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Infrared audio headphone link for TV By JIM ROWE Do you have trouble understanding what’s being said on the TV? Do you need the volume cranked up too loud for everyone else? Do you have a hearing aid as well? If you said yes to any other these questions, here is your answer: an infrared transmitter and receiver to let you listen to the TV sound via headphones. That way, you can listen as loudly as you like, without disturbing anyone else. I T HAPPENS all the time. One of the older members of the household is getting a bit deaf and needs the TV sound turned well up. But then it is too loud for everyone else. It’s worse at night when people go to bed but one family member wants to watch the late-night movie – or whatever. The problem can be even worse if you have a hearing aid because it also tends to pick up extraneous noises – coughs, heater fans, a radio in another room, toilets flushing, planes flying overhead, cars and trucks passing in 30  Silicon Chip the street and people washing up the dishes, to list just a few irritations. The real answer is to listen via headphones – preferably good “surround your ears” muff-type headphones which not only deliver the wanted sounds directly to your ears and hearing aid(s) but also cut back the competing sounds at the same time. And if you pick the right kind of headphones with some acoustic damping in the earmuffs, they don’t cause your hearing aid(s) to feed back and whistle either. The result is comfortable listening at a volume level that’s right for you, where you can hear and understand everything that’s being said. Headphone jack Some TV sets do have earphone jacks, so you could simply fit a pair of stereo headphones with their own volume control (if necessary), plus a long cord and plug to mate with the jack on the TV. But many sets do not have a headphone jack and many that do have it wired so that when headsiliconchip.com.au Fig.1(a): how the transmitter works. The left and right channel audio signals are converted to mono, amplified and fed to comparator stage IC5 where they are compared to a 90kHz triangle wave (the sampling signal). The resulting PWM signal then drives transistor Q1 to pulse a string of infrared (IR) LEDs. Fig.1(b): at the receiver, the transmitted signal is picked up by an IR diode and the resulting current pulses converted to voltage pulses (and amplified) by IC1b & IC1a. This amplified pulse waveform is then fed through a limiter and filtered to recover the audio waveform. This is then fed via volume control VR1 to an audio output amplifier (IC4). phones are plugged in, the speakers are disabled. That’s fine for you but no good for everyone else. In any case, being hooked up to the TV via a long cable has its own problems: you can forget to take ’em off when you get up for a comfort break or someone else can trip on the cable when they move about the room. Cordless headphones A much better solution is to use “cordless” headphones, either via a UHF or infrared link. This means that you have a transmitter or sender unit that sits on the top of the TV, plus a small battery-operated receiver to drive the headphones at your end. Of course, IR-linked cordless headphones are available commercially and these can give you some improvement. But there are drawbacks, the main one being that the receiver unit is built into the actual earphones and/ siliconchip.com.au or their headband, so it can’t be used with any other headphones. That means you’re stuck with the ones you get and in most cases, they are not the “surround-your-ears” muff type. Nor do they have any acoustic damping. As a result, you not only have to throttle back your hearing aid to stop it from whistling but the headphones allow quite a lot of competing sounds to enter as well. So that’s the reasoning behind the development of this project – by building it, you get to choose the best type of headphones. However, there is one more feature – it works in mono only. This has been done deliberately because stereo sound is a real drawback to those who have trouble making out speech from the TV. This applies particularly to those films, documentaries and sportcasts where there is a lot of background music or other sounds. By using a mix of the left and right channels, we can- cel most of these extraneous sounds, making the speech much easier to discern. In addition, we have applied a small amount of treble boost to the audio signal which further improves intelligibility on speech – see Fig.6. There’s one more bonus with