated as well.
To reiterate, to reset the Missed Call
Alert all you have to do is press the
reset button which turns off LED2 and
the relay, resetting the circuitry to wait
for the next call.
SC
Parts List
1 PC board, code 12103061,
122 x 58mm
1 ABS jiffy box, UB3 size
(130 x 67 x 44mm)
1 2.5mm concentric DC
connector, PC board
mounting (CON3)
2 RJ12 modular phone socket,
PC board mtg (CON1,CON2)
1 3-way terminal block, PC board
mounting
1 SPDT 10A PC board mounting
relay, 12V/200-285W coil
1 SPST momentary pushbutton,
panel mounting
4 25mm long M3 tapped spacers
4 M3 flat washers
4 6mm long M3 machine screws,
csk head
4 6mm long M3 machine screws,
round head
Semiconductors
1 4093B quad CMOS Schmitt
NAND (IC1)
1 4N25 or 4N28 optocoupler
(OPTO1)
1 2N7000 N-channel MOSFET
(Q1)
1 3.3V 1W zener diode(ZD1)
1 12V 1W zener diode(ZD2)
1 5mm green LED (LED1)
1 5mm red LED, flashing type
if desired (LED2)
6 1N4004 power diode (D1-D6)
1 1N4148 signal diode (D7)
Capacitors
1 1000mF 16V RB electrolytic
1 10mF 16V RB electrolytic
1 470nF 630V metallised polyester (or 250V AC Class X2
metallised polypropylene)
3 100nF multilayer monolithic
1 100nF MKT metallised
polyester
Resistors (0.25W 1% unless
specified)
1 1MW
2 100kW
1 22kW 1W carbon composite
2 1.2kW
2 100W
siliconchip.com.au
March 2006 89
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