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Build this telephone intercont Do you have a couple of old telephones sitting in your junkbox? This simple project will turn them into a useful intercom unit that works just like a real telephone system. By GREIG SHERIDAN Ever wondered how Commissioner Gordon managed to raise Batman so effortlessly on the hotline? Or have you ever been curious to know how the White House-Kremlin hotline works? Both systems are probably very similar in concept to this telephone intercom - ordinary telephones connected to 2-wire lines and featuring full-duplex operation (ie, simultane58 SILICON CHIP ous 2-way conversation). As well, this unit is simple to drive, easy to build and doesn't cost the earth! As well as an intercom, this interface is capable of testing telephones, modems, DTMF decoders and facsimile and answering machines. It is also ideal for theatrical applications. A stripped-down version could also be used to make role-playing training sessions more realistic at telephone counselling services. In fact, this is the purpose for which the circuit was initially designed. Telephones - an introduction A telephone in its on-hook (hung up) state exhibits a capacitance and series resistance between both legs of the line. This is the ringing circuit see Fig.1. An AC ringing signal will pass and cause the bells to ring or the "tone ringer" to warble. When the phone is taken off-hook, a DC loop (mainly resistive) is applied to the line and DC flows, fed from the exchange. The exchange equipment detects this current flow and either stops the ringing signal (for an incoming call) or sends dial tone (if it's an outgoing call attempt). When a phone is rung, the ringing signal is connected to one leg of the line and the return path is through and frequency are not critical. For applications where a higher ring voltage is required (generally the 800 series phones and their Bakelite predecessors), a larger ring transformer can be used. As well as the Arlec 75XXX and Know how 7VA transformers, the circuit board has been designed to accept the Farnell T 150-07X 6VA series. The I I Farnell 24-0-24 (150-076) rings ..J.. RL1 RL2 an 800 series telephone much more convincingly. For those who want to go L - - - - - - - - - - - 4 - - S~P~Y over the top, replace the power Fig.1: the basic telephone ringing circuit. supply shown here with an When one phone is taken off-hook, the ring ex-Telecom 50V supply (comrelays in the other loop close & a ringing plete with 75V ring). These signal is applied to the second phone. supplies occasionally pop up at electronics disposals stores the DC supply. The idle line voltage is for a fraction of their "new" value. usually around 48V, although this is To operate the intercom, you simnot critical. ply lift one phone and the other phone When the phone handset is off- rings. Lifting the second phone then hook, a current of 20-30mA flows. stops the ringing and conversation This is sufficient to power the phone's can commence. transmitter (microphone) , whether it The circuit does not reset until both is a modern electronic type with an telephone handsets are replaced. This in-built preamplifier or the original prevents the first phone to be hung up carbon granule type. from ringing until the second phone This loop current also powers any "clears". It also allows one party to dialling circuitry where appropriate. hang up and continue the conversaThis circuit emulates the above con- tion on another extension on the line. ditions, which makes it compatible Circuit details with just about all types of telephone. However, there are a couple of deviaFig.2 shows the full circuit details. tions from standard telephone prac- All the required voltages are derived tice. from two power transformers. One First, the circuit described here uses transformer (Tl) provides a +12V rail a negative earth whereas telecommu- (via D5 and a 3-terminal regulator) for nications equipment generally runs the logic and around 46V DC for