using mono sound – it also simplifies the circuit considerably. How it works The method of transmission is simple and effective. Basically, the signal is transmitted using pulsewidth modulation (or PWM). This converts the audio signal directly into a pulse stream of constant frequency but with the pulse width varying with the instantaneous amplitude of the audio signal. Fig.1(a) shows the method. First, the left and right stereo signals are mixed together to give a mono signal. This signal is then passed through an input amplifier stage (IC1b) and then via a December 2007  31 Fig.2: this diagram illustrates how the audio signal that’s fed into the transmitter is compared to a 90kHz triangular waveform (the sampling signal) to produce the pulse width modulated (PWM signal). As shown, the PWM output is high when the audio signal level is higher than the sampling signal. Its output current is then through a current-to-voltage (I/V) converter and amplifier stage (IC1b & IC1a) to boost its level. The resulting pulse waveform is then fed through a limiter stage (IC2) to produce a stream of clean, rectangular pulses of constant amplitude. Next, the pulses are fed through a multi-stage low-pass filter (IC3b & IC3a) to remove all traces of the 90kHz sampling/modulating signal. This simply leaves the audio signal which was carried in the average signal level of the pulses. From there, the recovered audio passes to a volume control pot and finally to a small audio amplifier (IC4) to drive the headphones. Power for the receiver circuit comes from four AA cells, which can be of either alkaline or NiMH rechargeables. Circuit description 4-pole low-pass filter (IC1a & IC4a), which sharply rolls off the response just above 12kHz. This is done for two reasons. First, if you are partially deaf, signals above 12kHz are not much use anyway. And second, it prevents any spurious “alias” signals from being generated during the digital modulation process – which is equivalent to digital sampling. We are using a fairly high sampling frequency of about 90kHz which tends to reduce aliasing but the low-pass filtering is also worthwhile because it ensures that virtually no signal frequencies above 15kHz are fed to the modulator. 90kHz sampling signal Next, the audio is fed directly to the non-inverting input of a comparator (IC5) where it is compared with a 90kHz triangular wave “sampling” signal on the inverting input. This 90kHz triangular wave signal is generated by feeding a 180kHz clock signal into a D-type flipflop. This then produces a very symmetrical square-wave signal at half the clock frequency, or 90kHz. This 90kHz signal is buffered and fed through an active integrator stage which converts it into a linear and very symmetrical triangular wave. But how does the comparator use this 90kHz triangular wave to convert the audio signal into a PWM stream? To see how this works, take a look at the waveforms of Fig.2. Here the green sinewave represents the audio signal fed to the positive input of the com32  Silicon Chip parator, while the higher frequency red triangular wave shows the sampling signal fed to the comparator’s negative input. In operation, the comparator’s output is high when the audio signal level is higher than the 90kHz sampling signal. Conversely, the comparator’s output is low when the sampling signal’s level is the higher of the two. A switching transition occurs when ever the two waveforms cross. The resulting PWM output waveform from the comparator is shown as the lower black waveform. Note that the comparator output is a stream of 90kHz pulses, with the pulse widths varying in direct proportion to the audio signal amplitude. The average value of the pulse stream is directly proportional to the instantaneous value of the incoming audio, as shown by the dark blue dashed curve. Referring back to Fig.1, this PWM pulse stream is fed to a PNP switching transistor which drives a string of IR-emitting LEDs. As a result, the digitised audio is converted into a stream of IR light pulses, directed towards the receiver unit. Receiver block diagram The receiver is even simpler than the transmitter because of the fact that the average value of the PWM pulse stream varies in direct proportion to the audio modulation. As shown in Fig.1(b), a silicon PIN photodiode is used to detect the IR pulse stream from the transmitter. Refer now to the full circuit for the transmitter – see Fig.3. As shown, the incoming line level stereo signals are mixed together using two 47kW resistors, while trimpot VR1 sets the