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Need to do a bit of selfdiagnosis? Make sure your heart is still beating or check other body sounds? Maybe you would like to sort out some unusual rattles or other noises in your car’s engine or other machinery? This electronic stethoscope will do the job – and you can listen via headphones or a loudspeaker. It has switchable frequency shaping in four bands so you can hone in on sounds which might otherwise be masked out. Features • Portable • Battery supply • Volume control • Boost or cut control • Selectable tone frequency • Reverse supply protection • Power and battery condition indicator Electronic Stethoscope By JOHN CLARKE S o why have an electronic stethoscope when a traditional cheap and cheerful stethoscope might be all you need? Well, a conventional stethoscope is OK if you have keen hearing and you are listening in a quiet environment but its sound levels are quite low, particularly at low frequencies. Secondly, on a cheap stethoscope there is no way of tailoring the frequency response of sound heard at the earpiece (apart from choosing the diaphragm or bell on the chestpiece of a medical stethoscope). The SILICON CHIP Electronic Stethoscope has plenty of gain – you can adjust the volume level to suit and you can use switchable filtering to cut or boost a particular band of frequencies. As well, it can be connected to headphones siliconchip.com.au or a loudspeaker, in which case more than one person can hear the sounds. If you wanted to, you could record the monitored sounds and display the waveforms on computer screen. Our Electronic Stethoscope comprises a chestpiece (sound pickup) that connects to a small amplifier box via a shielded cable. It has a headphone socket, knobs for volume and equaliser (EQ) and switches for power and for frequency band. The equaliser provides boost or cut in the frequency band selected by the 4-position slide switch. These bands are centred on 63Hz, 250Hz, 1kHz and 4kHz and are labelled Low, Mid1, Mid2 and High respectively. To simulate the bell sensor (of a medical stethoscope) August 2011  21 1k 4.7k ELECTRET BIAS LK1 +8.6V 100nF 10 10 33pF 9 IC1c VR3 100k +8.6V K K 470 F VOLUME VR1 10k LOG 100nF 5 6 4 IC1b 10k 7 3 1nF 1k  LED1 A 10 F 8 100k IC1a 2 68pF 1M A ZD1 4.7V A 2.7k 100 F IC1: TL074 INPUT CON1 K D2 1N4148 1k 150 100 F 1 220nF 10k 100k GAIN 47 47 1nF VR2 50k 10 F 10k CUT 10 F BOOST +4.3V 820nF 10k 10 F LOW BAND: 63Hz MID1 BAND: 250Hz MID2 BAND: 1kHz HIGH BAND: 4kHz 220nF 56nF 15nF S2 220k LOW MID1 MID2 1.8k 13 HIGH 'BAND SELECT' 12 IC1d 11 SC 2011 ELECTRONIC STETHOSCOPE 18nF 4.7nF 1nF 270pF Fig. 1: the circuit is based on two low-cost ICs – an amplifier (IC1a), buffer (IC1b), frequency band selection (IC1c and d) and finally, a power amplifier capable of driving a set of headphones or ear buds (IC2). It’s all powered by a 9V battery. where low frequency sounds are more prominent, the Mid2 band can be selected and the equaliser control set for an amount of signal cut. Or the low band could be selected with the equaliser control set in the boost position. To simulate the chestpiece diaphragm, the high band can be selected and the “EQ” pot set to the cut position. Alternatively, any one of the bands can be selected by the switch and the equaliser pot can be set to boost or cut. Boosting the frequency band selected will make more prominent any sounds of interest within that band. Conversely, the cut position will remove prominent sounds in that band that may otherwise mask out the sounds of interest. The chestpiece is adapted from a low cost stethoscope but with a piezo transducer fitted inside. For use with car engines or other machinery, the chestpiece is further modified to provide a more direct contact with the piezo transducer element. Circuit details The Electronic Stethoscope is based on two low-cost ICs: a TL074 quad op amp IC and an LM386 power amplifier IC. The op amps are used for amplification and filtering of the signal from the piezo element in the chestpiece. The power amplifier drives the head22  Silicon Chip phones or a loudspeaker. The full circuit is shown in Fig.1. Signal from the piezo element is applied via CON1, 3.5mm socket and a 100nF capacitor to op amp IC1c. The associated 33pF capacitor and 10resistor attenuate high frequencies and thereby reduce the possibility of picking up radio signals. IC1c is biased at +4.3V via the 1Mresistor connected to the 10kvoltage divider resistors across the 8.6V supply. The 1Mresistor also sets the input impedance of the amplifier. Note that bias for an optional electret microphone is included and is fed via link LK1 and 4.7kresistor to the 8.6V supply via a 1kresistor and 100F bypass capacitor. (Electret bias is included so that the stethoscope can be used in a different application. See the section entitled “Using the stethoscope as an audio eavesdropper”). IC1c is connected as a non-inverting amplifier with a gain that can range from about two when trimpot VR3 is set at 100kup to 101 when VR3 is set to its minimum resistance. IC1c’s output is coupled to the volume control potentiometer, VR1, via a 10F capacitor. SPECIFICATIONS The output of VR1 (wiper) is fed to IC1b Supply voltage:......... 