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Music on hold for your telephone How many times have you just sat down to a meal and the telephone rings? Annoying, isn't it? This little project won't solve that problem but it will allow the person on the other end to at least wait with music on the line, until you finish your meal. By JEFF MONEGAL & BRANCO JUSTIC Seriously, how often do you have to put the phone down during a conversation to go and get something? Or maybe you've had to discuss something with someone in your household while the caller "hangs on" 40 SILICON CHIP With this project, you can put the incoming caller on hold with music. When a call comes in and you have to put the phone down for a while, just press the ON button and hang up the phone. The unit will hold the line and send music to the other end. After about 5 minutes, if you have not come back to the phone, it will automatically hang up. In use, the unit connects in parallel with the existing phone and does not interfere with normal phone operation in any way. The music can be from any convenient source and an on-board power switch is provided to power it up when needed. The prototype used a small Walkman-style radio. How it works The complete circuit is shown in Fig.1. We'll begin the description with the "line siezing" section centred around the RS flipflop consisting of IC2a and IC2b. This controls a number of transistors in the +12V I T10. 220pF II-- M•r LYBUZZER 39k i ~-r'" 1M 01 BC327 r- -B 0.1 o--r 47k 02 1N914 -i '"'H '-.1./ 0.47 AUDIO INPUT i T 4-, 39kt 01 1N914 I 03 1N914 ..,. 10 "'-t' ..J.: + B~ 02 BC337 --·-i 330pF .__ .,. 1k I RADIO POWER SUPPLY + ~ T I I ...L. -i 33k TO RADIO V+ 47+ .,. 0.1 010 1N914 PHONE LINE 0.1+ + 07 BC548 EOc ON .,. c0E :ll ..... co co 0 0 B It 14 CLKIC3 4024 AST ~ 100k 1M 100k j1K t s21 OFF 0 IC2d 1 .,. C t:'1 ::: .,. MPSA42 >-'J b:l t:'1 .,. S1 LED1 BC C/J .,. IC2a 4093 100k E B t:'1 "O 100! .00331 39k -:+ 10? D13 1N914 VIEWED FROM BELOW MUSIC-ON-HOLD ii>, I-" Fig.1: the circuit siezes the phone line and switches on the radio when you press the ON button. .,. Fig.2: you can install the parts on the PCB in any order you like but be sure to use the correct part at each location. Use PC stakes to terminate external wiring leads. ;fJ 1 2 ~ □11 ~ rl1igi'i~~~®® .l~ - Ll]J :<at>~ l ~ ®~ I 04 ~ 10 ~ 0 1Q ~ ~ ~ oou{;:) 1 07 [J E:} • c,, ~0• • 88.® "' I0,._®~:,:® ® m L!} ~ ~ TORADIOV+ TO RADIO POWER SUPPLY 05 ~ 01 ~~ ~~ .[Jo,. ~ ~ -· 010 o--c:::D-4 □12 01~ . o 18 ~~ ~ ,.,;:; !.! o ~ t□i =i oo?~ l;Jo6~ ~ ~ TT oOM:J[v' 01 a _\ \ . " "" 02 ®~~ ~ [!,,'7 □2 01 ~ ~ ~ {.-1 ~ s;,J o[J[}, 275L40B 4 ~ 7 u F10uF ~ -c:=n---ru:o,. .----i, - .... r;§ 1GzD e • ~ P HONE 0 +• DJ 220pF~ 1 LINE AUDIO INPUT + ®o[illJe ~ ~ □1 ~7k 0.1 ,.,i-_;_-i 12V circuit. When the hold button, S1, is pressed, pin 3 of IC2a goes high. This turns on Q6 and connects Q7 to the phone line via the diode bridge (D6-D9). Q7 functions as a constant current load for the phone line and thus "holds" the line. LED 1 provides a reference voltage of about 1.7 volts to the base of Q7 and it therefore feeds a constant voltage of about 1V to the 4711 emitter resistor. By this means, Q7 draws a more or less constant current of around 21 milliamps from the phone line. Once Q7 is on, the phone handset can be placed back on the hook and the line will be on hold. When Q6 switches on, it feeds a positive pulse to the reset input (pin 2) of counter IC3 via diode Dl 1 and the 0.1µ,F capacitor. This allows the counter to start counting pulses from Schmitt trigger oscillator, IC2c. Oscillator IC2c and counter IC3 are there for two reasons: (1) to stop you from forgetting that you have left the phone on hold; or (2) if you do forget, to take the line off hold so that you can receive the next incoming call. Let's see how. this happens. As well as providing clock pulses to counter IC3, IC2c turns Q3 on briefly, every two seconds or so. This causes LED 2 to flash briefly and the piezo buzzer, connected via transformer Tl, to click. Thus, Q3 gives an audible and visible indication that the phone is on hold. At the same time, counter IC3 is clocked by IC2c until, after 128 pulses, pin 3 (Q7) goes high. This causes pin 11 of IC2d to go low and reset the RS flipflop via diode D13. This turns off Q6 and Q7 to discon- RESISTOR CODES □ □ □ □ □ □ □ □ □ □ □ 42 No. 3 1 4 3 5 1 1 7 1 1 2 SILICON CH IP Value 1 OMO 1 Mn 1OOkfl 47k11 39k11 33k11 4 . 