level. The resulting mono signal is then fed to op amp stage IC1b which operates with a gain of 23, as set by the 22kW and 1kW feedback resistors. Next, the signal is passed through op amps IC1a and IC4a which form a 4-pole low-pass filter (or two 2-pole active filters in cascade, to be more precise). Together, these roll off the response above 12kHz. The filtered signal then emerges from pin 1 of IC4a and is fed directly to the non-inverting input of comparator IC5. The 180kHz “twice sampling clock” signal is generated by IC2b, a 4093B CMOS Schmitt NAND gate wired as a simple relaxation oscillator. A 12kW resistor and 680pF capacitor set the operating frequency. This is not particularly critical, although for best performance it should be between 160kHz and 200kHz (corresponding to a sample frequency of 80-100kHz). Flipflop stage IC3a is used to divide the clock pulses by two and generate the symmetrical 90kHz square wave. Its output at pin 1 is then passed through Schmitt NAND gates IC2a, IC2c & IC2d which are connected in parallel as a buffer. The buffer output is then coupled via a 100nF capacitor to op amp IC4b. IC4b is configured as an active integrator to convert the 90kHz squarewave into a linear symmetrical triansiliconchip.com.au siliconchip.com.au December 2007  33 Fig.3: the circuit for the transmitter. The incoming stereo audio signals are first mixed together to form a mono signal which is then amplified by IC1b. IC1a and IC4a then filter this signal and drive pin 3 of comparator stage IC5. IC2b is the 180kHz clock. Its output is divided by two using IC3a, buffered by IC2a, IC2c& IC2d and fed to integrator stage IC4b to produce the 90kHz triangular waveform. This waveform is then fed to the other input of IC5 and compared with the audio waveform. The resulting PWM waveform from IC5 then drives transistor Q1 which in turn pulses a string of six infrared LEDs plus a power indicator LED. Fig.4: the receiver circuit. Photodiode PD1 picks up the incoming PWM IR signal and IC1b converts the resulting current pulses to voltage pulses. IC1a then amplifies these voltage pulses, while IC2 is the limiter. The resulting PWM signal from the limiter is then fed to low-pass filter stages IC3b & IC3a and finally to audio amplifier stage IC4. gular waveform of the same frequency. This triangular wave is then fed directly to the inverting input of comparator IC5, to sample and convert the audio signal into the PWM pulse stream. IC5’s PWM output appears at pin 7 and is used to drive transistor Q1 (BC328). This in turn drives seriesconnected infrared LEDs (LEDs1-3 & LEDs5-7), along with LED4 (green) which serves as a “power on” indicator. The 47W resistor in series with the LED string limits the peak pulse current to around 45mA, resulting in an average current drain for the complete transmitter circuit of about 25mA. Transmitter power supply Power for the transmitter circuit is derived from a 12V AC or 15V DC plugpack. This feeds diode bridge D1D4 which rectifies the output from an AC plugpack. Alternatively, the bridge rectifier allows a DC plugpack to be used with either polarity. The output from the bridge rectifier is filtered using a 1000mF capacitor and 34  Silicon Chip then fed to 3-terminal regulator REG1 to produce a 12V DC supply rail. Receiver circuit OK, so much for the transmitter circuit. Let’s take a look now at the receiver circuit – see Fig.4. In operation, the transmitted PWM infrared signals are picked up by PIN photodiode PD1 (BP104). This device produces output current pulses in response to the incoming IR signals and these are then fed to the inverting input (pin 6) of op amp IC1b. The non-inverting input (pin 5) of IC1b is biased to half-supply (ie, 4.5V) by two 22kW resistors connected in series across the 9V supply rail. IC1b operates as an active I/V (current-to-voltage) converter. In operation, it converts the input current pulses to voltage pulses which appear at its pin 7 output. These pulses are then coupled via a 2.2nF capacitor to op amp stage IC1a which operates with a gain of -10. The resulting amplified output pulses appear at pin 1 and are fed directly to pin 3 of IC2. IC2 is an LM311 comparator and is used here as the limiter. Note that its non-inverting input (pin 2) is biased to half the supply voltage using the same voltage divider (2 x 22kW resistors) that’s used to bias IC1a and IC1b. This ensures that the pulses from IC1a are compared with a voltage level corresponding to their own average DC level. And that in turn ensures that the limiter “squares up” the