the on a positive earth. This convention telephone "speech" voltage. The secwas chosen to minimise electrolytic ond transformer (T2) produces around action in earth stakes and the like but 90V peak-to-peak (nominally 30V as this is not applicable here, it has RMS) for the ring signal. been omitted. Besides, I hate trying to The whole circuit uses only four think and design for a positive earth. CMOS chips and two optoisolators to The ring voltage and frequency have provide the logic and generate the also been altered to keep the project required ringing signal. Let's see how easy to build and the cost as low as it works. Schmitt inverter stages ICla & IClb possible. The normal exchange ring signal is 75-90V RMS at 25Hz but, by provide buffering and false triggering experimentation, it has been found protection for phone 1. In the idle that this voltage and frequency are state, there is no DC flowing in the only required when driving older tele- phone line and so the LED in the phones with real bells. associated optoisolator (IC5) is off. Modern telephones usually rectify ICla's input is thus held high, which the incoming ring signal and regulate means that pin 12 ofIClb is also high. it to about 12V, which then becomes Similarly, if phone 2 is on-hook, the power supply for the ringer chip . pin 10 of ICld will be high. These So, for these phones, the ring voltage logic highs are fed to pins 5 & 6 of PARTS LIST 1 metal case, 256 x 155 x 76mm (W x D x H) 1 PC board, code SC12105921, 132 x 102mm 1 PC board, code SC12105922, 105 x 103mm 1 mains cord & 3-pin plug 2 15-0-15V 7VA PC-mounting transformers (see note 1) 1 600:600 ohm transformer (Arlec 45035, Altronics M1000, Jaycar MA-1510). 2 12V DPDT relays (Jaycar SY4061 or Altronics S-4165) 2 LED bezels 1 4-way screw terminal panel 1 100kO 10mm horizontal trim pot 19 PC stakes 1 cordgrip grommet 1 solder lug Semiconductors 1 74C14/40106 hex Schmitt inverter (IC1) 1 4081 B quad AND gate (IC2) 1 4001 B quad NOR gate (IC3) 1 4017B decade counter (IC4) 2 4N25 optoisolators (IC5,IC6) 4 BC547 NPN transistors (01 ,02,03,05) 1 BC557 PNP transistor (04) 7 1N4004 diodes (D1-D7) 4 1N914/1 N4148 diodes (D8D11) 2 3.3V 1W zener diodes (ZD1, ZD2) 1 5mm green LED (LED 1) 1 5mm orange LED (LED 2) Capacitors 1 3300µF 63V PC electrolytic 1 1000µF 35V PC electrolytic 2 10µF 25V PC electrolytic 3 1µF 25V PC electrolytic Resistors (0.5W, 1%) 3 100kO 2 6800 2 47kO 3 2200 6 10kO 2 1500 21.2kO1W Miscellaneous Machine screws, nuts & washers; hook-up wire; two telephones; 2way telephone cable. Note 1 : if a higher ring voltage is required, use a 24-0-24V transformer (eg, Farnell 150-076) for the ring supply instead of one of the specified 15-0-15V transformers. MAY1992 59 + 1 2 V • - - - - - - - - - - - - + - - - - - - - - -....- - - - - - - - . . - - - ~ - - - , RING 202 3.3V PHONE 2 100k 15 R .,. +12V 16 D10 VR1 100k IC4 4017 Q3 BC547 37 240VAC ":' 3300 63VW 14 + 1000l25VW+ T2 15V-0-15V OR 24V-0-24V D5 +12v---------- 8 rO.c 1N4004 VIEWED FROM BELOW TELEPHONE INTERCOM Fig.2: the final circuit uses optoisolators IC5 & IC6 to isolate the ring circuits from the control logic. When one phone is taken off-hook, then either Ql or Q2 turns on to activate the ring relay in the other circuit. Transformer T2 provides the ring signal, while IC4 & its associated parts generate the ring sequence. AND gate ICZa, to pins 5 & 6 of NOR gate IC3a, to pin 13 ofIC2c, and to pin 8 ofIC2d. Assuming that both phones are initially idle (ie, on-hook), the output of ICZa will also be high. This holds the RS flipflop formed by IC3b & IC3c in its reset state, with "Q" low & "Q-bar" high. The Set input of the RS flipflop 60 SILICON CHIP (pin 13 of IC3c) is fed from pin 4 of IC3a which is currently at logic 0. If phone 1 is taken off-hook, current flows through the LED in optoisolator IC5 and turns on the internal transistor. This pulls pin 1 of ICla low and thus the previous high on the reset of the flipflop is also toggled low (via IClb & ICZa), which means that the