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Ever wanted to be able to listen to the ‘unlistenable’ – sounds that are way beyond the range of normal human hearing? Like the supersonic whine of a gas leak, or the echo-location ‘chirps’ of bats? Here’s a low-cost project that will let you do just that. It’s a down-converter which shifts ultrasonic sound signals down into the frequency range where they can be heard (or recorded). by Jim Rowe ULTRASONIC EAVESDROP A FEW WEEKS AGO, I found myself watching a wildlife doco on TV in which naturalists were studying the behaviour of bats. They were using infrared lighting to photograph them and a down-converter so that they could hear and record the ultrasonic ‘chirps’ that the bats use for navigation in the dark – and often for tracking down their insect prey. My curiosity was aroused and I decided to ‘have a go’ at coming up with a low cost down-converter of my own. This project is the end result, presented so that other readers can indulge their curiosity as well. I won’t claim that the project has all kinds of uses, because it’s mainly going to be useful for listening to the ultrasonic sounds emitted by bats and one or two other nocturnal insect-eating creatures. But you should also be able to use it to track down the exact location of high-pressure gas leaks -- which apparently also produce an ultrasonic whistle or whine. You could even use it to make sure an ultrasonic dog whistle is working, if Fido seems to be ignoring it (perhaps his hearing has deteriorated like mine)! How it works Most of the sounds emitted by bats are in the frequency range from about 15kHz to 50kHz, with a few extending 72  Silicon Chip up to about 150kHz and a small number extending down below 10kHz. So most of them are above the range of human hearing, and some well above. (Young people can often hear up to about 18-20kHz but this upper limit generally falls as we grow older.) The idea of the eavesdropper is to shift the ultrasonic sounds down in frequency, so they fall within our comfortable hearing range. This is done by using the heterodyne principle, in much the same way as it’s used in many radio receivers. Or more accurately, in exactly the same way as it’s used in ‘direct conversion’ receivers: we mix the incoming ultrasonic signals with a continuous ultrasonic signal from a ‘local oscillator’. In the mixer the two signals heterodyne or ‘beat’ together, generating signals which correspond to the sum and difference of the two frequencies. The ‘sum’ signal will be very high in the ultrasonic range – and thus even more inaudible – but the ‘difference’ signal is easily arranged to be much lower in frequency and therefore in the audible (to humans!) range. You can see how this down-conversion system works from the block diagram in Fig.1. The ultrasonic sounds are picked up by a small electret siliconchip.com.au PPER microphone, which turns them into small ultrasonic electrical signals. This type of microphone has a frequency response which extends well up into the ultrasonic region. The signals are then passed through a preamplifier to boost them to a more useful amplitude (or level), where they can be passed into one input of a balanced mixer. The other input to the mixer is fed with a continuous ultrasonic signal produced by a tuneable ‘local oscillator’, so it can be varied in frequency from about 15kHz to 50kHz. As a result the output of the balanced mixer contains three main frequency components: the difference signals (FIN - FOSC) and (FOSC - FIN), and the sum signal (Fin + FOSC). The sum signal is obviously even higher in the ultrasonic range than FIN and FOSC, so it’s of no interest to us. We filter it out, anyway. But by adjusting the tuning of the local oscillator the difference signals can be placed down in the audible range, so all we have to do is feed them through an audio amplifier (via a volume control), before they can be either heard in a pair of headphones or sent to a tape or other recorder (even recorded on a computer hard disk or memory card for later analysis). What’s with the dish? Fig.1: the block diagram shows the various functional elements of the Ultrasonic Eavesdropper. siliconchip.com.au Used by