pulse stream in a symmetrical fashion. In addition, the 2.2MW feedback resistor and the 10kW resistor in series with the bias for IC2 together provide a small amount of positive feedback hysteresis, to ensure clean switching. Because the LM311’s output (pin 7) is an open collector, it must have a resistive pull-up load. This is provided by power-on indicator LED1, together with its 390W series resistor. The restored PWM pulse stream appears at pin 7 of IC2 and is then fed through the receiver’s low-pass filter circuitry. This comprises passiliconchip.com.au sive 47kW/180pF and 100kW/100pF RC filter stages, voltage follower IC3b, active low-pass filter stage IC3a and finally, a 4.7mF coupling capacitor and a 1kW/10nF passive filter connecting to the top of volume control VR1. As a result, the signal appearing across VR1 is a very clean replica of the original audio signal fed into the transmitter unit. IC4 is the audio amplifier output stage and is based on an LM386N. It amplifies the signal from the volume control (VR1) and drives a stereo phone jack via a pair of 33W current limiting resistors (one to the tip and one to the ring). Finally, the receiver is powered from a 6V battery consisting of four AA cells connected in series. These cells can be either standard alkaline primary cells or rechargeable NiMH (or Nicad) cells if you prefer. The average current drain is typically around 20mA, so even ordinary alkaline cells should give at least 80-100 hours of listening. Construction Building the SILICON CHIP Infrared Audio Link is straightforward, with all the parts mounted on two PC boards – one for the transmitter (code 01112071) and one for the receiver (code 01112072). The transmitter board fits inside a standard low-profile ABS instrument box measuring 140 x 110 x 35mm, while the receiver board goes inside a standard UB3-size jiffy box (130 x 68 x 44mm), along with its 4xAA cell battery pack. Fig.7 shows the assembly details for the transmitter unit. Begin by installing the resistors and diodes D1-D4, taking care to ensure that the latter are all correctly oriented. An accompanying table shows the resistor colour codes but you should also check each resistor using a digital multimeter before installing it, just to make sure. Next, install the small ceramic and monolithic capacitors, then install trimpot VR1, transistor Q1 and the electrolytic capacitors. Make sure that the electrolytics and transistors all go in the right way around. Follow these parts with the five ICs. Be sure to use the correct IC type at each location and again check that they are all oriented correctly. IC sockets were used on the prototype but we suggest that you solder the ICs directly to the PC board. Regulator REG1 is next on the list. siliconchip.com.au Fig.5: this screen grab (taken on our LeCroy WaveJet 324 oscilloscope) shows three waveforms. The purple trace at top is the 90kHz “sampling” triangular waveform (the carrier frequency), as measured at TP2. The yellow trace is the audio input to the transmitter, in this case a 10kHz sinewave (at TP1). And the red trace shows the signal across the 47W resistor at the emitter of Q1 (this signal is proportional to the current driving the transmitter’s infrared LEDs). As can be seen, the pulse width of this waveform is modulated by the audio input. Fig.6: this graph plots the audio frequency response of the system. Note that a small amount of treble boost is applied from about 1kHz (rising to a maximum of 7dB at 8kHz) to improve intelligibility on speech. As shown, this is fitted with a small U-shaped heatsink and mounted flat against the PC board. The correct procedure here is to first bend the regulator’s leads down by 90° about 5mm from the device body (use a pair of needle-nose pliers to grip the leads while you bend them). That done, the regulator and its heatsink are secured to the PC board using an M3 x 6mm machine screw, nut and lock washer. Mounting the LEDs As can be seen on Fig.7 and in the photos, LEDs1-7 are all mounted with their leads bent down through 90°. This is done so that the LED bodies later protrude through their matching holes in the front panel. In each case, it’s simply a matter of bending the leads down through 90° exactly 5mm from the LED’s body, then installing the LED with its leads 8mm above the PC board (see photo). Make sure that each LED is correctly orientated – the anode lead is the longer of the two. The easiest way to get the LED lead spacings correct is to cut two December 2007  35 Capacitor Codes (Trans.) Value mF Code IEC Code