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Want to listen to headphones without being connected to your TV or CD player? Now you can do it with this infrared stereo link. It takes the stereo signal from any source, converts it to a modulated infrared beam and then converts it back to an audio drive signal for standard headphones. Infrared stereo headphone link PART 1 – THE TRANSMITTER How often have you wanted to watch that “special program” on TV while others have wanted to read, or even worse, sleep. Sure, most modern TV sets have a headphone socket but sitting close to the TV on a cushion or hard chair is not our idea of relaxed viewing. Perhaps you or another member of the household is a little hard of hearing (aurally challenged?) and likes the volume louder than the rest. This new project will also solve this problem. Our new infrared stereo link allows you to relax in your favourite chair and listen to stereo sound at a level that suits you. Individual volume controls allow you set the left to right balance, especially for those who may be a little deaf in one ear. The infrared stereo link consists of By RICK WALTERS 54  Silicon Chip two parts. There is the transmitter unit which is powered from the 240VAC mains and takes the signal from the CD player, TV or whatever. And there is the receiver; it is battery powered and it picks up the IR beam and converts it into an audio drive signal for your headphones. As the receiver is battery operated, it will switch itself off after about half an hour to extend the battery life. If you want to use it for longer periods, you will have to press the ON button every half hour. The transmitter has two infrared LEDs at one end and these should point in the direction of the receiver. Naturally, for best results the receiver’s pickup lens should face the transmitter. For good noise-free reception, the distance between the transmitter and receiver should be no more than about three to four metres. The challenge in producing an infrared circuit like this is that it must have adequate dynamic range, low distortion, good frequency response and adequate separation between channels. Let us state, at the outset, that the quality of reproduction is quite good and most people will judge it perfectly adequate for watching TV. However, it is not as good as CD-sound quality although many people with hearing difficulties will not be concerned with this aspect. The inside story on the infra red stereo transmitter, showing almost all components mounted on a pc board attached to the lid. Note the shrouding around the mains switch – this is essential to prevent you coming into contact with 240V, either directly or through your hifi if a mains wire comes adrift! Transmitter operation Fig.1 shows the circuit of the infrared transmitter. It is essentially a pulse width modulator (PWM) which produces a stream of varying width pulses at 44kHz. The stereo signal is multiplexed into the PWM stream and the left and right channels must be separated by the receiver circuit. Notice that the transmitter can be split into two halves. At the top lefthand corner is the right channel input, feeding into volume control VR1 and then into op amp IC4b. Still on the lefthand side of the circuit but about half way down is the left channel input, feeding into the volume control VR2 and then into op amp IC4c. Let’s concentrate on the right channel to begin with. Op amp IC4b has a gain of 10 at mid-frequencies. IC4b’s output, pin 7, drives a two section low-pass RC filter and then IC4a which has a gain of two. The output of IC4a is fed to the non-inverting input of comparator IC5. The inverting input (pin 3) is driven with a triangle waveform running at 88kHz. This signal comes from pin 1 of IC3a. IC1, a 555-type timer, is wired as a free running oscillator to operate at 176kHz. Its output frequency at pin 3 is divided by two in IC2a, producing symmetrical 88kHz square waves at pins 1 and 2. The signal at pin 1 is fed to IC2b’s clock input, its output being 44kHz square waves at pin 12 and 13. Pulse width modulation The square wave at pin 2 of IC2a is fed to integrator IC3a, which produces a very linear triangular waveform at its output, pin 1. Thus at the input of IC5 we have an audio signal on pin 2 and an 88kHz triangle wave on pin 3. The output of IC5, pin 7 is normally high but will pull down to ground whenever the voltage at pin 3 exceeds that at pin 2. Therefore the output of the comparator will be a train of varying width pulses (Pulse Width Modulation or PWM), the width varying in sympathy with the frequency and volume of the audio. The left audio channel is identical to the right in function. IC4c and IC4d provide the signal gain and IC4d feeds the audio signal to the non-inverting input of comparator IC6. The 88kHz ramp comes from IC3a, as before. The two comparators IC5 and IC6 are open-collector output and they are wired in parallel to a common 1kΩ resistor; when one output goes low it will pull the other low as well. What we wish to do is to transmit the right channel signal for a short period, then transmit the left, then the right etc. This is known as multiplexing. “Wait a minute”, you are thinking, September 1996  55 Fig. 1: the circuit diagram of the transmitter. Quite complex operation is simplified through the use of a number of integrated circuits. “if we chop the audio like this we should only hear half the program.” Luckily, this is not the case. Compact 56  Silicon Chip discs operate on a similar multiplexing principle. If