itself, the electret microphone insert works – but not very well. To make it more effective, we concentrate the ultrasonic sound waves with a small, “somewhat” parabolic dish. As you may recall from previous SILICON CHIP projects, a parabolic dish reflects all the waves which strike it back to its focal point. With the miAugust 2006  73 74  Silicon Chip siliconchip.com.au 1nF 220k 10nF 4.7k 4 6 9 8 VR2 5k 8 470Ω 220k 1 5 6 IC3: LM833 8 O5-9 12 O0 O1 O2 O3 O5 O6 O7 O8 O9 1 5 6 9 11 3 2 4 7 IC2 4017B O4 10 CP1 Vss MR CP0 IC3a 470nF 2 3 PREAMP GAIN 180k 13 15 10 14 6.8k 7 14 Vdd ULTRASONIC EAVESDROPPER MIC INPUT CON1 47 µF 5 VR1 5k IC1b 11 OSCILLATOR FREQUENCY 1.2k MIC1 ELECTRET INSERT 2 IC1a 13 IC1d IC1c 16 4 IC3b 16k 10k 16k 120k 120k 7 22 µF 100nF 100nF 680Ω 100nF 1k +12V 4.7nF 11k 11k 30k 30k 100nF 1k 560Ω 470Ω 8 10 1.5k +12V 3 1k 14 MIXER BALANCE VR4 1 6 4.7nF 12 470Ω 4 5 10k λ LED1 1k IC4 MC1496 2 220Ω K A Fig.2: the circuit beats the “bat” frequency against the supersonic generator formed by IC1 and IC2. SC 2006 + 100nF 1 3 12 IC1: 4093B 100nF 4.7nF 4.7nF 1k 2x 3.3k 100 µF 100Ω A ZD1 K 2 VOLUME VR3 10k 3 2.2 µF 6 1 4 10 µF IC5 LM386N ZD1 12V 1W 220 µF A K 100Ω 7 8 5 10Ω 47nF 220 µF 2200 µF K A D1 1N4004 A K 1N4004 LED 12-15V DC INPUT STEREO PHONES CON2 RECORD OUT CON4 1k A K CON3 Fig.3: the entire project mounts on a single PC board, with the electret mic insert connected via an RCA socket on the left side. The sockets on the right connect power (12VDC), earphones and some form of audio recorder. We used IC sockets (as seen in the photo below) but these are not really necessary. crophone insert mounted at the focal point (or as close as we can guess!), pick-up becomes much more efficient and effective. This dish can be made from just about any material which will reflect sound waves – we used a laminated wood cereal or salad bowl, bought from a ‘bargain store’ for just a couple of dollars. It is about 155mm in diameter and about 39mm deep but this is not at all critical – a larger dish should be even better but would start to become unwieldy. A similar (hard) plastic or even stainless steel salad bowl could also be used. We said a moment ago that it was “somewhat” parabolic in shape – it has a flat bottom. This might not be technically ideal but it is good enough for our purposes – and certainly makes it a lot simpler to attach things to! You can work out the focal point of a parabola by formula (but it is complicated by the flat bottom), or you could line the bowl with aluminium foil and aim the bowl at the sun to enable you to get it spot on (as we did for the dishes used in our WiFry articles). Another way of finding the focal point would be to connect the mic insert to an audio amplifier and aim the dish at a single point sound source (such as a speaker connected siliconchip.com.au to an oscillator). Moving the microphone back and forward along the centre axis would reveal one point where the maximum signal was found. Having said all that, we found near enough (an educated guess) was good enough – but feel free to experiment with distances! We’ll look at mounting the dish and microphone a little later. The circuit Now let’s look at the circuit diagram (Fig.2) for a more detailed understanding of how it works. The ultrasonic sounds are picked up by the electret microphone insert, MIC1. The fairly small signals from MIC1 are fed in via CON1 and first amplified by IC3a, half of an LM833 dual low-noise op amp. It’s used here as a preamp with its gain variable between about 40 and 400, using trimpot VR2. This allows the project to be set up for either short or long range bat monitoring, and with bats having either loud or soft ‘chirping’ (they do vary, between species). After amplification, the signals are passed through IC3b, the ‘other half’ of the LM833, connected as a unity-gain August 2006  75 The completed PC board screwed to the lid of the UB-3 box, which becomes the base. Actually, this photo is a tad premature in the assembly sequence because you need to screw the lid to the timberwork, then fit the PC board to the lid. buffer to provide a low impedance source feeding the mixer IC4, via a 1kW series resistor. The ultrasonic signal used for our ‘local oscillator’ is generated using IC1 and IC2. This signal (a) needs to be tuneable over a fairly wide frequency range; (b) should be reasonably low in harmonic content and (c) should also be fairly constant in amplitude. However, this combination of qualities is not easy to produce using conventional audio oscillator circuits. So we generate it in a slightly unusual fashion. Gates IC1a, IC1b & IC1d are used as a relaxation-type oscillator, producing a square wave clock signal which is variable between 150kHz and 500kHz using pot VR1. This clock signal is buffered by gate IC1c and fed into the clock input of IC2, a 4017B Johnson-type decade counter. This IC therefore counts the clock signals so that its 10 outputs, O0 - O9, switch high in turn, on a continuous cyclic basis. These outputs are used to drive a simple digital to analog converter (DAC) using a set of resistors. While it may appear that output O7 is not used, it is – its “infinite value” resistor (ie, open circuit!) actually sets the zero point. The values of the resistors are carefully chosen so that as the outputs of IC2 go high in turn, a 10-sample approximation of a sinewave is developed across the output (ie, the 680W resistor between pins 10 and 8 of IC4). The 4.7nF capacitor which is also across the output provides a measure of low-pass filtering and further ‘smoothing’ of Here you can see how the plastic case needs to be drilled and slotted . . . 76  Silicon Chip the sinewave. The result of this simple digital waveform synthesis is a fairly smooth sinewave signal of reasonably constant amplitude, with a frequency exactly one tenth that of the clock signal from IC1. So as the clock signal is varied between 150 and 500kHz via VR1, the ‘local oscillator’ sinewave signal at the pin 10 input of IC4 is varied between 15kHz and 50kHz. IC4 is an MC1496 double-balanced mixer, expressly designed for this kind of use. When we feed our amplified ultrasonic sound signal into its pin 1 input and our local oscillator signal into its pin 10 input, it performs analog multiplication between them and provides the corresponding sum and difference frequency signals at its outputs (pins 6 and 12, which are simply dual polarity outputs). By the way, the mixer strictly only produces just the sum and difference signals at its outputs when it is carefully balanced using trimpot VR4. If it is not truly balanced, both of the input signals can also be present in the outputs – although this is not a major problem here because both of these input signals are inaudible. All the same, it’s a good idea to have the mixer reasonably close to balance, to reduce distortion in the audio amplifier. We’ll explain how to do this later. . . . so that the PC board is an easy fit. Again, the lid is screwed to the handle before the board is placed inside the box. siliconchip.com.au As you can see in this project we take the mixer output signal from pin 6 of IC4 and then pass it through a simple low pass filter using the 1kW series resistor and 4.7nF capacitor (across volume control VR3). This filtering attenuates the ‘sum’ frequency components quite significantly, leaving mainly just the audible ‘difference’ components that represent the downshifted version of our ultrasonic sound signals. We then pass these through audio amplifier IC5, after adjusting their volume level via pot VR3. The amplified output of IC5 is used to drive a standard pair of stereo headphones via CON4 and/ or an audio recorder via line-level output CON2. The complete circuit is designed to operate from almost any source of 12-15V DC, which is fed in via CON3 and can come from either a small AC plugpack supply or a nominal 12V battery such as that in a car or motorcycle. The total current drain is less than 35mA, so you could also run it from a pair of 6V lantern batteries connected in series or even a pack of eight ‘C’ cells. Zener diode ZD1 limits the voltage which can be fed to ICs1-3, while LED1 is an indicator that power is applied. Construction All of the Eavesdropper circuitry is mounted on a single PC board, measuring only 122 x 57mm and coded 01208061. As you can see the board has rounded cutouts at each corner so it can be mounted snugly inside a standard