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If you do a lot of multi-conductor cable wiring or cable troubleshooting, you’ll find this project especially handy. It consists of two very compact units: a signal injector which produces a very distinctive ‘warbling whistle’, and a sensitive signal tracer which can help you easily identify the cable conductor(s) carrying the warbling test signal. The signal tracer unit can easily be adapted for other kinds of signal tracing as well. ‘WHISTLE & POINT’ CABLE TRACER By Jim Rowe I t should be easy but it’s often a pain. Most multi-conductor cables are colour coded, so it should be a snack to connect each one to the appropriate pins of an RJ45 wall socket or whatever. But some of the colours are often a bit hard to distinguish — especially in poor lighting, when you’re crouch-ed down behind a desk or in some other awkward location. It is surprisingly easy to mistake the blue-and-white for the green-andwww.siliconchip.com.au white, or the white-and-orange for the white-and-brown. And then you find that somebody’s PC doesn’t seem to want to talk to the network server, because you’ve swapped some of the pair returns. . . Or you might be running a length of six-pair telephone cable and need to make sure that you get the pairs properly matched at each end. It can be trickier than you’d expect. What you need in your toolbox is a compact little signal injector giz- mo to squirt an easy-to-identify test signal along the conductors from one end, plus an equally compact little ‘sniffer’ or signal tracer gizmo so you can make sure which conductor is carrying the test signal at the other end. These two handy little gizmos are exactly what you get when you build this project, which we’ve dubbed the ‘Whistle & Point’ Cable Tracer. (Get it? One device produces the ‘whistle’ to draw attention to the wire you want, October 2002  53 and the other then ‘points’ you to it...) We can’t take the credit for designing the gizmos themselves, because they were dreamed up by the team at Oatley Electronics — who are selling kits for the two PC board assemblies inside ’em. However when they showed them to us, we were so impressed that we decided to work out how to house them in low-cost cases. This turned them into rugged little devices capable of being carried around in the usual toolbox and used reliably ‘on the job’. We also dreamed up that weird name for the project too, so it would get your attention. (So blame us for that, not Oatley!) You’ll be able to buy both PCB kits from Oatley for only $24 plus post and packing (typically $7.00 within Australia). We’ve calculated that you’ll only have to spend an extra $16 or so at most, to fit both boards into the more expensive of the boxes we’ve used with on-off slider switches and batteries. So the total cost for the complete project as a cable tracer set should still be no more than $32, buying everything from scratch. Not bad for such a handy pair of tools, wouldn’t you agree? By the way if you want to turn the signal tracer unit into a more general-purpose unit, this mainly involves using a larger speaker and building it into a larger box. And Oatley Electronics can even help you out there, too: as you’ll find in the ‘Wheredyageddit?’ box, for only $2.00 more they can supply husky little 50mm speakers complete with a larger plastic case which can be used to house the complete tracer. The box even contains an optional power amp IC which can be used to get more ‘grunt’. How they work Let’s look first at the signal injector unit. The circuit for this is shown in the upper part of the schematic diagram and, as you can see, it’s based on a couple of very low cost 555 timer ICs, plus a C8050 NPN transistor. The first 555 (IC1) is connected as S1 ON/OFF 8 3 9V BATTERY 10F 5 B 6 K 8 C 3 2 10F E 7 1 100nF Q1 C8050 10k 4 IC1 555 a simple relaxation oscillator, with its frequency of oscillation set by the 10kΩ feedback resistor from pin 3 to pins 2 and 6, and the 10µF capacitor from pins 2 and 6 to ground. With these values IC1 oscillates quite slowly at around 6Hz, producing a square wave at pin 3 and a ‘rounded sawtooth’ waveform at