EIA Code 220nF 0.22mF 220n 224 100nF 0.1mF 100n 104 10nF .01mF   10n 103 3.3nF .0033mF  3n3 332 2.2nF .0022mF  2n2 222 1nF .001mF   1n0 102 680pF    NA     680p 681 470pF    NA     470p 471 cardboard templates – one 5mm wide and the other 8mm wide. The 5mm template is then used as a lead bending guide, while the 8mm template is used to correctly space the LEDs off the board. The transmitter board assembly can now be completed by installing the two RCA connectors (CON1 & CON2) and the DC power socket (CON3). Fig.7: install the parts on the transmitter board as shown here, taking care to ensure that all polarised parts are correctly orientated. Below is a full-size photo of the assembled PC board. 36  Silicon Chip Receiver board assembly Fig.8 shows the assembly details for the receiver board. Once again, begin by soldering in the resistors and the small non-polarised capacitors, then install the larger electrolytics and the ICs. Note that the large 2200mF electrolytic capacitor is mounted on its side, with its leads bent down through 90°. Note also that the ICs are all different, so don’t mix them up. Take care to ensure they are correctly orientated. Once the ICs are in, install the volume pot (VR1), the headphone socket and power switch S1. Follow these by installing PC pins at the A & K positions for PD1 (the BP104 photodiode) and at the power supply inputs. The BP104 photodiode can now be installed by soldering its leads to its PC pins (see side-view diagram in Fig.8). Be sure to install this part the right way around. Its cathode lead has a small tag, as shown on its pin-out diagram in Fig.4. It’s also vital to install this device with its sensitive front side facing out from the PC board. Finally, LED1 can be mounted in position. This part must be mounted with 13mm lead lengths, so that it will later protrude through the lid of the case. A 13mm wide cardboard template makes a handy spacer when mounting this LED. Be sure to orientate siliconchip.com.au The completed transmitter PC board is installed in a low-profile instrument case and secured using four selftapping screws that go into integral mounting posts in the base. We used IC sockets for the prototype but you can solder the ICs directly to the PC board. it with its anode lead (the longer of the two) towards IC2. Final assembly – transmitter The final assembly involves little more than installing the PC boards inside their respective cases. If you are building the unit from a kit, the transmitter’s front and rear panels will be come pre-drilled (and with screen-printed lettering). In this case, it’s just a matter of first slipping these panels over the LEDs and input sockets on the PC board. That done, the entire assembly is then slipped into the bottom section of the case and secured using four self-tapping screws that go through the PC board and into integral matching stand-offs in the base. If you are not building from a kit, then you will have to drill these panels Resistor Colour Codes (Transmitter) o o o o o o o o o o o o o o siliconchip.com.au No. 2 2 4 2 1 1 1 1 1 1 3 1 2 Value 2.2MW 100kW 47kW 22kW 20kW 12kW 5.6kW 4.7kW 2.4kW 2.0kW 1kW 270W 47W 4-Band Code (1%) red red green brown brown black yellow brown yellow violet orange brown red red orange brown red black orange brown brown red orange brown green blue red brown yellow violet red brown red yellow red brown red black red brown brown black red brown red violet brown brown yellow violet black brown 5-Band Code (1%) red red black yellow brown brown black black orange brown yellow violet black red brown red red black red brown red black black red brown brown red black red brown green blue black brown brown yellow violet black brown brown red yellow black brown brown red black black brown brown brown black black brown brown red violet black black brown yellow violet black gold brown December 2007  37 Fig.8: here’s how to assemble the receiver board. Note how the BP104 diode is mounted by soldering its leads to two PC pins. Make sure it’s installed the right way around. yourself. Fig.10 shows the drilling details. The best approach is to photostat these diagrams and then attach them to the panels so that they can be used as drilling templates. Note that hole “D” is the adjustment access hole for trimpot VR1. Once the panels have been drilled, they can be dressed by attaching the relevant artworks (the files can be downloaded from the SILICON CHIP website and printed out on a colour printer). These artworks are attached using double-sided adhesive tape. Once they are attached, they