9V <at> 12mA quiescent current which is connected as Battery life: ...............Typically 30 hours a unity gain buffer. The output from IC1b drives          (with alkaline battery) the equaliser (EQ) stage Selectable bands:......63Hz, 250Hz, 1kHz and 4kHz consisting of op amps Boost or cut range:...±15dB IC1a and IC1d. siliconchip.com.au 14 4 10 F 100k 4.7k 1M VR1 10k VOLUME 47 BAND SELECT 18nF S2 10 Fig,4: here’s how it all goes together on the PCB. Note how the twin wires from the battery snap pass through the front of the board and back out again – that’s for strain relieve on the solder joints (they could otherwise snap off). Watch the IC, LED and electrolytic capacitor orientation; also make sure the two pots aren’t mixed up. Finally, the jumper for the link (LK1) is only placed in position for use with an electret microphone – it is not used at all for the “normal” piezo version. ZD1 A K 1N4148 LED A K 1N5819 K A A K Switchable single band equaliser These two op amps form a single band equaliser which can boost or cut the signals in a defined frequency band selected by the 4-position slide switch, S2. The concept for the single band equaliser can be seen in Fig.2. In essence, we have an op amp (IC1a) connected as a non-inverting amplifier and a feedback network with a potentiometer (VR2) which sets the amount of boost or cut. The frequency band is defined by the resonant frequency of the series-connected capacitor C1 and inductor L1. With VR2 wound fully to the left, the tuned series LC circuit is connected to IC1a’s input via a 47 resistor. At SIGNAL IN CHESTPIECE 1nF 47nF 270pF 15nF 7 CON2 OUTPUT 1 1 1 8OUTPUT 0140 CON1 56nF 470 F 220k 4.7nF 5 LK1 A K 1nF 220nF 470 F 10 47nF 820nF OUTPUT CON2 10 1.8k IC2 LM386N 8 9V BATTERY 100nF 1 LED1 47 IC1 TL074 33pF 1k VR3 100nF 1nF 68pF 100 F 10 F S1 POWER 10k 10k 100 F + 2 6 ZD1 100k 100 F 3 4.7V VR2 50k EQ 220nF 10 F D2 1k 10 F 10 F IC2 LM386 D1 4148 EP O C S O HT E T S 9V BATTERY 100 F 5819 2.7k 150 10 F 1k 470 F POWER 10k + + A – K 10k D1 1N5819 (–) S1 +8.6V 10k SIGNAL OUT IC1a IC1c 10k 47 47 Fig.2: the essence VR2 50k of an equaliser. A series-resonant CUT BOOST LC network C1 (comprising L and C1) and potentiometer (VR2) is L connected within the IC1c op amp feedback network. siliconchip.com.au Vin Iin C2 the resonant frequency, the impedance of the LC network is at a minimum. Thus, the signal applied to IC1a will be shunted to ground, reducing the signal at the IC1a output. When VR2 is rotated to its boost setting, the LC network is connected directly to the inverting (-) input of the op amp via another 47resistor, shunting the negative feedback to ground. At the resonant frequency, the low impedance of the LC network reduces the feedback and the gain of IC1a will increase. The centre frequency of the circuit can be obtained from the formula: F0 =    1 2  L C1 In fact, our circuit does not use an inductor in the equaliser as it would be very large and bulky. Instead we have replaced the inductor with a gyrator. A “gyrator” is a pseudo-inductor using an op amp and a capacitor, as shown in Fig.3. In an inductor, the current lags or is delayed by 90° with respect to the voltage waveform. With a capacitor, however, the voltage lags the current by 90°. To simulate the inductor, the voltage lag of the capacitor must be converted to a leading voltage compared to the current. With an AC signal applied to the input of the circuit (Vin) of Fig.3, current will flow through capacitor C2 and the resistor R2. Because it is connected as a voltage follower, I out R1 1.8k the op amp will reproduce the voltage across R2 at its output. IC1d R2 220k Fig.3 (left): circuitry of a gyrator. The op amp IC1d simulates an inductor by a phase transformation of the current through C2. The resulting inductance is equal to the product of R1, R2 and C2. August 2011  23 The three basic components of our new Electronic Stethoscope. At left is a pair of standard heaphones – it will also work with ear buds but we find ear-covering headphones best, as they mask more external noise. Top right is the “works” while at lower right is the chest-piece, itself made by modifying a low-cost medical (acoustic) stethoscope. Inset top left is the “mechanic’s” attachment we made to listen into machinery etc. This voltage will now cause a current to flow in R1 and this adds to the input current. The resulting total current lags the input voltage by 90°. So as far as the signal source is concerned, the circuit behaves like an inductor. The value of simulated inductance is given by the equation: L = R1 x R2 x C2. By substituting the gyrator for the inductor in the circuit of Fig.2, we have the basis for a complete equaliser. The 4-position slide switch, S1, selects different values for C2 and C1 for each of the frequency bands. +20 Stethoscope Frequency Response 06/22/11 10:29:09 +17.5 +15 +12.5 Amplitude Variation (dBr) +10 +7.5 +5 +2.5 +0 -2.5 -5 -7.5 -10 -12.5 -15 -17.5 -20 20 LOW 50 MID1 100 200 MID2 500 1k HIGH 2k 5k 10k 20k Frequency (Hz) Fig.5: boost and cut graph for each band as set for maximum boost and cut. Note that only one single band can be used at one time in either boost or cut position. Boost or cut can be set to any level between the two extreme boost or cut levels. 