flipflop can now be toggled. Similarly, if phone 2 is taken offhook, pin 3 of IClc goes low and toggle~ the Reset of the flipflop low via ICld & ICZa. IC3d detects any difference between the states of the two phones. When a difference is detected (ie, when one phone is taken off-hook), its output at pin 3 goes high and turns on LED 1 via transistor Q5 (actually, ICZa, IC3a & IC3d together form an XOR gate to detect the different phone states). TABLE 1 Symptom Possible Fault No sidetone in either phone Is green "power-on" LED lit? Check all voltages. If either + 12V or +46V rail missing, switch off & di$COnnect the logic PC board from the supply. Power up & check the supply rails again. If supply rail(s) still missing, check the supply board; if supply rai ls now correct, check for a snort on the logic board. No sidetone in one phone only (1 ). Check that the associated 220-ohm protection resistor in the loop circuit has not burnt out. (2) . Do cabling & telephone test OK? Check by swapping phone lines over on the back of the interface. If the same phone still has no sidetone, then the fault is either in the phone itself or in the cable. No ring to either phone (1 ). Check that the 220-ohm ring protect resistor (connected to transformer T2) has not burnt out due to excessive current. Check the loop circuitry carefully before replacing this resistor. (2). Are interrupted ring counter IC4 & transistor 03 operating? Test by earthing collector of 03. (3). Check that the orange LED (LED 2) is off when both phones are on-hook. If LED is on & logic circuitry is OK, then one phone is faulty or there is a cabling fault. Test this by removing the wires from the interface. No ring to one phone (1 ). Check ring supply voltage from transformer T2. (2) . Is associated ring relay (RLY1 or RLY2) operating? If phone 1 does not ring, check for high on pin 11 of IC2c. If pin 11 high, suspect switching transistor 01 & RLY1. If phone 2 does not ring, check for high on pin 1O of IC2d. If pin 1O high, suspect 02 & RLY2. In addition, when IC3d's output goes high, pin 3 of IC2b also goes high. Depending on which phone was taken off-hook, then pin 11 of IC2c or pin 10 ofIC2d will switch high. (Note: IC2 is a 4081 quad 2-input AND gate. When both inputs of these AND gates are high, the gate output will be high). Let's assume that phone 1 has been taken off-hook. In this case, pin 10 of IC2d switches high and forward biases Q2. Q2 then subsequently turns on and activates ring relay RLY2 whenever Q3 in the ring circuit turns on. Similarly, if phone 2 is taken offhook first , pin 11 of IC2c switches high and forward biases Ql. In other words, taking phone 1 offhook closes RLY2 and rings the bell on the other phone. And vice versa. the beginning the ringing cycle, rather than having to wait for possibly up to two seconds for the ringing cycle to begin. IC4 is clocked by Schmitt oscillator stage IClf and turns on Q3 and ring relay RLY2 each time its "1" and "3" outputs go high. It also switches interrupt transistor Q3 (via its "2" output) to generate the required ring cadence. The exact operation of the ring cadence generator is described a little further on. Each time RLY2 operates, its contacts close and the AC ring signal from transformer T2 flows through phone 2, ZD2, the series 1.2kQ and 220Q resistors, and the 3300µF filter capacitor on the +48V supply rail. If the calling phone now goes back on-hook, pin 3 ofIC3d goes low again Ring circuit and the circuit reverts to the idle conCounter stage IC4 (4017) and its · dition. Alternatively, if the called party associated parts form the ring timer answers, pin 4 of IC3a goes high and circuit. It is normally held reset by toggles the flipflop (IC3b & IC3c). This ICle but is activated when pin 3 of sets Q-bar of the flipflop (pin 11 of IC2b goes high and pin 8 of ICle IC3c) low, which in turn switches pin switches low. This ensures that the 3 of IC2b low and stops the ring. Conversation can now proceed, counter is only one "clock tick" off g QUALITY NO BRAND DISKETTES 5.25" 5.25" 3.5" 3.5" DSDD DSHD DSDD DSHD $4.70 $8.95 $8.50 $16.95 II I ----------EPROIIS 2716 .... ..... .. .. $9.95 2732A .. .. .. .... . $7.50 27C32 .. ....... $11.50 2764 ............. $6.50 27C64 .......... . $7.00 27128 .... .. .. .. . $7.50 27C128 ......... $8.00 27256 ........... $8.50 27C256 .. .. ..... $9.00 27512 ......... $12.50 27C512 .... .. . $13.50 27C1001 ... .. $23.50 RAMS 6116 .... .. ....... $4.50 6264 ........ ..... $9.50 62256 .... .... . $17.50 628128 ...... . $60.00 SERIALPOFIT CHIPS 8250 .. .... ... ... . $7.50 82450 .. ....... $17.50 16C450 ....... $18.00 16C550 .. .. ... $29.50 *************** ~ ~ ~-~ - n r PRIMES~ S DYNAMIC RAMS 4164-10 ........... $3.25 41464-08 ... ... ... $2.75 41256-08 ......... $2. 75 414256-08 ....... $7.95 411000-08 ....... $7.95 256KSIM,SIP. $23.50 IM,SIM,SIP .... $79.00 4M SIM,SIP . $335.00 llfGUUTORI 7805T ... ........... $0.50 7812T ...... ........ $0.50 7815T .......... .... $0.50 7905T .............. $0.60 7912T ...... ........ $0.60 7915T .............. $0.60 317T ................ $1.50 337T ................ $2.10 723 .................. $0.80 DIDDEI IN4148 .......... .. $0.04 IN914 ......... ..... $0.05 IN4004 .......... .. $0.08 IN4007 ............ $0.10 IN5404 ............ $0.20 IN5408 ............ $0.25 TIIMIISTORS BC547/8/9 ........ 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LEDS 5mm RED ........ 5mm Green ..... 5mm Yellow .... $0.50 $1.50 $2.50 $3.50 $3.95 FREE PRICE LIST UPON REQUEST $0.15 $0.25 $0.25 ELECTRONICS & COMPUTER SUPPLIERS 289 LATROBE ST MELBOURNE, VICTORIA 3000 PHONE: (03) 602 3499 FAX: (03) 670 6006 MAIL ORDER HOTLINE: (008) 33 5901 ALL MAJOR CREDIT CARDS WELCOME POSTAGE RATES $1.00-$9.99 ......... $3.00 $10.00-$24.99 ..... $3.50 $25.00-$49.99 ..... $4.50 $50.00-$99.99 .... $5.50 $100.00-$199.00 $6.00 $200.00+ ............. FREE M AY1992 61 CORD GRW GROMMET PHONE 1 PHONE 2 REAR PANEL ACTIVE (BROWN) NEUTRAL (BLUE) • • • 240VAC T1 • • • 01000uF • • • T2 • ,. G• 7812 e+46V O• • • FRONT PANEL Fig.3: this diagram shows how the parts are installed on the PC boards. Note that the two 1.2kQ 1W resistors on the logic board are soldered to PC stakes (see text). The two boards are connected together using 4-way telephone cable. with the audio signal coupled by the 600:600 ohm transformer (T3). When one party subsequently replaces the handset, pin 3 of IC3d will go high and attempt to ring the idle phone. Ringing will not proceed, however, because the flipflop is set (Q-bar low), thus depriving IC2b of the required logic high. The flipflop remains set until both handsets are replaced and IC2a resets the logic to its standby state, ready for another call attempt. Ring sequence The ring cadence can be easily customised to suit your applicmtion. Some constructors may choose to build the 62 SILICON Cf:IIP unit as close to the Australian standard as possible, whereas others may opt for an" American-sounding" ring, or some other sequence. The standard ring cycle that we are familiar with has the following pattern: 400ms on, zooms off, 400ms on, 2s off, and so on. To generate this sequence, we would normally require a zooms clock period and a counter with 15 outputs (ie, we would have to use two counter !Cs in cascade). Another way is to use a single standard decade counter, the 4017, and addJogic to make it suit the application. In this circuit, the 4017 (IC4) is fed from a 2.5Hz clock (IClf) which pro- vides a period of 400ms per step. However, as the output corresponding to the zooms "off" period goes high (output "2", pin 4), the clock is doubled in speed to give the required zooms period. This is easily accomplished using PNP transistor Q4 and the lOµF capacitor wired between its emitter and collector. In operation, Q4 is normally conducting and the lOµF capacitor across it is short circuited. However, when output "2" (pin 4) of IC4 goes high, it turns off Q4 and this switches the lOµF capacitor in series with an existing lOµF capacitor in the clock's timing