Build A Mains Music Transmitter & Receiver - May 1995

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Contents
Publisher's Letter: Australia can do better with greenhouse gases
Feature: CMOS Memory Settings - What To Do When The Battery Goes Flat by Greg Swain
Feature: Electronics In The New EF Falcon; Pt.3 by Julian Edgar
Feature: Introduction To Satellite TV (Build A Satellite TV Receiver; Pt.1) by Garry Cratt
Order Form
Project: Build A Mains Music Transmitter & Receiver by Jeff Monegal
Project: Guitar Headphone Amplifier For Practice Sessions by John Clarke
Feature: Remote Control by Bob Young
Project: Build An FM Radio Trainer; Pt.2 by John Clarke
Project: Low-Cost Transistor & Mosfet Tester For DMMs by John Clarke
Serviceman's Log: All it needs is a new fuse plus the set that fell by The TV Serviceman
Vintage Radio: A console receiver from junk by John Hill
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Build a mains music transmitter & receiver How many times have you wanted to listen to music while working in the garage or a room in the house but didn’t want to move the stereo speakers. With this system, you can listen to your music anywhere there is a power point. By JEFF MONEGAL This project is actually a variation of a mains intercom circuit. It consists of two parts: (1) a transmitter unit which is connected to the program source to feed the signal into the 240VAC mains wiring; and (2) a receiver which is plugged into another power point anywhere within your home. The transmitter is housed in a plastic box which has two RCA panel sockets for the input signal. It is powered from the mains supply and it couples its frequency modulated carrier signal into the 240VAC supply via its mains cord. The receiver is housed in another plastic box which is connected to the mains supply. On the front panel it has tone and volume controls and two LEDs, one to indicate that it is 32 Silicon Chip locked to the carrier signal and the other a power indicator. On the rear panel are a pair of screw terminals for connection of a loudspeaker to an internal 10W amplifier. Circuit description Let’s look at the circuit of the Music Transmitter which is shown in Fig.1. The heart of the circuit is IC2, a 4046 phase lock loop (PLL) IC. This oscillates at around 300kHz, as set by the 22kΩ resistor at pin 9 and the 100pF capacitor between pins 6 & 7. Pin 9 is the input to the voltage controlled oscillator (VCO) within IC2 and this is driven by IC1, a TL071 op amp connected as an inverting amplifier. This op amp has a gain of 4.7, as set by the 10kΩ and 47kΩ feedback resistors connected to pin 2. The input signal for IC1 comes via the volume control, VR1, which is preceded by a 0.1µF capacitor and two 1kΩ resistors which mix the left and right channels from the program source. This could be a CD player, tape deck, FM tuner or just the tape monitor output from your stereo amplifier. The latter gives access to all the program sources you have connected – pretty simple, really. The output of the VCO is at pin 4 of IC2 and it is used to drive the gate of Mosfet Q1. Q1 provides a high impedance buffer for the VCO and its drain drives both the Active and Neutral lines of the 240VAC mains supply via .01µF 5kV ceramic capacitors. IC1 is biased so that its output at pin 6 sits at half the supply rail of 8V (ie, at +4V), by virtue of the voltage divider consisting of R4 & R6. The voltage divider is bypassed by 10µF capacitor C3 and further supply de coupling is provided by resistor R8 and capacitor C5. The DC supply is provided by an 8V 3-terminal regulator and this feeds a bridge rectifier (BR1) and a 2200µF R8 1k C6 100pF C5 10 R4 47k RIGHT AUDIO INPUT LEFT C1 0.1 R1 1k R2 1k VR1 50k R6 47k IC1 2 TL071 4 6 R5 47k C3 10 +8V C7 10 6 CA 7 3 R3 10k C2 0.1 R11 270  7 16 R7 CB 3.3k 9 BIN 