can be protected by covering them with clear self-adhesive film (eg, wide sticky tape) and the holes cut out with a sharp utility knife. Final assembly – receiver Now for the final assembly of the receiver. Once again, kit versions will come with a case that’s pre-drilled and screen printed. If you’re not using a kit, use Fig.11 as a drilling template and attach the front panel artwork as described above. Fig.9: these full-size front panel artworks can be photocopied and applied to front & rear panels of the transmitter and to the lid of the receiver. Use a wide strips of self-adhesive film to protect them from damage – see text. 38  Silicon Chip siliconchip.com.au Capacitor Codes (Rec.) Value 100nF 47nF 10nF 2.2nF 1nF 470pF 180pF 100pF 15pF The receiver board is mounted on the lid of the case on M3 x 14mm tapped spacers and secured using M3 x 6mm screws (see text) mF Code IEC Code EIA Code 0.1mF 100n 104 .047mF   47n 473 .01mF   10n 103 .0022mF  2n2 222 .001mF   1n0 102    NA     470p 471    NA     180p 181    NA     100p 101    NA    15p   15 As shown in the photos, the PC board is mounted on the underside of the lid on four M3 x 15mm tapped spacers. Four M3 x 6mm countersink-head screws secure the spacers to the lid, while the PC board is secured using four M3 x 6mm pan-head screws. The power LED (LED1) and toggle switch (S1) both protrude through matching holes in the lid. Once the PC board is in place, one of the switch nuts is fitted to the top of the threaded ferrule, to help hold everything securely together. The two holes in the side of the box accept the shaft of the volume control (VR1) and the collar of the headphone socket (CON1). Another hole at one end of the box provides the “window” for photodiode PD1. As shown in the photos, a short length of PVC conduit was fitted around this hole, on the end of the box, to make a light shield “hood”. Although not strictly necessary, it does improve the signal-to-noise ratio of the link when you are using it in a fairly large room that’s lit with compact fluorescent lamps (CFLs) – ie, when there’s a long link path. CFLs produce a significant amount of noise at IR wavelengths and the hood stops most of this noise from reaching PD1. For the prototype, the hood was made using a 15mm length of 16mm OD PVC conduit. This was glued to the box end (concentric with the hole) using fast-setting epoxy cement. The battery holder, with its 4 x AA cells, is mounted at the other end of Resistor Colour Codes (Receiver) o o o o o o o o o o o o o o siliconchip.com.au No. 1 4 1 2 1 2 1 1 1 1 2 2 1 Value 2.2MW 100kW 47kW 22kW 20kW 10kW 2.0kW 1kW 390W 100W 47W 33W 10W 4-Band Code (1%) red red green brown brown black yellow brown yellow violet orange brown red red orange brown red black orange brown brown black orange brown red black red brown brown black red brown orange white brown brown brown black brown brown yellow violet black brown orange orange black brown brown black black brown 5-Band Code (1%) red red black yellow brown brown black black orange brown yellow violet black red brown red red black red brown red black black red brown brown black black red brown red black black brown brown brown black black brown brown orange white black black brown brown black black black brown yellow violet black gold brown orange orange black gold brown brown black black gold brown December 2007  39 Here’s another view inside the completed transmitter. Note the lead dress on the infrared LEDs and the green indicator LED, so that they protrude through their matching holes in the front panel. The rear panel of the receiver has clearance holes for the two RCA audio input sockets, plus access holes for the “Set Level” trimpot and the power socket. Power can come from a 12V AC or 15V DC (regulated) plugpack. the box. This can be held in place using a strip of electrical insulation tape. It’s then wedged firmly in position by the end of the PC board when the lid goes on. Note that the lid assembly must be introduced into the box at an angle, so VR1’s shaft and the headphone 40  Silicon Chip socket can enter their matching holes. It’s then swung down and fastened to the box using the self-tapping screws supplied. Set-up & adjustment Getting the transmitter unit going is straightforward. Basically, it’s just a matter of connecting the audio input leads and applying power. However, if you have an oscilloscope or a frequency counter, it’s a good idea to check the frequency of the clock oscillator before you close up the case. This is easiest done by checking the frequency