circuit . . Because the two capacitors are in series and of the same value, the total capacitance seen by IClf is now halved. The clock therefore doubles its frequency, giving one zooms burst to step the counter past output "2". When output "3" goes high, Q4 turns back on again and the clock reverts to its 400ms period. Diode DB resets the counter when output "9" (pin 11) goes high to limit the off period to 2s following the second 400ms ring. The ring sequence is then repeated. Although Fig.2 shows the circuit arrangement for the Australian ring standard, you can easily customise the ring to suit your own requirements. For example, the clock frequency can be adjusted over a wide The two PC boards are housed in a · metal case & secured on 5mm spacers using machine screws & nuts. The screw terminals on the rear panel provide the connections for the lines to each telephone. range using VR1, or the ringing sequence can be changed by using different counter outputs. If you don't require the standard "ring-ring" cadence, omit Q4 and replace the lOµF capacitor between its emitter and collector with a wire link. check the circuit boards for undrilled holes and damaged tracks. In particular, check the logic board around ICZ & IC3, since the tracks here are very fine. Check also that the mounting holes and the holes for the transformers, filter capacitor, relays, PC stakes and trimpot are large enough. Fig.3 shows the wiring details. Begin by installing PC stakes at all exter- Construction Construction is straightforward, with most of the parts accommodated on two PC boards. One board (code SC12105921) carries all the logic circuitry, while the second board (code SC12105922) carries the power supply components. Before mounting any of the parts, TABLE 1: RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 No. 3 2 6 2 2 3 2 Value 4-Band Code (1%) 5-Band Code (1%) 100kQ 47kQ 10kQ 1.2kQ 680Q 2200 150Q brown black yellow brown yellow violet orange brown brown black orange brown brown red red brown blue grey brown brown red red brown brown brown green brown brown brown black black orange brown yellow violet black red brown brown black black red brown brown red black brown brown blue grey black black brown red red black black brown green black black brown MAY 1992 63 0 - ( \J (]\ -- If) C) ( \J u (I) 0 0 0 0 0 (\J (\J (J\ If) 0 -( \J u (I) 0 0 Fig.4: here are the full-size patterns for the two PC boards. 64 SILICON CHIP nal wiring points on the power supply board, then fit the 7812 voltage regulator, capacitors and transformers. Take care with the orientation of the voltage regulator - see Fig.2 for the pin connection details. Moving now to the logic board, install the five wire links before mounting any of the other parts (one link runs underneath two ICs). Once these are in, fit PC stakes to the external wiring points and install the remaining parts as shown, leaving the relays and transformer until last. Note that you should also fit PG stakes at the mounting points for the two 1.2kQ current limiting resistors (in series with ZD1 & ZD2), since these values may have to be adjusted when the circuit is operational. The prototype was housed in a metal case measuring 256 x 76 x 155mm and fitted with an adhesive aluminium label. After attaching the label, drill mounting holes in the front panel for the two LEDs, then drill the rear panel to accept the mains cordgrip grommet, an earth lug mounting screw and the screw terminals for the telephone lines. The two PC boards can be used as templates to mark out their mounting holes on the bottom of the case. Once the holes have been drilled, secure the mains cord to the case using the cordgrip grommet and solder the Active (brown) and Neutral (blue) leads to the power supply board. The Earth lead (green/yellow) is connected to the earth lug on the rear panel. This lead should be made longer than the Active and Neutral leads, so that it will be the last to break if the cordgrip grommet comes adrift. Once the mains wiring has been