3 IC2 VCIN VC 4 4046 R1 INH OUT 11 8 5 R9 22k R12 100  R10 68  D G Q1 P222 S Fig.1: the circuit of the Music Transmitter. The 4046 is used as a voltage controlled oscillator to frequency modulate a 300kHz carrier. C4 330pF I GO A .01 5kV K A .01 5kV REG1 IN 7808 OUT 12.6V 240VAC GD S BR1 W04 T1 C12 2200 25VW N GND C10 10 C11 .0033 +8V A  LED1 K R13 560  E MUSIC TRANSMITTER filter capacitor from a 12.6VAC transformer. Music receiver While the Music Transmitter is fairly simple, the circuit for the receiver is a little more complicated, as shown in Fig.2. Again, the heart of the circuit is a 4046 PLL, IC1. This takes the FM signal which has been impressed onto the mains wiring and recovers the audio signal. In this case though, IC1 operates as a PLL and not simply as a VCO, as we shall see. Two .01µF 5kV capacitors couple the 300kHz FM carrier from the mains to the base of transistor Q1. Q1 is a common emitter amplifier roughly tuned to 300kHz by C1 and L1. The output signal from its collector is AC-coupled to the input of IC1. IC1 is also set to run at around 300kHz, as set by the 100pF capacitor between pins 6 & 7, and the 12kΩ resistor and 50kΩ trimpot (VR1) connected to pin 11. VR1 is set so that when a 300kHz carrier is present on the 240VAC mains, the PLL locks onto it. Resistor R3 and capacitor C7, connected to pins 2 & 9 respectively, form a filter which sets the “capture range” of the PLL; ie, the ease with which it locks to the incoming 300kHz carrier. When the VCO is locked to the incoming frequency, an error signal is generated by the PLL at pin 10. This voltage is propor tional to the difference between the free-running frequency of the PLL and the incoming carrier frequency. So as the incoming 300kHz carrier deviates from its nominal centre frequency, pin 10 generates a voltage which is proportional to the difference. The AC component of this error signal at pin 10 is actually the same as the modulating audio signal back at the transmitter. After filtering by R5 and C10, the signal is coupled via C9 to trimpot VR2. From there, the signal is fed to IC2, a TL071 op amp connected as an inverting amplifier with a gain of 10. Its frequency response is rolled off Where To Buy A Kit Of Parts A kit of parts for this project is available from CTOAN Electronics. The kits will be available in two forms with prices as follows: Kit 1 is a short form transmitter which contains the PC board and all onboard components excluding the power transformer. Price $ 20.00. Kit 2 is a short form receiver which contains the PC board plus all on-board components excluding the power transformer. Price $39.00. Kit 3 is a full transmitter kit containing all components, transformer, mains cord, case and adhesive front panel. Price $43.00. Kit 4 is a full receiver kit containing all components including the transformer, case, mains cord and adhesive panel. Price $65.00. All the above prices include postage within Australia. Kits may be ordered over the phone using a credit card or by sending a cheque or money order to CTOAN Electronics, PO Box 211, Jimboomba, Qld 4280. Phone (07) 297 5421. CTOAN Electronics will also be offering a repair service for this project. The cost will be $30.00 including return postage. Fully built and tested units will also be available. May 1995 33 L1 100uH C1 .0033 B E Q1 BC327 C R1 33k 3 BIN 14 AIN C5 .0033 C4 330pF 6 4 VC OUT D1 1N914 16 PCP Q2 LED1 BC558 E A B K  C 1 R6 22k CA C6 100pF 7 R3 120k 2 PC1 VCIN 9 R1 INH 11 5 SF R4 22k 8 R2 12k C9 0.1 R5 6.8k 10  K R8 470  C Q3 BC548 B E D2 1N914 C11 VR2 50k C10 .022 LOCKED LED2 R7 10k IC1 4046 CB C7 .001 A C8 10 0.1 VR1 50k R18 100  R11 22k R10 22k C12 47 R9 22k 3 R13 22k C13 47 2 7 IC2 TL071 6 C15 0.47 C18 0.47 C17 47 R14 10k R15 68k C20 0.1 C19 .047 4 R12 220k R16 33k C14 68pF C2 .01 5kV C3 .01 5kV A ZD1 