pins 2 and 6. It’s the rounded sawtooth that we make use of, but since pins 2 and 6 are operating at a fairly high impedance, we use transistor Q1 as an impedance matching emitter follower. This allows us to extract the sawtooth without loading down the oscillator and disturbing its operation. As you can see, the low impedance version of the sawtooth which appears at the emitter of Q1 is then coupled to pin 5 of the second 555, IC2. This is the ‘control voltage’ input of the 555, so as a result the sawtooth from IC1 is able to modulate the operation of IC2. IC2 is again connected as simple relaxation oscillator, just like IC1. 10F 4 IC2 555 5 6 2 A 47k CLIP A (+) 100 7 K 1 2.2k D1 1N4148 SIGNAL INJECTOR UNIT D2 1N4148 10nF CLIP B (q) A 1N4148 K A BC549 B ZD1 + C8050 q E 2N5484 B C C S E G D S2 ON/OFF 680 + PROBE TIP 150pF 39k Q2 2N5484 D G S 1.5nF 3.9k Q3 BC549 B IC3 LM386 1.5nF 2 SIGNAL TRACER UNIT SC 2002 1M 4.7k 10k 1k 100F q 6 100F 5 C 9V BATTERY 3 4 E 10pF ZD1 5.6V 100F 47k 4.7 PIEZO SPEAKER 15nF AWHISTLE & POINT˚ CABLE TRACER 54  Silicon Chip www.siliconchip.com.au The injector board (top) and the tracer board following assembly. Note the absence of on/off switches as shown in the drawings below: these were added when they were put into cases. clips should be accidentally connected to supply rails with voltages above or below the 9V battery rails. So that’s the injector unit. A simple, low cost circuit which generates a strong and very easy-to-recognise audio test signal, from a standard 9V battery. Now let’s look at the matching signal tracer unit. As you might expect this is basically just a fairly high gain audio amplifier, although it does have a few special aspects because of its being customised for this application. For example because we’re really only interested in tracing the injector unit’s warbling whistle signal, the amplifier’s frequency response is tailored to mainly respond to frequencies between 1100 and 1700Hz. This also TONE GENERATOR 2.2k IC1 555 + 10F 1 Figs. 1a (the injector board – top) and 1b (the tracer board – bottom) along with the wiring required. 100F 10F 1 D2 4148 D1 + CLIP A (+) CLIP B (–) – 10nF S2 3.9k 1 4.7 IC3 LM386 1.5nF ZD1 1k 220 47k Q3 39k + 10k 1.5nF Q2 680 5.6V BC549 2N5484 1M 4.7k 150pF 10pF PROBE 100F + + 100F + GND IC2 555 10k 100nF 10F 47k C8050 Q1 100 +9V 4148 S1 9V BATTERY allows us to use a very small piezo-electric speaker mounted directly on the board, as we’re not interested in reproducing frequencies below 1100Hz. At the heart of the amplifier is IC3, an LM386 audio output device. This provides the drive for the piezo speaker, as you can see, with the 4.7Ω resistor and 15nF connected across the output as a ‘Zobel network’ to ensure stability. As the LM386 has a fixed voltage gain of 20 in this configuration, transistor Q3 is used ahead of it to provide additional gain and make the tracer suitably sensitive. Q3 is a BC549, used in a standard common emitter stage. The emitter is fully bypassed to give a high voltage gain, while the use of 1.5nF coupling capacitors deliberately limits the low frequency response. + However in this case the main timing components are the 47kΩ feedback resistor and the 10nF capacitor from pins 2 and 6 to ground. This gives a basic oscillation frequency of around 1400Hz but because of the modulation from IC1 the actual frequency of IC2 varies up and down between about 1150Hz and 1700Hz. This time we use the square wave output from IC2, available at pin 3. This provides a waveform of almost 9V peak to peak, which becomes the injector’s ‘warbling whistle’ output signal. It’s fed to the active output clip (clip A) via the series 100Ω resistor — to protect IC2 from damage due to accidental short circuits. ‘Catcher’ diodes D1 and D2 are also connected so that pins 3, 2 and 6 of IC2 are protected against overvoltage damage if the test 15nF +9V PIEZO SPEAKER 9V BATTERY GND TONE DETECTOR www.siliconchip.com.au October 2002  55 Q3 and IC3 together would probably serve quite well alone as a cable tracer, providing plenty of gain plus a reasonably low input impedance (about 5kΩ). However, a JFET source follower stage has been