plastic utility box measuring 130 x 68 x 44mm. Microphone input socket CON1 is mounted on the lefthand end of the board, while the DC input, headphone output and recording output connectors are all mounted on the right-hand end. The local oscillator ‘tuning’ pot VR1, power LED1 and volume control pot VR3 are all mounted along the front side for easy accessibility. Begin construction by checking the PC board for any etching problems or undrilled holes and fixing these before you proceed. Then it’s a good idea to fit the various connectors (CON1-CON4), as these sometimes require a small The “gun” assembly immediately before the UB-3 case lid is secured. You can clearly see the way that the piece of coat hanger wire which supports the electret microphone is attached. amount of fiddling and board hole enlargement. There is only one wire link to be fitted to the board, so I suggest you fit this next to make sure it isn’t forgotten. It’s located just behind CON4, at lower right as viewed in the PC board overlay diagram. Next fit the various fixed resistors, taking care to fit each one in its correct position. These can be followed by trimpots VR2 and VR4, making sure you don’t swap them around. The 5kW trimpot is VR2, while the 1kW trimpot is VR4. Don’t fit the two larger pots at this stage, though – they’re best fitted later. Now you can fit the capacitors, starting with the two 100nF multilayer monolithics (near IC1 and IC2) and then progressing through the small MKT caps, the 2.2mF tag tantalum electrolytic (just to the front of IC5) and then the other electrolytics. Remember that all the electrolytics are polarised, so make sure you orient them correctly (as shown in Fig.3, the PC board overlay diagram). After these you can fit the semiconductors, starting with diode D1 and zener diode ZD1 – again make sure you don’t swap these accidentally and that they are both fitted with the correct orientation as shown in the overlay diagram and Fig.4: use this diagram in conjunction with the photo above to work out which bit goes where! siliconchip.com.au August 2006  77 photos. Then fit the ICs, preferably in reverse numbered order (ie, IC5 first, then IC4, working your way back to IC2 and IC1). Even though we did, there is no need to fit any of the ICs in sockets unless you wish to. All five ICs must be oriented as shown. If you are soldering IC2 and IC1 directly into the board, take care to protect them from the possibility of electrostatic damage. Use an earthed soldering iron, earth yourself if possible (or at least discharge yourself before handling the ICs) and solder the supply pins of the ICs first (pins 7 and 14 on IC1, pins 8 and 16 on IC2) to enable their internal protection circuitry as early as possible. After the ICs are all in position, it’s time to fit power LED1. This is fitted to the board vertically to begin with, with its longer anode lead to the right and both leads soldered to their pads underneath with the LED’s body about 18mm above the board. Then using a pair of needle-nose pliers or similar, bend both leads forward by 90°, 12mm above the board. This will position the LED facing forward and ready to protrude through the matching hole in the box, after final assembly. The last two components to mount on the board are control pots VR1 and VR3, which are both fitted along the front of the board on either side of LED1. You may need to cut the pot spindles to about 10-12mm long before they’re fitted, if they’re not already this length. Make sure you fit Fig.5: hole drilling diagram for a UB-3 plastic box. 78  Silicon Chip siliconchip.com.au the 5kW linear (B) pot as VR1, and the 10kW log (A) pot as VR3, as shown in the overlay diagram. Your Eavesdropper board should now be complete and ready to be fitted to the lid of the UB3 box, which is used here as the base. But before doing this, you may need to prepare both the lid and the box itself, by drilling and cutting the various holes that are needed for mounting, assembly and access to the various connectors and controls. The location and dimensions of all of these holes are shown in the drilling diagram (Fig.5), so you shouldn’t have any problems if you use this as a guide. The hardware It would