12V C23 100 R17 68k C21 .047 C16 .0015 TONE VR3 50k VOLUME VR4 50k R19 47k C22 0.47 240V AC E C VIEWED FROM BELOW BR1 W04 12.6V N CASE  A K 34 Silicon Chip C28 1000 5 R23 10  8 C27 0.1 R21 1k C24 10 1 9 MUSIC RECEIVER Fig.2: the receiver circuit of the uses a 4046 PLL IC to recover the audio modulation on the 300kHz carrier which is picked up from the mains supply. above 10kHz by the 68pF feedback capacitor (C14). Immediately following IC2 we have a passive “twin-T” notch filter (C1921 & R15-17) which attenuates 50Hz, necessary in a system which is directly connected to the 50Hz mains supply. The output of the twin-T filter is connected to potentiometer VR3 which together with C18 forms a simple top-cut tone control. Potentiometer VR4 is the volume control. From here, the audio signal is coupled to IC3, a TDA1520 power amplifier which is capable of delivering 20W. In this cir- 4 R22 270  C26 680pF R20 47k R24 1.5k POWER LED3 E 6 IC3 9 TDA1520 2 +18V C29 2200 25VW 8 1 B T1 +18V C25 .0033 cuit though, the nominal supply rail of +18V means that it can only deliver about 3W, which is adequate for this application. Resistors R20 & R21 set the gain of IC3 to 48 while capacitor C24 sets the low frequency response of the amplifier to about 20Hz. The remaining components around the amplifier are for high frequency stability, while C28 is the output coupling capacitor. Power for the circuit comes from a 12.6V transformer driving a bridge rectifier and 2200µF capacitor to give about +18V. This supplies the power amplifier directly and also feeds 12V zener diode ZD1 via a 100Ω resistor. The resulting +12V rail supplies IC1 and IC2. The remaining components to be discussed are those involved with Q2 and Q3. Q2’s job is to ensure that the audio path is disabled if the PLL loses lock at any time, as would be the case if the transmitter was switched off. In this situation, the PLL has no signal to lock onto and so the VCO will free run. This has the result of producing all sorts of noise and rubbish in the audio section and so it must be muted. This is done using the signal available at pin 1 of IC1. RCA SOCKETS 0.1 VR1 47k 1 A 10uF LED1 22k 68  100pF 3.3k IC2 4046 1 1k 270  K 10uF IC2 TL071 10k 47k 330pF 1k R IN 1k L IN 10uF .0033 10uF G D S Q1 2200uF REG1 BR1 100  SECONDARY Construction Let’s discuss the construction of the Music Transmitter first, since it is the most straightforward. It is assembled onto a PC board measuring 127 x 77mm and coded CE/MUSA/94. This is then mounted on the base of a standard plastic utility box using 9mm insulated spacers and secured using short screws. The component layout for the PC board is shown in Fig.3. After checking the PC board carefully for any defects, you can begin the assembly by installing PC stakes at the external wiring points for the RCA sockets and the LED. This done, install the resistors and capacitors, followed by the ICs and the 3-terminal regulator. Take care with the orientation of the semiconductors and electrolytic capacitors. The last component to be mounted is the small power trans former which is bolted to the board, along with a solder lug. The PC board can now be used as a template for drilling out its mounting holes in the base of the case. You will also have to drill holes for the RCA sockets, the LED and for the cordgrip grommet. Be sure to carefully shape the cordgrip grommet hole so that the grommet will be a tight fit. The mains cable should now be passed through the hole in the case and secured with the cordgrip grom met. Its Active and Neutral terminals can then be wired directly to the board, while the Earth lead is connected to a 47k 0.1 GND 560  When the system is in lock, pin 1 of IC1 is high. This high output is fed via D1 to R6 and is used to reverse bias LED1 and the base of Q2 so that the transistor is