we limit the bandwidth (and thus the slew rate) of the audio signal and sample it at a rate faster than the signal can vary, then no information will be Fig. 2 (left): the component layout and wiring of the transmitter, with its associated PC board pattern shown above. Use this to check your own PC board thoroughly before commencing construction. lost. This is the reason we have low-pass filters in each channel. Multiplexing So, how do we switch between the right and left signals? Fortunately LM311 comparators have a gating pin (pin 6) which allows us to do just that. If this pin is held low, the output at pin 7 will stay high. Thus by applying a square wave signal to pin 6 pin of IC5 we can alternately enable and disable the chip. By feeding the complement (opposite polarity) of the squarewave to pin 6 of IC6, we gate them on alternately, just as we require. The gating signal is 44kHz, as supplied by the Q and Q-bar outputs of IC2b. The commoned output of IC5 and IC6 switches Q1 on when it goes low, turning on LED1 and LED2 which will emit pulses of infrared light of fixed intensity and varying duration. Pilot tone OK, we are now transmitting two audio channels multiplexed in a continuous stream but we will have a problem at the receiving end, for we will not know which channel is which. This is where IC3b comes into the picture. It is configured as a 10Hz square wave oscillator. Its output at pin 7 is fed via a two-section low pass filter to produce a 10Hz sine wave signal. This sine wave is injected at a low level into pin 13 of September 1996  57 PARTS LIST - TRANSMITTER 1 PC board, code 01109961, 106 x 80mm 1 plastic box, 150 x 90 x 50mm, Jaycar HB-6011 or equiv. 1 30V centre-tapped transformer Altronics M-2855 or equiv. 1 DPDT 250VAC miniature toggle switch, Jaycar ST-0552 or equiv. 1 3-core mains core with moulded 3-pin plug 2 chassis-mount RCA sockets 1 TO-220 heatsink (see text) 2 3mm x 15mm machine screws 2 3mm x 12mm countersunk screws 1 3mm x 6mm machine screw 5 3mm nuts 5 3mm star washers 2 6mm spacers 4 12mm square stick-on feet 1 cable clamp, Jaycar HP-0716 or equiv 1 6.5mm crimp lug 1 solder lug 1 100mm cable tie 150mm twin screened cable 100mm green/yellow mains wire Semiconductors 1 555 timer (IC1) 1 4013 dual JK flipflop (IC2) 1 TL072 dual op amp (IC3) 1 TL074 quad op amp (IC4) 2 LM311 comparators (IC5,6) 1 7815 +15V regulator (REG1) 1 7915 -15V regulator (REG2) 1 BC640 PNP 1A transistor (Q1) 2 100mA IR transmitter diodes (LED1,2) Jaycar ZD-1950 or equiv 4 1N4004 1A diodes (D1-D4) Resistors (0.25W 1%) 2 2.2MΩ 6 10kΩ 1 330kΩ 1 5.6kΩ 1 150kΩ 3 4.7kΩ 1 75kΩ 1 3.3kΩ 1 56kΩ 3 1kΩ 3 47kΩ 1 470Ω 4 39kΩ 2 100Ω 1 36kΩ 2 68Ω 2 50kΩ horizontal mounting trimpots (VR1,VR2) IC4d via the 36kΩ resistor and will only appear in the left channel. Thus the left channel will always contain a low level 10Hz sine wave and this becomes the pilot tone used by the receiver to differentiate between the left and right channels. process of high frequency reduction in the receiver is called de-emphasis and again, is standard in FM receivers. A small power transformer and two IC voltage regulators provide the positive and negative 15 volt supplies for the transmitter. High frequency pre-emphasis Putting it together To improve the signal to noise ratio we boost the higher frequencies in the audio signal by increasing the gain of IC4b and IC4c. This is done by the .047µF capacitor in series with the 100Ω resistor. At high frequencies the impedance of the capacitor reduces, thus increasing the gain of the amplifier. Increasing the high frequency signals in this way is called pre-emphasis and it is a standard technique in FM radio transmissions. Then, when the signal is received, the high frequencies are reduced by the same amount as they were boosted in transmission. This reduction in high frequencies also reduces the hiss which is naturally present in high gain circuits and this improves the overall noise performance. The There are three PC boards to be assembled for this project, two in the receiver and one in the transmitter. The transmitter PC board measures 106 x 80mm and is coded 01109961. Before you begin assembling the PC boards, check all three for etching problems, open circuit or bridged tracks and undrilled holes. Fix any defects before proceeding further. Now let’s describe the transmitter assembly. The PC board layout and wiring diagram is shown in Fig.3. Begin assembly of the PC by inserting and soldering the six PC stakes and nine links, followed by the resistors, diodes, trimpots, transistor, capacitors and finally the regulators. The positive regulator (REG1) is fitted with a small heatsink. For our prototype we 58  Silicon Chip Capacitors 2 470µF 25VW electrolytic 1 100µF 25VW electrolytic 5 10µF 25VW electrolytic 1 3.3µF 25VW electrolytic 2 3.3µF 25VW non-polarised (NP) electrolytic 1 0.22µF MKT polyester 4 0.1µF 63VW MKT polyester 1 0.1µF 50VW monolithic 2 .047µF 63VW MKT polyester or ceramic 1 .01µF 63VW MKT polyester or ceramic 1 .0022µF 63VW MKT polyester or ceramic 1 820pF 63VW MKT polyester or ceramic 4 680pF 63VW MKT polyester or ceramic 2 150pF 63VW MKT polyester or ceramic 2 100pF 63VW MKT polyester or ceramic 1 39pF 63VW MKT polyester or ceramic Note: ceramic capacitors must be within ±10% tolerance. used a U-shaped heatsink with the sides straightened out so that it fitted between the two 470µF capacitors. Put a smear of thermal compound on the heatsink before screwing it to the regulator. Finally, insert and solder the ICs checking that