also be a good idea at this stage to make the Eavesdropper’s wooden ‘handle’ and attach to its front the small dish we mentioned earlier. The dish is simply attached to the front of the wooden handle using a couple of 15mm long self-tapping screws, passing through 3mm holes drilled in the centre of the bowl. Two further 3mm holes were drilled just above these mounting holes to allow the mic support ‘bracket’ and its shielded lead to pass through. The mic support bracket was bent up from a 200mm length of 2.2mm diameter steel wire, salvaged from a coat hanger. After straightening and cutting to length, the wire was bent into a small loop at one end (around the shank of a 4mm twist drill). Then the straight section of wire was passed through the matching hole in the back of the bowl, and the loop end attached to the top of the wooden handle about 45mm behind the bowl using a 15mm long woodscrew, with a small flat washer under the screw head. The front end of the bracket was then bent around and downwards in an open ‘J’ shape, about 20mm in diameter, so the end was aligned very closely with the centre axis of the bowl and about 65mm in front of the bowl’s inside centre – corresponding to an approximation of this bowl’s likely ‘focus’ as an ultrasonic reflector. Then the mini electret mic insert was cemented to the side of the wire’s end using epoxy cement, with its ‘front’ facing the centre of the bowl (ie, it looks backwards, not forwards). After the epoxy cement has cured, solder the wires at one end of a 300mm length of light duty, screened microphone cable to the mic insert connection pads, with the cable screen wires connected to the insert’s earthy/case pad and the inner wire to the other ‘+’ pad. This is a slightly tricky job, as the pads are very small and closely spaced. So take your time, and take care not to overheat the mic insert in particular. If you’re new to soldering, it might surprise you to find that a hot, well-tinned iron poses much less danger than a cooler iron. The solder job is completed much more quickly – before the insert has had a chance to realise it’s getting hot! It’s also a good idea to connect the cable screen to the wire support bracket just near the mic using a short length of fine tinned copper wire, to minimise hum pickup. The free end of the mic cable is then passed back through the remaining hole in the centre of the bowl and fitted with a metal-shelled RCA plug at the other end ready to plug into the Eavesdropper. To prevent the cable from flapping around it can be fastened to the mic supporting wire using three short siliconchip.com.au Parts List – Ultrasonic Eavesdropper 1 1 2 1 1 1 1 1 4 4 8 2 PC board, code 01208061, 122 x 57mm Plastic utility box, UB3 size (130 x 68 x 44mm) RCA socket, PC-mount (CON1, CON2) 2.5mm DC socket, PC-mount (CON3) 3.5mm stereo socket, PC-mount (CON4) Electret mic insert, miniature type 300mm length of screened mic cable RCA plug, metal screened type 10mm long M3 machine screws, countersink head M3 star lockwashers M3 nuts Small control knobs (for VR1 and VR3) Semiconductors 1 4093B quad Schmitt NAND gate (IC1) 1 4017B decade counter (IC2) 1 LM833 dual low noise op amp (IC3) 1 MC1496 double balanced mixer (IC4) 1 LM386N audio amplifier (IC5) 1 12V 1W zener diode (ZD1) 1 3mm green LED (LED1) 1 1N4004 1A diode (D1) Capacitors 1 2200mF 16V RB electrolytic 2 220mF 16V RB electrolytic 1 100mF 16V RB electrolytic 1 47mF 16V RB electrolytic 1 22mF 16V RB electrolytic 1 10mF 16V RB electrolytic 1 2.2mF 35V TAG tantalum 1 470nF MKT metallised polyester 5 100nF MKT metallised polyester 2 100nF multilayer monolithic 1 47nF MKT metallised polyester 1 10nF MKT metallised polyester 4 4.7nF MKT metallised polyester 1 1nF MKT metallised polyester Resistors (0.25W 1%) 2 220kW 1 180kW 2 120kW 2 30kW 2 16kW 2 11kW 2 10kW 1 6.8kW 1 4.7kW 2 3.3kW 1 1.5kW 1 1.2kW 5 1kW 1 680W 1 560W 3 470W 1 220W 2 100W 1 10W 1 5kW linear pot, 16mm or 24mm PC-mount (VR1) 1 5kW mini trimpot, horizontal PC-mount (VR2) 1 10kW log pot, 16mm or 24mm PC-mount (VR3) 1 1kW mini trimpot, horizontal PC-mount (VR4) lengths of ‘gaffer’ tape (visible in the photos) wrapped around them both. At this stage, we gave the whole assembly a couple