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(*) UPi Newsletter Volume #1, Issue #4 (*)
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(*) The Extension Silencer & The Tele Mon9jo
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This article was origin published in Popular Electronics - Electronics
Hobbyists Handbook. This article was republished without permission.
The Extension Silencer
By Terrance Kelly
This project keeps your modem communications and important private
conversations from being interrupted by locking out extension phones when the
line is in use.
Are you tired of others listening in on your conversation on an extension
phone? Do you live with a computer hacker who always seems to want to dial out
with his modem in the middle of your of your conversations? Fear no more,
three inexpensive parts - which comprise the Extension Silencer - will solve
all of your problems.
The Extension Silencer described in this article is a simple is a simple
circuit that, when placed in series with each of your extension telephones,
allows privacy at each location without interference from the others. All
phones will ring normally and the first one to answer locks out the others.
Someone picking up an extension will hear nothing if the phone is already in
use. If you need to switch extension all you have to do is pick up one
extension and hang up the other.
In order to fully understand the operation of the Extension Silencer, a
little discussion on telephone operation is in order.
Telephone Operation
Each subscriber telephone is connected in what's called a local loop to
central office, which contains switching equipment, signalling equipment, and a
power source that supplies direct current (DC) for the operation of the
telephone system within its coverage area.
Switches in the central office respond to string of dial pulses to connect
the calling station with the station being called. The Switching array within
the central office feeds an AC ring signal down the line to the station being
called. When a link is established (the telephone receiver is taken off-hook),
the two stations communicate via transformer-coupled loops using DC power
supplied by the central office.
When the telephone receiver is on-hook, the receiver holds down the
hook-switch buttons in the base of the unit, creating an open circuit to DC
between the receiver and the central office. The signaling circuit, which is
AX operated, is always connected to the central office; a capacitor in the
signaling circuit blocks the flow of DC, while passing the AC ring signal.
As long as the hook-switch buttons on the telephone are depressed, no
communications can take place. However, when the receiver is taken off-hook,
the hook-switch closes contacts, passing DC to the telephone, activating the
communications circuits, and causing the normal DC line voltage to drop due to
the loading effect of the telephone.
Circuit Operation
The circuit is little more than an electronic cut-off switch built around a
Triac (the switching element). Two phone station, A and B, are show to make
the circuit's operation easy to understand (Component designations for only one
circuit are given in the Parts List). Note that an Extension Silencer circuit
is connected in series with each station.
Normally when a phone is on hook, the voltage across the phone line is
48-volts DC; when the receiver is taken off hook (is picked up), the DC voltage
across the line drops to between 2 and 12 volts DC (typically about eight
volts). That's due to the off-hook impedance of the telephone and the loop
resistance between the subscriber station and the telephone company's central
office.
As long as both Station A and Station B receivers are on-hook, the voltage
at the gates of TR1 and TR2 is sufficient to trigger TR1 and TR2 into
conduction. That means that both Extension Silencer circuits connected to that
line are conducting. But when one of the receivers - say station A - is lifted
from it's cradle, closing the hood-switch contacts, that telephone draws
considerably more power, causing the line voltage at TR2 to drop to around
0.6-volt DC.
The operational characteristics of the Triac are key to the operation of
the Extension Silencer circuit. Remember that once a Triac is triggered (via a
gate trigger voltage), it continues to conduct as long as the current through
it remains above it holding-current requirement, even if the trigger source is
removed. And will cease to conduct, only when the current through the device
is brought below its holding-current level. (In the case of an AC signal, a
Triac would be triggered into conduction twice during a single cycle: once
during the positive half-cycle, and again during the negative half-cycle.
Therefore the 20-Hz ring signal is passed to the phone uninhibited.)
The 0.6-volt level at the main terminals of TR2 is too small to generate
sufficient current through TR2 to satisfy it's holding-current requirement, so
it turns off, disallowing any communications through station B.