of the triangular wave siliconchip.com.au Parts List Transmitter Unit 1 low profile ABS instrument case, 140 x 110 x 35mm 1 PC board, code 01112071, 117 x 102mm 2 PC-mount RCA sockets (CON1, CON2) 1 2.5mm PC-mount DC socket (CON3) 1 19mm square heatsink, 6073 type 3 8-pin DIL IC sockets (optional) 2 14-pin DIL IC sockets (optional) 1 M3 x 6mm machine screw, pan head 1 M3 nut with star lockwasher 4 self-tapping screws, 4g x 6mm long 3 PC board terminal pins, 1mm diameter 1 50kW vertical trimpot, 5mm (VR1) Semiconductors 1 LM833 low-noise op amp (IC1) 1 4093B quad CMOS Schmitt NAND (IC2) 1 4013B dual flipflop (IC3) 1 TL072 dual op amp (IC4) 1 LM311 comparator (IC5) 1 7812 +12V regulator (REG1) 1 BC328 PNP transistor (Q1) 6 5mm IR LEDs (LED1-LED3, LED5-LED7) 1 3mm green LED (LED4) 4 1N4004 1A diodes (D1-D4) Capacitors 1 1000mF 25V RB electrolytic 1 220mF 16V RB electrolytic 2 100mF 16V RB electrolytic signal at test point TP2 (just behind IC5). The frequency here should be between 80kHz and 100kHz. If it’s well outside this range, then you’ll need to change the value of the 680pF oscillator capacitor to correct it. The capacitor concerned is easy to find on the transmitter board – it’s just to the right of IC2. In practice, a value of 680pF (as shown on the circuit) should be suitable if a Motorola MC14093B device is used for IC2. However, if an ST Micro 4093B is used, this capacitor will probably have to be reduced to 470pF or 390pF. Conversely, for a Philips 4093B, siliconchip.com.au 1 22mF 16V RB electrolytic 1 220nF MKT metallised polyester 3 100nF MKT metallised polyester 3 100nF multilayer monolithic ceramic 1 10nF metallised polyester 1 3.3nF metallised polyester 1 2.2nF metallised polyester 1 1nF metallised polyester 2 680pF disc ceramic 1 470pF disc ceramic Resistors (0.25W 1%) 2 2.2MW 1 4.7kW 2 100kW 1 2.4kW 4 47kW 1 2.0kW 2 22kW 3 1kW 1 20kW 1 270W 1 12kW 2 47W 1 5.6kW Receiver unit 1 UB3-size jiffy box, 130 x 68 x 44mm 1 PC board, code 01112072, 57 x 84mm 1 battery holder, 4 x AA cells (square) 1 SPDT mini toggle switch (S1) 1 PC-mount 3.5mm stereo jack socket (CON1) 4 8-pin DIL IC sockets (optional) 1 small knob, push-on (for VR1) 1 15mm length of 16mm OD PVC tubing (optional) 4 M3 x 6mm machine screws, CSK head 4 M3 x 6mm machine screws, pan head the capacitor may need to be increased to 820pF or even 1nF. The basic idea is that you increase the capacitor’s value to lower the clock frequency, and reduce its value to increase the frequency. If you don’t have a frequency counter but have a modest uncalibrated oscilloscope, you can still check and adjust the clock frequency fairly easily by using the waveform at TP2 as a guide. The waveform here should be a very linear and symmetrical sawtooth, with a peak-to-peak amplitude of about 10.5V and only a very tiny “pip” on each positive and negative peak. 4 M3 x15mm tapped spacers 4 PC board terminal pins, 1mm diameter 1 10kW log pot, 9mm square PCmount (VR1) Semiconductors 1 LM833 dual low noise op amp (IC1) 1 LM311 comparator (IC2) 1 LM358 dual low power op amp (IC3) 1 LM386N audio amplifier (IC4) 1 BP104 IR sensor diode (PD1) 1 3mm green LED (LED1) Capacitors 1 2200mF 16V RB electrolytic 1 470mF 16V RB electrolytic 2 220mF 16V RB electrolytic 1 47mF 16V RB electrolytic 1 10mF 16V RB electrolytic 1 4.7mF 25V tag tantalum 1 100nF MKT metallised polyester 1 47nF MKT metallised polyester 2 10nF metallised polyester 1 2.2nF metallised polyester 1 470pF disc ceramic 1 180pF disc ceramic 2 100pF disc ceramic 1 15pF disc ceramic Resistors (0.25W 1%) 1 2.2MW 1 1kW 4 100kW 1 390W 2 22kW 1 100W 1 20kW 2 47W 2 10kW 2 33W 1 2.0kW 1 10W If you find that the waveform is a clean sawtooth but much lower in amplitude than 10.5V p-p, this means that the clock oscillator’s frequency is too high. To fix this, simply increase the value of the 680pF capacitor (eg, to 820pF). On the other hand, if the waveform does have an amplitude of 10.5V p-p or more but is clipped or truncated rather than being a clean sawtooth, this means that your clock oscillator’s frequency is too low. That’s fixed by reducing the value of the 680pF capacitor. If you don’t have a counter or an December 2007  41 Fig.10 (above): these are the drilling diagrams for the front and rear panels of the transmitter case. They can be photostated or downloaded from our website and directly used as drilling templates if required. Fig.11: here are the drilling details for the receiver case. It’s important to get all holes in their correct locations, so that everything lines up correctly when the