added at the input, to give the tracer a much higher input impedance (nearer 1MΩ). This will make the unit also very suitable for signal tracing in low-frequency electronic circuits, where its high input impedance won’t cause unnecessary loading. (When using it for signal tracing in such circuits, you could use either the injector unit to provide a suitable signal for tracing, or a standard audio generator set to produce a tone of about 1200-1400Hz.) The input stage uses a 2N5484 N-channel JFET, with the signal from the tracer’s probe tip coupled to its gate via a 150pF capacitor. The 1MΩ resistor provides the gate’s bias return, while the 10pF capacitor shunts away any RF that may also be picked up by the probe tip. Both Q2 and Q3 are powered from a regulated 5.6V supply rail, which is derived from the 9V battery via a simple regulator circuit using the 680Ω resistor and zener diode ZD1. The LM386 chip runs directly from the 9V Two different views of the “opened out” tracer case showing how everything is “shoe-horned” in. It’s a tight fit but it will all go in! Note the probe, the tinplate shields, the insulating tape and also the polystyrene packing around the battery. All these are explained in the text. battery rail but both supply rails have 100µF reservoir capacitors to ensure low frequency stability. Construction Apart from the 9V batteries and on-off switches, virtually all of the circuitry for both the injector and the tracer units is fitted on two very small PC boards. The board for the injector unit measures only 41 x 25mm and has the Oatley code K181A, while the board for the tracer measures 72 x 25mm and is coded K181. Despite the small size of both boards, fitting the components should be very straightforward as there’s not all that many of them in either case. The location and orientation of each part is also shown clearly in the board overlay and wiring diagram, so if you follow this carefully you shouldn’t have any problems. As usual it will be easier if you fit the low profile components (resistors and diodes) first, then follow with the smaller and larger capacitors, and finally the transistors, ICs and the piezo speaker. Just make sure you fit each polarised component in the correct way around, to prevent problems later. When it comes to housing each board in a protective case, you have a range of choices. The injector unit in particular can go in virtually any small case, as long as there’s room for the board assembly itself, on-off switch S1 and the 9V battery and its snap lead. The two output leads are simply taken out through a grommetted hole and fitted with small shrouded alligator clips. To illustrate at least one of the packaging options for the injector, we Here’s how the injector board, battery and switch all fit inside a piece of 32mm PVC electrical conduit. Again, it’s a pretty tight fit inside the pipe! 56  Silicon Chip www.siliconchip.com.au housed the prototype unit in a 120mm length of 32mm outside diameter PVC conduit fitted with push-on plastic end caps. This length was plenty to fit both the board and battery end-toend, with the on-off switch mounted in one end cap and the output leads emerging through a grommetted hole in the other end cap. This makes a practical and quite rugged little package, which can also be easily opened when you need to replace the battery. There are fewer options for packaging the tracer unit, because we found that its PC board really needs to have a certain amount of shielding. This means that a very small metal case would be quite OK, although there aren’t too many suitably sized and proportioned metal cases available — especially at a reasonable price. You might have to make one up yourself, or modify an existing metal utility box. Of course you can always use a low cost plastic case which lends itself to fitting some shielding inside. This can be quite practical, as we’ve tried to show with our housing of the prototype tracer unit shown in the photos. The case we’ve used is a modular ABS unit measuring 90x50x32mm, and sold by Dick Smith Electronics (Cat. No. H-2832). As you can see from the photos this has just enough room to fit