If the station B extension is take off-hook and station A is hung up, a
similar, but reversed action occurs, with station B absorbing the majority of
the line-fed power, reducing the current through TR1 to a level insufficient to
sustain conduction through that unit. And that in turn locks out Station A.
A 15-volt Zener diode was used because that is all the local Radio Shack
had in stock, but anything between 12 and 24 volts would have done just fine.
The value specified for R1 (1000-ohms) was chosen to yield 35 mA of Zener
current. That amount of Zener current may seem excessive, but keep in mind
that it is on for a very short time. before a call is answered there is no
loop current because the phone-hook switch is open, so there is no Zener
current.
Construction
There is nothing critical about the construction of the Extension Silencer.
In fact the author's prototype of the circuit was assembled on a small section
of perfboard - half of Radio Shack's 276-148 snap-apart perfboard - measuring
about 1 3/4 inches square. The circuit-board assembly was then housed in a
modular quick-connect phone-jack cover.
Before assembling any components on the board, the two sections must be
separated and a hole drilled (through the center of the board) for a wood screw
The wood screw will be used for mounting the assembly to the wall. Next,
solder the three parts onto the perfboard, interconnecting the components as
they are installed.
Cut the black and yellow leads from the modular jack as thnce
all telephones use the center two conduction of the sres ander them to the boar
line. Observe the proper polarity of those wires because the one that attaches
to the red wire of the modular jack must connect to the minus terminal of the
phone line.
Installation
WARNING: Remember that all equipment installed on the telephone line must
meet FCC part 68 requirements. The equipment must be certified by an
authorized agent before it is used on the network. (This article is meant to
be of instructional value and not as a certification for FCC approval.)
Since my house had phone jacks with screw terminals inside all I had to do
was remove the face-plate and measure the voltage across the connection-block
terminals. The connection block has four screw terminals labeled L1-L4. In
most cases, the terminals of the connection block are tied to the telephone
line via a quad color-coded cable - red, green, yellow, and black. (Older
installations use twisted twin lead tied into the telephone line.) Normally,
the read and green wires are used as the communications pair, and are connected
to the L1 and L2 terminals.
With my VOM, I determined the polarity of the phone line. If the positive
lead of the meter is on L1 and the negative lead is on L2 and the meter reads a
positive 48-volts DC, then L1 is positive with respect to L2. If it reads
negative, then L2 is positive with respect to L1. In my own installation, the
minus lead was attached in parallel with the L2 terminal (you may find it to be
the L1 terminal) on the phone jack.
The plus terminal was then connected in parallel with the L1 Lead. Remove
the phone from the normal hack and insert its plug into the Extension Silencer
jack, and you are ready to go. Pick up the phone with the silencer and listen
for dial tone and hang up. Pick up the extension without the silencer and
leave it off hook.
Pick up the phone with the silencer and it should be "dead". Hang up the
phone without the silencer and there should be audio from the "dead" phone.
You can now add as many Extension Silencers as you have phones. For those
readers who have a "smart modem", set it to return "NO DIALTONE" if the line is
in use. Also set it to keep retrying so that you can capture the line should
it ever become free.