held in the off state. In this situation, the audio signal from volume control VR2 has an uninterrupted path to IC2 and IC3. The same voltage from pin 1 of IC1 turns on Q3 and so LED2 will be lit to indicate the “locked” condition. When the PLL loses the carrier signal, pin 1 will go low. Now base current for Q2 can flow via LED1 and R6 so that the transistor turns on. Its collector now pulls the non-inverting input of IC2 high, via diode D2 and R9. This is a rather brutal way of shutting down IC2 and thus prevents any extraneous signals from being fed to power amplifier IC3. Because pin 1 of IC1 is low, transistor Q3 will be off and LED2 will be extinguished. POWER TRANSFORMER .01 5kV PRIMARY A .01 5kV E ACTIVE BROWN Fig.3: this diagram shows the parts layout for the transmitter PC board. N EARTH GREEN/ YELLOW NEUTRAL BLUE CORD GRIP GROMMET The transmitter PC board is mounted on the base of the plastic case using 9mmlong insulated standoffs. Note that the mains cord must be anchored securely with a cordgrip grommet in the end of the case. May 1995 35 Fig.4: this is the component overlay for the receiver PC board. Note that LEDs 2 & 3 are actually mounted on the front-panel, while the output terminals (near IC3) go to an RCA socket on the rear panel – see photo. BR1 2200uF 100uF LED3 270  10  680pF .0033 Q1 12k 330pF 22k 33k IC1 4046 47uF 47k .047 VR3 68k 68k .047 .015 0.47 1 ZD1 IC2 TL071 0.1 .01 5kV 33k D1 10k 22k D2 A Q3 10uF LED1Q2 10k 22k 0.47 47uF .01 5kV 470  68pF E GREEN/YELLOW VR2 0.1 N 100pF 22k 22k BLUE 0.1 .001 1 6.8k A VR4 120k .0033 .022 BROWN VR1 220k PRIMARY 100 POWER TRANSFORMER 10uF A LED2 0.47 47uF L1 CORD GRIP GROMMET 0.1 A .0033 1000uF IC3 TDA1520 1.5k 47k 47k 1k OUTPUT solder lug – see Fig.3. An additional earth wire is then run from this solder lug to the earth terminal on the board. Finally, the board can be mounted in the case and the RCA sockets and LED connected using short lengths of hook-up wire. Music receiver The receiver board accommodates all the components, including the tone & volume control pots. Note the small aluminium heatsink for the TDA1520 power amplifier IC. Use plastic cable ties to lace the primary leads of the power transformer so that there is no possibility of them coming adrift & contacting other parts. The low voltage wiring should also be secured with cable ties. 36 Silicon Chip The receiver is assembled onto a PC board measuring 137 x 117mm and coded CE/MUSB/94. The component layout is shown in Fig.4. With the exception of wires to the LEDs and rear speaker terminals, all the wiring and components are on the PC board. Mount the small components first, such as resistors, capacitors, diodes and transistors, followed by the ICs and inductor. The TDA1520 should be mounted on a small heatsink as shown in the photographs. We used a small scrap of 5mm aluminium. Drill a couple of mounting holes that correspond with the two mounting holes in the power amplifier. Bend the leads at right angles so that IC3 can mount flat on top of the heatsink. Be careful not to allow the leads to touch along the edge of the aluminium. The last component to be mounted is the small power transformer which is bolted to the board. The mains cable should be passed through a hole in the rear panel of case which is fitted with the correct size cordgrip grommet to anchor it. It can then be wired directly to the board. The board can then be mounted in the base of the case and the wiring completed. Fit cable ties to both the mains wiring and to the low-voltage wiring to prevent shorts if a wire comes adrift – see photo. Testing & setup The first step is to turn on the transmitter and check that the +8V supply is present at the output of the 3-terminal regulator and at pin 7 of IC1 & IC2. Similarly, turn on the receiver and check that +18V (or