the PC board assembly and battery on-edge and side by side, with the on-off switch S2 mounted in the rear panel (offset to the side so it allows space for the PC board) and the probe tip mounted in the centre of the front panel. As for the probe tip itself, we gave this a bit of thought and ended up buy- Parts List – Whistle & Point Cable Tracer INJECTOR UNIT 1 PC board, code K181A, 25 x 41mm 1 Plastic case 90 x 50 x 32mm, or 120mm length of 32mm PVC conduit plus end caps (see text) 1 Miniature slider switch, SPST (S1) 1 9V battery, 216 type 1 Snap lead for 9V battery 2 Small alligator clips, red and black 1 Rubber grommet, 10mm hole diameter Semiconductors 2 LM555 timers (IC1,IC2) 1 C8050 NPN transistor (Q1) 2 1N4148 silicon diode (D1,D2) Capacitors 3 10µF PCB electrolytic 1 100nF (0.1µF) metallised polyester 1 10nF (.01µF) metallised polyester Resistors (0.25W 1%) 1 47kΩ 1 10kΩ 1 2.2kΩ 1 100Ω ing one of the low-cost ‘solderless’ test probes sold by DSE (Cat. No. P-1755). It proved to be quite easy to remove the metal probe tip from the plastic body — they simply pull apart. Then we used a small jeweller’s hacksaw to cut off all but about 3mm of the larger-diameter rear section of the metal tip, leaving the remaining section as a short ‘bolt head’ to go TRACER UNIT 1 PC board, code K181, 72 x 25mm 1 Plastic case 90 x 50 x 32mm, or 150mm length of 32mm PVC conduit plus end caps (see text) 1 Miniature slider switch, SPST (S2) 1 9V battery, 216 type 1 Snap lead for 9V battery 1 Test probe (see text) 1 Piezo speaker, 19mm diameter Semiconductors 1 LM386 amplifier (IC3) 1 BC549 NPN transistor (Q1) 1 2N5484 N-channel JFET (Q2) 1 5.6V 400mW zener diode (ZD1) Capacitors 3 100µF 10VW PCB electrolytic 1 15nF (.015µF) metallised polyester 2 1.5nF (.0015µF) metallised polyester 1 150pF ceramic 1 10pF ceramic Resistors (0.25W 1%) 1 1MΩ 1 47kΩ 1 39kΩ 1 10kΩ 1 4.7kΩ 1 3.9kΩ 1 1kΩ 1 680Ω 1 220Ω 1 4.7Ω behind the plastic case front panel. The details of this should be clear from the small diagram. We drilled the front panel so that the threaded section of the tip could be passed through it, and the round knurled ‘nut’ (used originally to fasten the test probe lead’s conductor) could then be used to fasten the tip to the front panel. The tracer’s own input wire was cut Resistor Colour Codes No.   1   1   1   5   2     2     2     2     2     2     1   1 www.siliconchip.com.au Value 1MΩ 47kΩ 39kΩ 10kΩ 4.7kΩ 3.9kΩ 2.2kΩ 1kΩ 680Ω 220Ω 100Ω 4.7Ω 4-Band Code (1%) brown black green brown yellow violet orange brown orange white orange brown brown black orange brown yellow violet red brown orange white red brown red red red brown brown black red brown blue grey brown brown red red brown brown brown black brown brown yellow violet gold brown 5-Band Code (1%) brown black black yellow brown yellow violet black red brown orange whiteblack red brown brown black black red brown yellow violet black brown brown orange white black brown brown red red black brown brown brown black black brown brown blue grey black black brown red red black black brown brown black black black brown yellow violet black silver brown October 2002  57 5 SILICON CHIP 40 12.5 (ALL DIMENSIONS IN MILLIMETRES) 20 ‘WHISTLE & POINT’ CABLE TRACER 10 Dia INJECTOR UNIT 30 35 (BEND UP AT ABOUT 70°) Panel labels for the injector unit (above) and tracer unit (below). As there are no on-panel controls, placement is nto critical. 12.5 (BEND UP AT ABOUT 70°) 22 SHIELD PLATE BEHIND FRONT PANEL SHIELD PLATES FOR TOP & BOTTOM OF CASE (2 OFF) SILICON CHIP MATERIAL: 0.2mm TINPLATE (SEE TEXT) We found that a shield was necessary inside the plastic case to prevent the high-gain amplifier picking up too much RF energy. Ours came from an empty pineapple tin – if you don’t like pineapple eat something else. very short and soldered directly to the rear of the probe tip, behind the panel. It all worked out quite neatly. To provide the shielding, we cut three small shield plates out of a strip of 0.2mm tinplate salvaged from a small tin which until recently contained pineapple pieces(!). The shield plates were shaped as shown in the small diagram, with the barrel-shaped piece designed