Part List For The Extension Silencer
TR1 1-amp, 50-PIV Triac
R1 1000-ohm, 1/4 watt, 5% resistor
D1 1N4744, 15 volt, 1-watt Zener diode (Radio Shack 276-564, or similar)
Dual Board Perfboard (Radio Shack 276-148)
Quick-Connect Modular Jack (Radio Shack 279-355)
Clear View Speaker Wire (Radio Shack 287-008)
Solder
Hardware
To <DDDDD*DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD> To
Telephone 3 MT1 MT2 Telephone
Line <DDDDD3DDDDD*DDDDD*DDDDDDDDDDDDDDDDDDDVVVDDDDDDDDDD*DDDDD>
3 3 3 ZDDDDDDVVV TR1 3
3 3 3 3 1A 3
3 3 3 R1 3 50 PIV 3
3 3 3 1k 3 3
3 3 @DDDDVVVVDDDD*DDDDDDDDDVVVVDDDDDDY
3 3 D1
3 3 Station A 1N4744
3 3 15v
3 3
@DDDDD3DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD> To
3 MT1 MT2 Telephone
@DDDDD*DDDDDDDDDDDDDDDDDDDVVVDDDDDDDDDD*DDDDD>
3 ZDDDDDDVVV TR1 3
3 3 1A 3
3 R1 3 50 PIV 3
3 1k 3 3
@DDDDVVVVDDDD*DDDDDDDDDVVVVDDDDDDY
D1
Station B 1N4744
15v
The Tele Monitor
By Charles R. Ball Jr. And W.K. Ball
Now you can keep an audible record of your telephone conversations with the
aid of this simple circuit.
WARNING: The use off the Tele Monitor and the parts therein is governed by
Federal and State law. Federal law prohibits the taping of telephone
conversation by one party without the knowledge and/or consent of the other
party. Many states have adopted identical or similar provisions. Prospective
users are advised to obtain independent advice as to the propriety of the
purchase and use of the parts and use of the Tele Monitor, including (but not
limited to) taping conversations therewith. We make no representations
concerning the parts referred to in this article of their use or the legality
of the use of the Tele Monitor, or any other such equipment referred to in this
article. Readers are advised to obtain independent advice as to the propriety
of their use of the parts, the Tele Monitor, or similar equipment based on
their individual circumstances and jurisdiction.
Remember when your sister called to tell you about this year's family
reunion, but in the excitement of hearing from her, you forgot to write down
the date, time, or even the city? How often have you talked on the phone with
a client for an extended period and after hanging up, remembered that you hadn
Well, such memory lapses need not be a problem if you build the Tele
Monitor described in this article. The Tele Monitor automatically activates a
recorder to keep a perpetual log (within the limits of the tape's capacity) of
all call, incoming or outgoing. Best of all, it can be built for less than
$25, excluding the recorder.
How It Works
The circuit is connected between the telephone line and the tape recorder.
The telephone line - the conductors are designated tip and ring - normally has
-48 volts across it when the telephone is on hook. When the telephone is off
hook, that voltage changes to about -10 volts, depending on the subscriber loop
resistance.
A full-wave bridge rectifier (consisting of D1 through D4) connected across
tip and ring allows the circuit to be tied to the telephone line without regard
to polarity. The voltage applied across tip and ring appears, polarity
corrected, at the + and - ends of the diode bridge. When the phone is on hook,
the voltage through D5 keeps Q1 on and Q2 off, so the recorder is in an idle
state.
When any telephone tied to the circuit is picked up (an goes off-hook), the
voltage across tip and ring drops to about -10 volts, causing Q1 to turn off
and Q2 to turn on, activating the recorder through J2. Incoming calls also
activate the recorder with each ringing cycle.
When the phone is off-hook and the recorder is running, voice is coupled
through R1 and the C1/C2 combination. Because the central-office ringing
voltage is 105-volts AC, C1 and C2 - which must have a voltage rating of
150-volts or better - are connected back-to-back to simulate a non-polarized
capacitor. The diodes, D1 through D4, should have a peak inverse-voltage (PIV)
rating 200 volts of better.
Power for Tele Monitor is derived from the telephone line. A very small
current (less than 80uA) is derived from the telephone line through the diode
bridge when the phone is on-hook, so that Q1 remains off, keeping the recorder
is an idle state. (CAUTION: For use in this application, the recorder must be
battery operated or powered with a UL/CSA/FCC approved adapter. Nether the
telephone company or the FCC take kindly to having 120-volts AC suddenly appear
on the telephone line.)
It is recommended that the Tele Monitor circuit be coupled to a recorder
with automatic volume control to compensate for varying signal levels on
different calls and varying speech characteristics. The recorder mentioned in
the Parts List is one such recorder. Other recorders can be used, but you must
be sure that the internal remote-jack connections have the proper polarity for
Q2 to turn on the recorder.