their orientation is correct. The same comment about polarity and orientation applies to the diodes, transistor and electrolytic capacitors. The PC board is actually mounted on the lid of the plastic case which is then turned upside down for normal use. The power transformer, power switch and RCA input sockets are mounted in the body of the case, as shown in the photos. The PC board is mounted centrally on the lid of the box, stood off on 6mm spacers. Once you drill the holes make sure you fit the board the correct way, as the mounting holes are not symmetrically placed on the PC pattern. Drill two holes for the RCA sockets on the box centreline 55mm and 70mm from the corner, on the side adjacent to the switch. We made the one on the right (believe it or not) the right RESISTOR COLOUR CODES    No. Value    4-Band Code (1%)    5-Band Code (1%) ❏ 2 2.2M red red green brown red red black yellow brown ❏ 1 330k yellow yellow yellow brown yellow yellow black orange brown ❏ 1 150k brown green yellow brown brown green black orange brown ❏ 1 75k violet green orange brown violet green black red brown ❏ 1 56k green blue orange brown green blue black red brown ❏ 3 47k yellow violet orange brown yellow violet black red brown ❏ 4 39k orange white orange brown orange white black red brown ❏ 1 36k orange blue orange brown orange blue black red brown ❏ 6 10k brown black orange orange brown black black red orange ❏ 1 5.6k green blue red brown green blue black brown brown ❏ 3 4.7k yellow violet red brown yellow violet black brown brown ❏ 1 3.3k orange orange red brown orange orange black brown brown ❏ 3 1k brown black red brown brown black black brown brown ❏ 1 470 yellow violet brown brown yellow violet black black brown ❏ 2 100 brown black brown brown brown black black black brown ❏ 2 68 blue grey black brown blue grey black gold brown channel input and the other one the left input when the box is inverted, as it is when it is in use. The holes for the infrared LEDs are on the end opposite the mains entry, 10mm from the open edge and 15mm either side of the centreline. Once these holes are drilled you can push the two LEDs into their mounting holes on the PC board and adjust them to protrude through the box. When you are satisfied, remove the bolt near them, swing the PC board around and solder them in place. The mains lead is held securely with a cable clamp where it enters the plastic box about 20mm from the bottom and 25mm from the edge. The mains cord is wired directly to a double pole mains switch which is mounted about 40mm to the right of the mains entry point. The mains transformer and metal bush of the mains switch must be earthed. Slip the mains lead earth wire and a 100mm length of green/yellow earth wire into the 6.5mm lug and crimp it securely or solder the wires in it, then slip the lug on the switch, add the star washer and mount the switch. The mains wires and the transformer wires are individually sleeved with 2.5mm heatshrink and soldered to the switch. They must be run through a 50mm length of 20mm heatshrink sleeving before they are soldered. After soldering the four wires slide the sleeves right up over the switch contacts and shrink them, then slide and shrink the large sleeve over the switch. Finally secure a cable tie around this sleeve and tighten it, to anchor the wires CAPACITOR CODES    ❏ ❏ ❏  ❏  ❏  ❏ ❏ ❏ ❏ ❏ Value IEC Code 0.22uF 220n 0.1uF 100n .047uF 47n .01uF 10n .0022uF 2.2n 820pF 820p 680pF 680p 150pF 150p 100pF 100p 39pF 39p EIA Code 224 104 473 103 222 821 681 151 101 39 securely. Should a lead come off the switch it will then be contained and cause no hazard. The switch is wired so that it is ON when the toggle points towards the mains lead. Using countersunk screws, mount the mains transformer about 40mm from the end remote from the mains entry, checking to ensure that it does not foul the PC board components when the lid is fitted. The two orange wires from the transformer are soldered to the bottom stakes adjacent to the power diodes, while the white centre tap lead is soldered to the stake closest to the centre of the PC board. This completes the assembly of the transmitter. Next month we will give the full SC details of the receiver. IREE Medal of Honour to Neville Thiele . . . continued from p53 Radio and Electronics Engineers Australia since 1947 and was elevated to Fellow, in 1969. He has been a Councillor from 1963 to 1973 and from 1982 to the present time. During that period he has held many positions, including that of Vice-President (1972/73), Deputy President (1984/86) and President (1986/87). He has served on the Publications Board and other Boards and Committees and as a Member and Chairman of the Sydney Division of The Institution. He has twice been presented with the Norman W. V. Hayes Medal. He is a Fellow of the Institution of Engineers Australia and of the Audio Engineering Society. During his career he has been a member of the Australian delegations to the CCIR. In addition, Neville has been active for many years in Standards Australia technical committees. Neville is also a prolific author, having published 48 technical papers. While at the ABC he prepared 25 reports on his design and development projects. He has attended 24 conferences at which he has presented at least one technical paper. His personal achievements and his contribution to the affairs of The Institution over many years make him a worthy recipient of the Award of Honour from The Institution of Radio and Electronics Engineers. SC September 1996  59