of coats of matte black spray paint. It looks 100% better than leaving it “au naturel”, which looks like a wooden salad bowl screwed to a piece of timber . . . If you do this, don’t forget to completely cover the electret mic insert in adhesive tape to stop it getting painted. Masking tape is preferable because ordinary adhesive tape can be a real pest to remove! Once the handle-dish-mic assembly is complete, you August 2006  79 01208061 Fig.6 (above): the same-size PC board pattern, while below, (Fig 7) is the same-size front panel artwork. We simply laminated and glued the paper label to the box, leaving about a 2mm border around the edge. can attach the Eavesdropper’s lid/base plate to the top rear of the wooden handle using a couple of 15mm long woodscrews through the two 3mm holes in the centre. As you can see the lid is orientated at right angles to the handle axis, and centred over it. With the box lid attached to the handle, you can fit the Eavesdropper’s finished PC board assembly to the lid. It’s attached using four 10mm long M3 machine screws with countersink heads, passed up from below and each then fitted with a star lockwasher and M3 nut. These nuts act as spacers, so the screws and nuts should be firmly tightened before the board assembly is fitted. Then when it is in position, four further nuts are used to hold it in place. Checkout and adjustment When the PC board assembly is fixed in place, it’s time to fire up the Eavesdropper and give it a quick functional checkout. Set both of the main control pots to roughly their midrange positions and also set both trimpots to their midrange positions using a small screwdriver or alignment tool. Then plug the mic cable into CON1, a pair of standard stereo headphones into CON4 (but don’t put them on yet, just in case something is Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o No.   2   1   2   2   2   2   2   1   1   1   1   1   5   1   1   3 1   2   1 80  Silicon Chip Value 220kW 180kW 120kW 30kW 16kW 11kW 10kW 6.8kW 4.7kW 3.3kW 1.5kW 1.2kW 1kW 680W 560W 470W 220W 100W 10W 4-Band Code (1%) red red yellow brown brown grey yellow brown brown red yellow brown orange black orange brown brown blue orange brown brown brown orange brown brown black orange brown blue grey red brown yellow purple red brown orange orange red brown brown green red brown brown red red brown brown black red brown blue grey brown brown green blue brown brown yellow purple brown brown red red brown brown brown black brown brown brown black black gold 5-Band Code (1%) red red black orange brown brown grey black orange brown brown red black orange brown orange black black red brown brown blue black red brown brown brown black red brown brown black black red brown blue grey black brown brown yellow purple black brown brown orange orange black brown brown brown green black brown brown brown red black brown brown brown black black brown brown blue grey black black brown green blue black black brown yellow purple black black brown red red black black brown brown black black black brown brown black black gold brown siliconchip.com.au Two views looking for’ard and aft. If you paint the whole shebang black, like we did, make sure you wrap a piece of masking tape around the microphone insert first. They don’t like being covered in paint! wrong!) and the cable from your 12V battery or plug pack into CON3. Power LED1 should immediately light up, to show that the circuit is operating. If the LED doesn’t light, this will probably be because one of three components is fitted to the board with reversed polarity: LED1 itself, D1 or ZD1. Either that or the plug on your DC input cable is wired with reversed polarity. With your multimeter you can check the voltage between the anode of D1 and the board’s ground – it should be the same as the incoming DC. Similarly the voltage at the cathode of D1 should Capacitor Codes Value 470nF 100nF 47nF 10nF 4.7nF 1nF μF Code 0.47µF 0.1µF .047µF .01µF .0047µF .001µF siliconchip.com.au EIA Code 474 104 473 103 472 102 IEC Code 470n 100n 47n 10n 4n7 1n0 be only 0.6V lower, while that at the cathode end of ZD1 should be a little lower again. You should also be able to measure the same voltage at pin 14 of