to go behind the front panel (its 10mm hole clearing the rear of the probe tip) and the two more rectangular pieces designed to go into the front sections of the top and bottom halves of the case itself. These latter pieces have a 12.5mm deep section on each side bent upwards at about 70°, so their centre sections lie flat inside each half of the case yet their shielding extends around the sides. When the shield plates had been cut out and any sharp burrs removed, we then fastened them to the rear of the (TRACER BOX FRONT PANEL) ORIGINAL WIRE CLAMPING NUT NOW ATTACHES TIP TO PANEL front panel and inside the case halves. We used small pieces of gaffer tape for this, but you might prefer to use a few dobs of epoxy cement (like 5-minute Araldite). Then we soldered some short lengths of insulated hookup wire to connect all three shields together, with another short length so they could be connected to the PC board’s earth line when it was placed in position. (They have to be connected to the board earth, to provide correct shielding.) Before mounting the PC board in the case, we applied a small strip of gaffer tape down the centre of each case-half shield plate, to make sure that the plates couldn’t cause any short circuits at the edges of the board. By the way, it’s OK to use Gaffer Tape as insulation for low voltage devices like this but only properly rated electrical tape should be used on higher voltages. As well as cutting holes in the front and rear panels to take the probe tip ‘WHISTLE & POINT’ CABLE TRACER TRACER UNIT and on-off switch, we also drilled some small (2mm diameter) holes in one side of each case half near the rear end, to allow you to hear the sound from the piezo speaker when the case is screwed together. You can hopefully see these holes in the photo. When the board assembly was fitted in the bottom half of the case, the front panel with probe tip mounted on it was slotted in too and the board input wire carefully bent around to go into the hole in the rear of the probe tip. Then the two were soldered together quickly, so as not to overheat either. The rear panel with switch S2 fitted was slotted in at the other end, and the wires from the PC board and battery clip lead soldered to the contacts of S2. After this the wire from the shield plates was carefully soldered to the earthy copper at the front end of the PC board. Then the battery was added and squeezed in alongside the PC board (on the copper side), with a small piece of PULL AWAY PLASTIC SLEEVE CUT OFF ALL EXCEPT 3mm OF TIP REAR SECTION TO ACT AS 'HEAD' BEHIND PANEL Here’s how we turned a multimeter probe into our tracer probe while at right is a close-up photo showing how it mounts to the tracer case. 58  Silicon Chip www.siliconchip.com.au Capacitor Codes Value Alt. Value IEC Code EIA Code 100nF  0.1uF 100n 104 15nF  .015uF 15n 153 10nF  .01uF 10n 103 1.5nF .0015uF  1n5 152 150pF – 150p 150 10pF – 10p 10 plastic material between the two to prevent shorts. Some small pieces of expanded polystyrene were added as ‘packing pieces’ at either end of the battery, to prevent it moving forward or backward and causing trouble. Finally the top half of the case was fitted, taking care to dress the battery leads so they weren’t squashed between the case halves. Using them Checking cables using the two devices is quite straightforward. All you need to do is connect injector output clip A to one end of the wire you’re trying to trace, and connect clip B to either another wire, or a number of other wires, or some earthy metalwork. Then you turn both units on, and start probing the far end of all of the wires with the tracer unit. When you contact the right wire with the probe tip, you’ll hear the injector’s ‘warbling whistle’ quite clearly. Note that you don’t really need an earth return wire for the tracer, because the tracer amplifier is very sensitive and there’s enough capacitance between the shield plates and your hand to provide a high impedance return path. Of course if you want to use the tracer to check signal paths in equipment PC boards, then you will have to fit it with an earth-return input lead. This could consist of a 