24  Silicon Chip Following the equaliser stage, the signal is fed via a 220nF capacitor to the non-inverting input (pin 3) of IC2, the LM386 audio power amplifier. IC2 can provide about 500mW into 8with a 9V supply and distortion is typically 0.2%. When using stereo 32headphones (with the earpieces connected in parallel to give a 16load), the power is about 250mW; more than enough to provide sufficient listening volume. IC2 drives the output load via a 470F capacitor and a Zobel network, comprising a 10resistor and 47nF capacitor, which helps prevents amplifier instability. The power for the stethoscope comes from a 9V alkaline battery, with diode D1 providing protection against a reverse polarity connection. A Schottky diode is used due to its low forward voltage loss (about 0.3V compared to a normal silicon diode’s 0.6V). LED1 has two functions: to show power ‘on’ and to show battery condition. It operates as follows. When power is first applied, current for the LED flows through the 4.7V zener diode (ZD1), the 1kresistor and the discharged 470F capacitor. If the battery is fresh, the 9V battery provides 8.7V at the anode of LED1. This voltage is reduced by about 1.8V by LED1 and 4.7V by ZD1, leaving 2.2V across the 1k resistor. LED1 lights with 2.2mA. At lower battery voltages, there is less voltage across the 1k resistor so the LED is dimmer. At a battery voltage of 7V, there is about 0.2V across the 1kresistor and the LED barely lights. As the 470F capacitor charges up, the LED current is reduced to much lower level, set by the 2.7kresistor across the capacitor. This indicates that the Electronic Stethoscope is switched on without wasting significant power. When power is switched off, diode D2 discharges the 470F capacitor so siliconchip.com.au LED Lighting - Saving Energy & the Environment ecoLED Tube The friendlier alternative to fluorescent lamps No mercury, no lead, environmentally friendly Less power, Longer life, Less maintenance Can retrofit T8 Fluorescent Lamps No strobing, no flicker, no buzzing, no irritation Half the power, energy cost saving Longer life, very low maintenance Flexible LED Lights RGB Multi-colour, White, Warm White. 24VDC. Cut to length. Remote controls for colour & dimming. With waterproof seal and adhesive taping (non-seal version also available) Inside the assembled Electronic Stethoscope “works”, reproduced here close to same size. Use this in conjunction with the component overlay (fig.4). the circuit is again ready to indicate the battery charge state when it is turned back on. The 8.7V supply is connected directly to IC2 but it is fed to IC1 via a 150resistor. A 100F capacitor decouples this supply and removes any supply modulation from IC2 which could otherwise cause instability. This would take the form of audible “motor-boating”. Cove lighting Construction With the exception of the piezo mounted on the chestpiece, all the Electronic Stethoscope components are accommodated on a single PCB, coded 01108111 and measuring 65 x 86mm. In turn, the PCB is housed in a black plastic “remote control” case measuring 135 x 70 x 24mm. The PCB is designed to mount onto the integral mounting bushes within the box. Make sure the front edge of the PCB is shaped to the correct outline so it fits into the box. It can be filed to shape if necessary using the PCB outline as a guide. Begin construction by checking the PCB for breaks in tracks or shorts between tracks or pads. Repair any defects, if necessary. Check the sizes for the PCB mounting holes and for the battery leads. These are 3mm in diameter. siliconchip.com.au Bar lighting Console Kickboard lighting Colour changing & effects via remote control. Sets the mood & atmosphere for your venue. Website: www.tenrod.com.au E-mail: sales<at>tenrod.com.au Sydney: Melbourne: Brisbane: Auckland: Tel. 02 9748 0655 Tel. 03 9886 7800 Tel. 07 3879 2133 Tel 09 298 4346 Fax. 02 9748 0258 Fax. 03 9886 7799 Fax. 07 3879 2188 Fax. 09 353 1317 August 2011  25 End-on view showing the three controls (Eq, Volume and Power) on the end panel and the four-way filter band selection switch on the front panel. The component overlay for the PCB is shown in Fig.4 You can start assembly by the inserting the resistors. Check each resistor value against the colour code table as you go and double-check with a digital multimeter. Next, install the two PC stakes followed by the diodes, mounted as shown. IC1 & IC2 can be directly mounted on the PCB, or