7k11 1 kfl 10011 4711 2211 4-Band Code brown black blue gold brown black green gold brown black yellow gold yellow violet orange gold orange white orange gold orange orange orange gold yellow violet red gold brown black red gold brown black brown gold yellow violet black gold red red black gold 5-Band Code brown black black green brown brown black black yellow brown brown black black orange brown yellow violet black red brown orange white black red brown orange orange black red brown yellow violet black brown brown brown black black brown brown brown black black black brown yellow violet black gold brown red red black black gold brown CAPACITOR CODES· Value D D D D D D IEC Code 470n 0.47µF 0.1µF 100n .01µF 1 On .0033µF 3n3 330p 330pF 220p 220pF EIA Code 474K 104K 103K 332K 331K 221K nect the line. So the period for which the unit will "hold" the line is about 4½ to 5 minutes, depending on circuit tolerances. Audio section The audio section of the project is centered around ICl, a TL072 dual FET-input op amp. ICla acts as a variable gain stage. It is fed with the audio signal from an external battery-powered radio. The audio signal is ideally taken from a portable (such as a Walkman-style radio) which has a headphone socket. This makes it easy to disconnect the radio for any reason. The gain of ICla can be varied between one (unity) and 22 by the gain control preset pot, VRl. ICla then feeds IClb, Ql and Q2. These are set up as an audio power amplifier to drive the phone line via a 0.47 µF capacitor, a lkQ resistor and the diode bridge, D6-D9. Radio supply switch As noted above, a batterypowered radio (or portable tape player) provides the music signal. To conserve the batteries, the radio only needs to be turned on while the phone is on hold. This can be achieved by placing a pair of switch wires in series with one of the cells in the battery holder. We'll tell you how to do this later. The two switch wires go to PNP transistor Q5 - positive ( + ) to emitter, negative ( - ) to collector. Q5 is an AD162 PNP germanium power transistor, a type now obsolete but still available from some sources. A germanium transistor is used as the switch because when it is saturated (ie, fully turned on), it has a very low voltage drop from emitter to collector. Q5 is turned on by Q4, a PNP silicon transistor driven by pin 4 of IC2b. Q4 is on when pin 4 of IC2b The PCB is mounted on the lid of the case and flying leads run directly to the DC plugpack, the radio and the telephone plug. Alternatively, the power supply, audio and radio supply switch leads can be terminated in sockets mounted on the rear panel. goes low, a complementary condition of pin 3 of IC2a going high. Thus, whenever the RS flipflop (IC2a & IC2b) turns on Q6 and Q7, to place the phone on hold, it also turns on Q4 and Q5 to power up the radio. Power Power for the circuit is provided by an external 12V DC plugpack. Very little current is drawn by the circuit when it is not active. The CMOS ICs (ie, IC2 & IC3) draw very little power anyway and ICl and its associated audio output stage is effectively choked off when the input to ICl (pin 2) is pulled low by IC2a, via diode Dl. Finally, a few words about phone line voltages. The normal DC voltage across phone lines when the phone is "off hook" is about 50 volts. The 25Hz ring voltage is about 70 volts RMS, superimposed on the 50 volts DC already there. This means that the maximum voltage that has to be withstood by Q6 is around 160 volts. This it can do quite happily since it is a 300 volt transistor. Even so, to provide a degree of protection for Q6, a 275V varistor is connected across the output of the diode bridge. Construction All the circuitry for this project is accommodated on a printed circuit board measuring 75 x 132mm. It is mounted on the lid of a plastic jiffy box measuring 159 x 96 x 52mm. Holes are drilled on one side of the case to take the two pushbutton switches and the two light emitting Where to buy the kit A kit of parts for this project is available for $44.90 plus $4 .00 postage and packing from Oatley Electronics, PO Box 89, Oatley, NSW 2223 . Phone (02) 579 4985. The kit does not includes the Telecom male plug, parallel adaptor socket, or 12V DC plugpack. The 12V plugpack is available for an additional $ 1 4 .90. Note : copyright of the PCB artwork associated with this project is retained by Oatley Electronics. SEPTEMBER 1990 43 MNS/t: on via screws and nuts and star washers. Watch the polarity and orientation of the diodes, transistors and integrated circuits. Sockets are recommended for the integrated circuits as they are a real problem if they have to be removed later on. Once the PCB has been assembled, carefully inspect all your work. IIOLD Wiring There is quite a lot of wiring to run inside the case to the printed board. The two wires to the phone line must go to pins 2 and 6 of a standard Telecom male phone plug. Polarity is not important as the bridge rectifier (D6-D9) takes care of that. Testing Keep the wiring tidy by lacing it at regular intervals with plastic cable ties. We used sockets for the ICs but these are optional. The audio input lead should be run using shielded cable to prevent hum pickup. A small Walkman-style radio can be used to provide the music. All you have to do is plug the Music On Hold unit into the headphone socket. Don't forget to wire the plug for mono operation. diodes. Adhesive