Assembly
The author's prototype of the Tele monitor was assembled on a printed
circuit board measuring about 2 3/4 X 2 1/4 inches. The layout provides for
two modular connectors that allow you to connect your phone and the Tele Monito
Once you have obtained or etched the board and are in possession of all the
components, begin assembling the project using the project. When stuffing the
board, be careful of component orientations, particularly the diodes,
capacitors, and transistors. Transistor Q2 should be heat sinked because the
combination of the saturation drop in Q2 and motor current in the recorder may
exceed the transistor's ambient thermal rating. Usually a small clip-on heat
sink is adequate.
Next, prepare a cable for connection to the remote jack of the recorder.
Select a plug that's suitable for mating with the remote jack of your recorder.
almost any wire will do, but for ease of soldering to the plug, 24 to 26 gauge
stranded wire is recommended. Make sure that the circuit-board pad marked "T"
is connected to the "tip" of the remote plug and and "R" to the other lug of
the plug.
After soldering the wires to the plug, twisting the wires about 2 turns per
inch will prevent the cable from tangling. If you are using a recorder other
than the once listed, check the recorder schematic to ensure that the circuit
is properly connected to the recorder.
Next, prepare a cable for connection to the mike jack. Again, you must
select a plug that's suitable for mating with the jack of your recorder. The
use of shielded audio cable is recommended. Solder the prepared cable to the
appropriate circuit-board pads.
Once the board is assembled and the cables connected, clean the flux from
the board, especially in the area of the modular connectors. Solder flux, when
damp, is conductive and can cause erratic operation. Also check for
misoriented or misplaced components, solder bridges, cold solder joints, and
all the other construction errors common to hobby electronic projects.
You must also prepare or purchase modular plug-to-plug telephone cable
(both ends terminated in modular plugs), which will be used to connect the
circuit to the telephone line. Radio Shack sells them in varying lengths, with
either coiled and flat cable. If a cable of greater length than is available
is needed, it will be necessary to make your own. All of the equipment and
material needed to handle that task is available from Radio Shack as well.
Preparing the Enclosure
A circuit is required for the modular telephone sockets, SO1 and SO2. Two
cutouts can be made (one for each socket), or a single long one, as shown. A
nibbling tool, available at electronic stores, is ideal for performing the
task.
Make sure that you check the orientation of the board before making the
cutout; the enclosure has only two posts for mounting the board. Position the
board, component-side down with the two mounting holes lining up with the
mounting posts to determine which part of the enclosure to cut.
Next, drill two holes for the recorder remote and mike cables. A 3/8-inch
hole for each will be adequate. It is recommended, however, that a 3/16-inch
hole be drilled and reamer used to enlarge the hole to the desired size.
Plastic is tricky to drill, and some plastics drill easier than others. The
plastic used to make the enclosure listed in the Parts List will chip, crack,
grab, or self destruct if you use large size bits. Drill speed is also
important; generally, the slower the drill sped the better.
Checkout
There are a number of ways to check out the circuit. First, check the
orientation of each component against the diagram and check all solder joints.
If you have a power supply capable of providing at least 50-volts DC, you
can check the Tele Monitor without connecting it to the telephone line. Attach
the power supply leads to the tip and ring leads of the modular sockets.
Connect J2 to the remote jack of the recorder. Press record and play as you
normally would to record. With the 50-volts DC power supply on, the recorder
should be "off;" with the supply off, the recorder should be running.
If you don't have access to be a 50-volts DC supply, it will be necessary
to use the telephone line as your power source. As before, connect the circuit
to the recorder. Install one of the modular plug-to-plug cable in one of the
Tele Monitor sockets and the other end in the wall. Either connect a phone to
the Tele Monitor or use another phone on the same line for the following test.