receiver board is installed. oscilloscope, leave the capacitor’s value at 680pF and wait to see if the link’s performance is satisfactory. We’ll discuss this option shortly. The receiver unit needs no adjust42  Silicon Chip ments; all you have to do to get it going is to plug in your headphones, switch it on and point it towards the transmitter. The small green power LED should light and it’s then simply a matter of adjusting the volume control for a comfortable listening level. Testing the link To test the link, first connect the left siliconchip.com.au & right channel audio signal leads to the transmitter’s inputs. These signals can come from the stereo line outputs on your TV. You can also use the line outputs on your VCR or DVD player but only if you are actually using this equipment. Note that piggyback RCA socket leads may be required to make these connections if the audio outputs are already in use (eg, Jaycar WA-7090). Next, use a small screwdriver to adjust the “Set Level” trimpot (VR1) at the rear of the transmitter to midposition. That done, position the transmitter (eg, on top of the TV) so that it faces towards your viewing position and apply power. The transmitter’s green centre LED should immediately light (assuming an audio signal is being applied) but the IR LEDs will remain dark to your eyes. It’s now just a matter of checking that the link actually works. To do this, initially set the receiver’s volume control to minimum, then plug the headphones in and switch the receiver on. The receiver’s green power LED should either blink briefly (if you’re not pointing the receiver towards the transmitter) or light steadily if PD1 is able to “see” the infrared signal. The idea now is to place the receiver in a convenient position so that it gets an unobstructed “view” of the transmitter. In most cases, it can simply be positioned on an armrest, an adjacent coffee table on even on the back of the sofa. Now turn up the volume control and you should be able to clearly hear the TV sound. If so, your link is finished and ready for use. If the sound is overly loud and distorted, even when the receiver’s volume control is down near zero, it’s probable that the audio input sig- Specification A cordless audio headphone link for the hard of hearing. Provides a single channel audio link via infrared (IR) light, using pulsewidth modulation (PWM). Overall frequency response restricted to 20Hz – 12kHz, with a small amount of treble boost (maximum of 7dB at about 8kHz). Signal-to-noise ratio approximately 50dB. Transmitter Unit Small set-top box accepts line level audio (either mono or stereo) from a TV receiver, VCR or DVD player, etc. Input impedance: 47kW. PWM output via six infrared LEDs Range: about five metres. Power supply: 12V AC or 15-18V DC, with an average current drain of approximately 25mA. Receiver Unit A small portable box which responds to the modulated IR light beam from the transmitter, demodulates the audio and drives a standard pair of stereo headphones (2 x 32W impedance). Power supply: four AA cells (either alkaline or rechargeable NiMH). Average current drain: approximately 20mA, giving a battery service life of 80-100 hours or more. Controls: local volume control and a power on/off switch, plus a power/ signal indicator LED. nals from the TV are overloading the transmitter. In that case, try adjusting trimpot VR1 anticlockwise using a small screwdriver, to lower the input level. This should allow you to remove any audible distortion and bring the volume down to a comfortable level. If you find that distortion is still present even when the audio level is turned well down, this probably means that your clock frequency is either too high or too low. This can occur if you weren’t able to previously check the transmitter’s oscillator frequency – eg, if you don’t have a counter or an oscilloscope. In this case, try altering the 680pF capacitor’s value one way or the other, to see if the distortion gets better or worse. If it gets worse, go back the other way. If it gets better, keep changing the value in that direction. In practice, you shouldn’t need to increase the capacitor value above 1nF or reduce it below 390pF in order to SC remove all audible distortion. Issues Getting Dog-Eared? Are your SILICON CHIP copies getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? REAL VALUE AT $14.95 PLUS P & P Keep your copies of SILICON CHIP safe, secure and always available with these handy binders Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. siliconchip.com.au December 2007  43