rubber feet are fitted to the lid of the case so that the box effectively sits upside down when in normal use. Assembling the printed circuit board is fairly straightforward although there are one or two points that need comment. The small audio transformer is secured to the printed board by two lugs 44 SILICO N CHIP which are bent over and soldered on the copper side. The four leads are then soldered to their respective pads on the board. The AD162 is a TO-66 metal case device with its collector connected to case. It is mounted right down onto the board and the base and emitter leads soldered to the board. The collector (case) connection is made Do not connect the unit to the phone line at this stage. Connect a 12V DC plugpack or power supply and connect an audio signal. If you have an oscilloscope, you should find the signal present at the junction of the 220 emitter resistors for Ql and Q2. If no audio signal is present, check all the circuitry associated with ICl. If you don't have an oscilloscope, you can still check that ICl is functioning by listening with a pair of headphones (the higher the impedance the better) via the 0.4 7µF capacitor at the output of the audio stage. The sound will be tinny but that is a consequence of the relatively small coupling capacitor. Press the ON button (Sl). LED 2 should flash and clicking should be heard from the piezo buzzer. Pushing button S2 will stop LED 2 from flashing and the piezo buzzer should cease clicking. Now press button Sl and check that LED 2 stops flashing after about 4 or 5 minutes. So far, so good. Radio switch Earlier on we mentioned connecting a pair of switch wires in series with one of the cells in the radio's battery holder. The way this is done is quite simple. All you need is a small piece of double sided copper laminate; it should measure about 3 to 5mm wide by about 10mm long. One switch wire is soldered to each side of the copper laminate. Use a red wire for the positive connection PARTS LIST 1 PCB, code OE 90 MOH, 75 x 132mm 1 small audio transformer 1 piezo disc buzzer 1 Telecom male telephone plug 1 Telecom parallel adaptor socket 1 plastic jiffy box, 1 59 x 96 x 55mm 3.5mm stereo jack plug (to suit headphone socket of Walkman-style radio) 2 momentary contact pushbutton switches 1 12V DC plugpack Semiconductors 1 TL072 , TL082 FET-input op amp (IC1) 1 4093 quad 2-input Schmitt NAND gate (IC2) 1 4024 7-stage counter (IC3) 1 BC327 NPN transistor (01) 1 BC337 PNP transistor (02) 2 BC559 PNP transistors (03,04) 1 AD162 germanium PNP transistor (05) 1 MPSA42 NPN high voltage transistor (06) 1 BC548 NPN transistor (07) and a black wire for the negative connection. The copper laminate is then pushed into the battery holder where it will be held in place by the positive connection of one of the cells. The side with the red wire soldered to it should go against the positive battery terminal. If the wires are not connected, the radio should not work. Then, when both This close-up view shows the wiring to the LEDs and pushbutton switches. The varistor can be installed either way around. 8 1 N914, 1 N4148 silicon diodes (D1 -D3, D5, D11-D13) 5 1 N4004 silicon power diodes (D4 ,D6 ,D7 ,D8,D9) 2 5mm red LEDs (LED1, LED2) 1 275L408 275VAC varistor Capacitors 1 1OOµF 16VW PC electrolytic 2 1 OµF 16VW PC electrolytic 1 4. 7µF 16VW PC electrolytic 1 0 .4 7 µF 1 OOVW metallised polyester (greencap) 4 0 .1µF ceramic or greencap 1 .01 µF ceramic or greencap 1 .0033µF ceramic/greencap 1 330pF ceramic 1 220pF ceramic Resistors (0.25W, 5%) 3 1 OMO 1 4.7k0 1 1 MO 7 1kO 4 1 OOkO 1 1oon 3 47k0 1 470 5 39k0 2 220 1 33k0 1 1 MO miniature trimpot, VR1 Miscellaneous Solder , screws, nuts , lockwashers, double sided copper laminate (see text) hookup wire. the wires are shorted together, the radio should work. Now connect the red (positive) wire to the emitter of the AD162. The black wire then goes to the collector of the AD162. Now, when the AD162 is turned on, it will allow the radio to work. With the circuit powered up, press button S1 and the radio should turn on. If all is OK so far, you are ready to connect the unit to the phone line. You will need a parallel adaptor plug so that the phone and the Music On Hold unit can be connected at the same time. Now apply power and press button S1. LED 1 should light. Now pick up your own phone. You should hear music. Have a friend call you and check that the call can be placed on hold and then reconnected with button S2. The calling party should hear the tuned station from the radio. Adjust the sound level with the radio's volume control and with VRl, if necessary. ~ Problems? ... and you don't have our NEW 1990/91 148 page electronic parts and accessories catalogue ... Its our latest TRADE catalogue for the consumer ARISTA ... Your one-stop problem solver. ... Stylus ... ... Plugs, Jacks and Sockets ... ... 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