500mm length of insulated hookup wire, with one end soldered to the earthy copper at the front of the tracer board, and the wire brought out of the case through a 1.5mm hole drilled in the side. The far end of the wire would be fitted with a shrouded alligator clip like those on the injector output leads. You’ll only need to add this lead to your tracer if you do want to use it for general signal tracing work on PC Wheredyageddit? This project and the PC boards are copyright Oatley Electronics. Oatley have available a kit (K181) with both PC boards and on-board compon-ents plus the 9V battery snap leads and the piezo speaker for $24.00 plus $7.00 for packing and postage if applicable. This does not include the on-off slider switches, plastic cases, batteries or probe tip as described in the text. However Oatley can supply a larger ‘surplus’ plastic case with a PC board containing a husky 50mm speaker and an optional audio power amplifier — all very suitable for ‘beefing up’ the signal tracer unit — for an additional $2.00. Oatley Electronics can be contacted by: phone (02) 9584 3563; fax (02) 9584 3561; mail to PO Box 89, Oatley NSW 2223; email (sales<at>oatleyelectronics .com); or via their website (www. oat-leyelectronics.com). boards, though. For cable tracing, it’s not needed. SC Book Review . . . by Leo Simpson Firsts in High Fidelity. The Products and History of H. J. Leak & Co, by Stephen Spicer. 1st edition published 2000 by Audio Amateur Press, USA. Soft covers, 195 x 260mm, 272 pages. ISBN 1-882580-31-1 Anyone who is over 45 probably is aware of the legendary English hifi company, H. J. Leak & Co Ltd, although unless you were reasonably well-heeled in the years preceding 1970, it is unlikely that you would have ever owned their products. I certainly knew of their products in those years but they were priced way above my means. So were the products of other notable English companies such as Quad, A. R. Sugden and Wharfedale. It was more a matter of admiring them from afar. So it is with considerable interest that I received this sample copy about the products of the Leak company. They made a range of amplifiers, tuners and loudspeakers and all were notable in some respect or other apart from high performance, for the day. Of particular interest was the Leak sandwich loudspeaker, based on a 13-inch woofer with a “sandwich” cone consisting of a core of expanded polystyrene foam sandwiched between aluminium skins. This very rigid cone was shown in adverts (in Radio TV & Hobbies) supporting the full weight of the founder, Harold Leak. One of the reasons why Leak amplifiers were so highly regarded was the quality of construction. Not only was the under-chassis layout beautifully symmetrical but the transformers were extremely well made, with diecast covers. They were very good performers too, with plenty of negative feedback (a supposed anathema to today’s valve amplifier enthusiasts) and harmonic distortion of less than 0.1%. www.siliconchip.com.au For me though, there was a certain English eccentricity about some Leak products and none more so than the “troughline” FM tuner. I had always assumed that this was another English oddity but it turns out that the “trough line” was quite innovative in its day and used a tubular transmission line in place of the conventional coil inductor used in the local oscillator. This method of construction gave very good frequency stability. Nowadays, AFC (automatic frequency control) largely achieves the same result. Another interesting chapter in the book is devoted to the Australian manufacture of Leak loudspeakers. This was started by Syd McClory, a well-known personality on the local Sydney hifi scene at the time. All told, these book is a wonderful source of information on Leak products, whether you just want to engage in nostalgia or whether you are involved in restoring or building a Leak amplifier. To that end, there are quite a few circuit diagrams for amplifiers and tuners. The book is priced at $59.95 plus $8 postage and packing. It is available from Evatco, PO Box 487, Drysdale, Vic 3222. Phone (03) 5257 2297. email evatco<at>mira.net October 2002  59