Pick up the telephone. The recorder should start; press "1" on the
telephone to eliminate the dial tone, and speak a few words into the mouthpiece
in a normal voice and hang up. The recorder should stop once you have hung up.
Remove the remote plug from the recorder, rewind the tape, press play and then
check the voice quality.
Final Assembly
Once the case is prepared and the circuit checked out, route the recorder
cables through the proper holes, position the board in the case with the
components facing down and the modular sockets lines up with the cutout.
Secure with the two screws supplied with the enclosure. Mount the cover using
the remaining four screws.
If everything checks out, you are ready to monitor. Make sure the remote
and mike plugs are connected, that the modular jack is connected to the
telephone wall jack, press record and play and you are all set. Happy
monitoring.
Telephone Subscriber Line Specifications
Parameter Value Condition
Central Office voltage, tip to ring -48 VDC On-hook
Central Office voltage, tip to ring -10 VDC Off-Hook
Central Office voltage, tip to ring 105 VAC, 20/30 On Hook, Ringing
Subscriber loop current, tip to ring -20 to -80 mA Off-hook, CO Seizure
Subscriber loop resistance, tip to ring 0 to 1200 ohms Off-hook
Parts List For The Tele Monitor
Semiconductors
Q1 TIP120, NPN darlington transitor
Q2 TIP42, PNP silicon power transitor
D1-D4 1N4004 1-amp, 400-PIV rectifier diode
D5 1N4748A, 22-volt, 1-watt Zener diode
Resistors
(All resistors are 1/4-watt, 5% units.)
R1 4300-ohm, 1/2-watt, 5%
R2 1-megeohm, 1/2-watt, 5%
R3 150,000-ohm
R4 390-ohm
Additional Parts And Materials
C1-C2 1uf, 200-WVDC, electrolytic
S01-S02 Modular telephone socket
PL1 See text
PL2 See text
Printed circuit
Enclosure (Radio Shack 270-283 or equivalent)
IC Socket (optional)
Heat sink (AAVID 5741B or equivalent)
Modular telephone plug with cable (Radio Shack 279-397, 279-374, or similar)
Recorder (Radio Shack CTR 70 or equivalant)
Note: The following items are available from BALLco, Inc., PO Box 1078,
Snellville, GA 30278-1078; 404/979-5900; etched, drilled, plated and silk
screened printed circuit board (#881101-R0), $9.95 post paid in the USA; a
complete kit (#TM88) less case and recorder plugs for $24.95 + 2.50 shipping
and handling. And assembled Tele Monitor is available for $39.95, plus $2.00
S/H. A case for unit is available for $3.99 post paid. Florida and Georgia
residents pleases add appropriate sales tax.
ZDDD*DDDDDD*DDDDDD?
3 3 3 3
3 D1 D2 D5 ZDDDDDDDDDDDDDDDDDD*DDDDD?
3 1N4004 1N4004 1N4748A 3 3 3
3 3 3 22V 3 3 3
*DDDDD*DDDDDEDDD* 3 3 3 ZDDDY 3
SO1 3 3 3 3 3 ZDDD4 ZDDDDD4Q2 TIP42 3
*DDDDDEDD*DDEDDDEDDDDDD* @DR3DDDDD*DD4 ZDYQ1 3 @DDD? 3
3 3 3 3 3 150K 3 @D4 TIP120 3 3 3
3 3 3 3 3 3 @DDDDDDDDDD* 3 PL2
3 3 3 D3 D4 3 3 @DDDDDY
3 3 3 1N4004 1N4004 R2 R4
3 3 3 3 3 1MEG 390Ohms
3 3 3 3 3 3 3
*DDDDDY 3 3 @DDDDDD*DDDDDD*DDDDDDDD*DDDDDDDDDDDDDDDY
SO2 3 3 3
*DDDDDDDDY 3 PL1
3 3
@DDDR1DDDDDC1DDC2DY
4.3K 1 1
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Last Updated: July 16, 1991
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