completed, mount the two boards in the case on 5mm standoffs and secure them using screws, nuts and star washers. The remainder of the wiring can now be completed as shown in Fig.3. This includes a 4-wire connection between the two boards, plus wiring from the logic board to the front and rear panels. Testing Before applying power, go over the project carefully and check for wiring errors. In particular, check that all parts are correctly oriented and that the mains cord is securely held by the cordgrip grommet. When you are satisfied that everything is correct, switch on and check the supply voltages. The output of the 7812 regulator should be at +12V, as should pin 14 ofIC1, IC2 and IC3, and pin 16 of IC4. The positive terminal of the 3300µF filter capacitor should be at about +46V. Exercise caution when making these measurements, as mains voltages are present tended life for carbon granule type transmitters. The level of sidetone (ie, the level at which you hear your own voice) is also be reduced. Of the phones tested, many gave reliable communication with as little as lOmA of loop current, although one Siemens model would not send DTMF tones from its keypad until it . had about 25mA flowing. Fault finding The two 1.2kQ lW loop current limiting resistors may have to be reduced in value for telephone lines of considerable length. Generally, you should aim for loop currents of 10-25mA. You can check this current by connecting your multimeter across each telephone while it is on-hook. on the underside of the power supply board. Assuming that the supply voitages check out, short pins 4 and 5 of one of the optoisolators (IC5 or IC6). One of ring relays should now begin operating, according to the programmed ring sequence. Trimpot VR1 can now be adjusted to give the correct clock period. If you want to simulate the Australian ring standard, just adjust VRl so that each complete ring cycle lasts three seconds (ie, 400ms on, zooms off, 400ms on, Zs off). Alternatively, you can connect the two telephones to the circuit, take one of them off-hook, and adjust Fig.5 (below): this full-size artwork can be used as a drilling template for the front panel. VR1 until you get the correct "sound". Installation The values of the current limiting resistors in series with ZD1 and ZDZ may have to be adjusted according to the lengths of the individual lines. Generally, a loop current of 30mA should be considered the maximum. For most in-house or house-togarage use, 1.ZkQ 1W resistors (as shown on Fig.2) will do the job. However, lines of considerable length require lower value resistors because the resistance of the cable itself provides a certain amount of current limiting. If built solely for use as an intercom, the loop current can possibly be reduced to as low as 10mA. The benefits of lower loop current include longer operating distances and ex- If you strike problems, first check that all ICs have+ 12V on their supply pins. You should also check the boards for missed solder joints and for solder shorts between adjacent IC pins (make sure that the power is off). Next, trace through the gates with a logic probe or a digital voltmeter to check that the input logic is operating correctly. Check that pin 12 of IClb, pin 10 of ICld and pin 4 of ICZ are all high when both phones are on-hook. Pin 4 ofICZa should switch low when one phone is taken off-hook. If you don't get the correct readings here, check the optoisolators and the input buffering circuitry (IC1a-IC1d). Table 1 lists a number of possible symptoms and their likely causes. By following this table carefully, you should have little difficulty in tracking down any likely faults. References (1). Telephony, Volumes 1-5. Postmaster-General's Department, Australia. (2). Telephony and Telegraphy. Sydney F. Smith. Oxford University Press. Caution This intercom must not be connected to Telecom lines. It is intended only for use on lines completely separate from Telecom installations. TELEPHONE INTERCOM 0 0 POWER CALLING MAY 1992 65