IC1, pin 16 of IC2 and pin 8 of IC3. Similarly at pin 6 of IC5 you should find the same voltage as you measured at the cathode of D1. Listen to the headphones without actually putting them on. If they are not shrieking, place the headphones on your ears and you should hear a small amount of noise and/or hum. If you turn up volume control pot VR3, this noise should increase a little, showing that the audio section of the circuit is working correctly. Now try returning VR3 to its midrange position and adjusting ‘tuning’ pot VR1 up or down. You may hear a faint heterodyne ‘whistle’ as you tune through one position in the tuning range. This is probably due to the mic preamp picking up a small amount of RF from a local AM radio station, which then heterodynes with the Eavesdropper’s local oscillator or one of its harmonics. This is not likely to interfere with the Eavesdropper’s normal operation but if nothing else it shows that the Eavesdropper’s local oscillator, ultrasonic preamp and mixer sections are all working. If all seems well at this stage, your Eavesdropper is probably working correctly and all that remains to be done before final box assembly is to set the mixer balance trimpot VR4 to the correct position. Got a ’scope? Mixer balance adjustment is easiest with an oscilloscope but if you don’t have access to one, you don’t really have to concern yourself about it; simply leave VR4 set to its midrange position, which is very likely to be ‘near enough’ for most purposes. If you do have access to a scope and you want to set the mixer for the best possible performance, the adjustment is quite easy. All you need to do is monitor the level of the Eavesdropper’s ‘local oscillator’ signal appearing at pin 6 of IC4 with your ’scope, while adjustAugust 2006  81 Here’s what the finished project looks like, ready to use (all you need is a 12V battery pack!). The headphones can be just about anything – including the bargain shop $2 cheapies! ing VR4 with a small screwdriver. At either end of the trimpot’s range the signal will increase in level, while it will pass through a minimum or ‘null’ somewhere near the middle of the range. The correct setting for VR4 is right at the centre of this null – this corresponds to the mixer being balanced. Final assembly The final assembly step is to fit the box itself down over the PC board assembly, as a protective cover. This is done by inverting the box and tilting it an angle of about 45° so that it can be offered up to the PC board with the control pot spindles and LED1 entering their matching holes on the box ‘front side’ from the inside. Then the box is moved towards the mic and reflector bowl, gradually tilting it down so the undrilled long side swings down outside the 220mF electrolytic and the other components along the rear of the board. The slots at each end of the box will allow the ends to clear the protruding sleeves of RCA connectors CON1 and CON2. When the box has been juggled into position, it can be attached to the lid/ base using the four small self-tapping screws supplied with it. Then the control pots can be fitted with their nuts, which can also be lightly tightened to help support the pots when the Eavesdropper is being used. After this you can fit the knobs, and your Eavesdropper should be ready for use. Using it! The top trace of this ’scope shot shows the synthesised sine wave coming from the ladder network of IC2. The lower (blue) trace shows the output at pin 6 of IC4. The very low mean voltage measurement of 5.38mV shows that the modulator is balanced. 82  Silicon Chip This is also very straightforward. You use ‘tuning’ pot VR1 to search for ultrasonic sounds over the Eavesdropper’s range and then when you find one the same control is used to shift the sounds down to a convenient frequency for listening or recording. Volume pot VR3 is used simply to adjust the output audio to a convenient level. You’ll probably find the Eavesdropper sensitive enough to pick up bat chirps, etc with the preamp gain trimpot VR2 left in its suggested midrange position. However if you want to have the highest possible sensitivity, VR2 can be turned up to its fully clockwise position. Happy bat tracking! SC siliconchip.com.au