thereabouts) is present at pin 6 of IC3 and that +12V is present at pin 7 of IC1 & IC2. For the initial setup, turn VR1 in the transmitter fully anticlockwise. Connect an audio source to the input – a CD player or cassette deck will do. At the receiver, turn VR1 to mid-position and VR2 fully anticlockwise. Connect power and turn on. Do not plug the receiver into the same power point as the transmitter. If another GPO (mains power point) is not within reach then use an extension lead from another GPO. The power LED should light on the receiver and the locked LED may or may not come on. Turn up the volume control. You should hear some noise and hiss at full volume. Now slowly turn VR1 until the locked LED comes on. Remember that the transmitter must be on but a music source is not necessary as the receiver will lock onto the carrier from the transmitter. When the receiver is not locked LED1 on the PC board should glow dimly and be out when it is in lock. Once the receiver is locked, turn on the music source to the transmitter. At the receiver, turn up the volume. If you are greeted with good clean music then no further adjustment is necessary. If not, then further adjustment of VR1 in the receiver is needed. At some point during the rotation of VR1 you should find that the receiver locks properly and produces good clean audio. If the audio is distorted, then reduce the level of audio at the transmitter by reducing VR1. This reduces the amount of modulation. VR2 in the receiver is set to give the PARTS LIST Transmitter 1 PC board, code CE/MUSA/94, 127 x 77mm 1 12.6V power transformer (Altronics Cat. M-2851) 1 plastic case, 158 x 95 x 55mm 1 3-core mains cord & moulded 3-pin plug 1 cordgrip grommet to suit mains cord 2 RCA panel sockets 1 50kΩ horizontal trimpot (VR1) 4 9mm tapped insulated standoffs plus 8 short screws to suit 1 5mm LED bezel 1 solder lug Semiconductors 1 TL071 FET-input op amp (IC1) 1 4046 phase lock loop (IC2) 1 P222 Mosfet (Q1) 1 7808 3-terminal regulator (REG1) 1 W04 1A bridge rectifier (BR1) 1 5mm red LED (LED1) Capacitors 1 2200µF 25VW electrolytic 4 10µF 16VW electrolytic 2 0.1µF monolithic 2 .01µF 5kV ceramic (do not substitute with lower rating) 1 .0033µF ceramic 1 330pF ceramic 1 100pF ceramic Resistors (0.25W, 5%) 3 47kΩ 1 560Ω 1 22kΩ 1 270Ω 3 10kΩ 1 100Ω 1W 1 3.3kΩ 1 68Ω 2 1kΩ Receiver 1 PC board, code CE/MUSB/94, 137 x 117mm 1 12.6V power transformer (Altronics Cat. M-2853) 1 plastic case, 152 x 64 x 158mm 1 100µH inductor (L1) 1 3-core mains cord and moulded 3-pin plug 1 cordgrip grommet to suit mains cord maximum recovered signal to IC2. To set it, set volume control VR4 to a low setting and then advance VR2 until the 2 knobs 1 set of speaker terminals 2 50kΩ horizontal trimpots (VR1,VR2) 2 50kΩ log PC mount potentiometers (VR3,VR4) 3 plastic cable ties Semiconductors 1 4046 phase lock loop (IC1) 1 TL071 FET-input op amp (IC2) 1 TDA1520 power amplifier (IC3) 1 BC327 PNP transistor (Q1) 1 BC558 PNP transistor (Q2) 1 BC548 NPN transistor (Q3) 2 1N914 signal diodes (D1,D2) 1 12V 1W zener diode (ZD1) 1 W04 1A bridge rectifier (BR1) 1 5mm yellow LED (LED1) 1 5mm green LED (LED2) 1 5mm red LED (LED3) Capacitors 1 2200µF 25VW electrolytic 1 1000µF 16VW electrolytic 1 100µF 25VW electrolytic 3 47µF 16VW electrolytic 2 10µF 16VW electrolytic 3 0.47µF monolithic 4 0.1µF monolithic 2 .047µF ceramic 1 .022µF ceramic 2 .01µF 5kV ceramic 2 .0047µF ceramic 2 .0033µF ceramic 1 .001µF ceramic 1 680pF ceramic 1 330pF ceramic 1 100pF ceramic 1 68pF ceramic Resistors (0.25W, 5%) 1 220kΩ 1 6.8kΩ 1 120kΩ 1 1.5kΩ 2 68kΩ 1 1kΩ 2 47kΩ 1 470Ω 2 33kΩ 1 270Ω 6 22kΩ 1 100Ω 1 12kΩ 1 10Ω 2 10kΩ Miscellaneous Scrap aluminium for heatsink (30 x 20 x 5mm), solder, hook-up wire. signal is overloaded. Finally, back off the trimpot to obtain a distortion-free SC signal. May 1995 37