if you wish can be mounted on DIP8 sockets. When installing ICs (and sockets if you use them), take care to orient them correctly. Orientation is with the notch positioned as shown. Switch S2 does not mount directly onto the PCB but is raised off the PCB using a 6-way dual row pin header. Remove a pair of pins so that there is a row of three pins, then a gap then two pins on each side of the DIL header. The header pins are longer at the top than the bottom. Push them down so that the tops are 5mm above the bottom of the plastic section and solder it in the switch mounting position. The switch is mounted by soldering its pins to the top of the header pins. The switch must be oriented correctly with the row of three pins toward the volume pot (VR1). The top of the switch body should be 12mm above the PCB. The capacitors can be mounted now. The electrolytic types must be oriented correctly – the polarity is shown on the component overlay. Make sure these capacitors are placed in the PCB so their height above the board surface is no more than 12.5mm otherwise the lid of the case will not fit correctly. The potentiometer (VR2) and the PCB mounted switch S1 can also be fitted now, along with the 3.5mm sockets. LED1 mounts horizontally but at a height of 6mm above the PCB. Bend its leads at 7mm back from the base of the LEDs at 90° making sure the anode lead is to the left. When assembled, the PCB is secured to the base of the case using four M3 x 6mm screws that screw into the integral mounting bushes in the box. Before putting this in place, drill out the small front panel for the LEDs, potentiometer and switch. A drill guide is provided with the front panel label. Holes are also required in the base and case lid for the 3.5mm sockets. A rat-tail file can be used to make these cut outs. The panel label for this project can be downloaded from the SILICON CHIP website (www.siliconchip.com.au). Go to the downloads section and select the month and year of publication. When downloaded, you can print onto paper, sticky backed photo paper or onto plastic film. Paper labels need protection, so cover them with self-adhesive clear plastic or, best of all, hot laminate film. When using clear plastic film (overhead projector film) you can print the label as a mirror image so that the ink is behind the film when placed onto the panel. Once the ink is dry, cut the label to size. The paper or plastic film is glued to the panel using an even smear of neutral cure silicone sealant or spray contact adhesive. If you are glueing a clear plastic film label to a black coloured panel, use coloured silicone such as grey or white so the label can be seen against the black. A rectangular hole in the panel is required directly above the slider switch S2. The positioning for this is shown on the label. This shape can be first drilled in the plastic lid and then once the panel label is affixed, the cut the panel hole out using a sharp hobby knife. The top of the switch can be coloured black using a permanent marker pen to improve the appearance through the switch hole. If you require the Stethoscope to be secured to a belt, a suitable belt clip is available from Altronics (cat no H0349). Contact www.altronics.com.au Chestpiece The chestpiece for the Electronic Stethoscope is cannibalised from a commonly-available (and low cost) acoustic medical stethoscope. Ours came from Jaycar Electronics (www.jaycar.com.au), cat no QM7255 <at> $14.95 but most chemists and medical supply houses have them. You can pay a lot more for a stethoscope – for example the Littman Cardiology III, manufactured by 3M, sells for more Fig.6: we used a commercially available stethoscope to obtain the chestpiece for our electronic version, then fitted it with a piezo transducer and a cable with 3.5mm jack plug to the amplifier. 26  Silicon Chip siliconchip.com.au PROBE STEM MADE FROM 2mm DIAM BRASS ROD 43mm DIAMETER DISC OF 1mm ALUMINIUM Parts List – Electronic Stethoscope PROBE TIP MADE FROM 6mm LENGTH OF 6.5mm DIAM. BRASS ROD 12mm LONG M3 TAPPED SPACER 6mm LONG M3 SCREW Fig. 7: to listen in to engines and other equipment you’ll need something like this probe. It transmits vibrations etc direct to the piezo transducer of the chestpiece. than $150. But we’re not interested in specialised models, the common or garden-variety stethoscope is what we’re after. The following applies specifically to the Jaycar model but you will probably find that most of the low-cost stethoscopes use a similar method of construction. The diaphragm section is removed from the chestpiece to access the inside of the casting. Unscrewing the outer annulus from the rear casting does this. The piezo element from a piezo transducer is used as the detector and is placed within the chestpiece diecast housing. The piezo transducer is available from Jaycar, cat. no AB-3440 or from Altronics, cat no S 6140. We did test the stethoscope using an electret microphone (Jaycar AM4008) for the chestpiece pickup sensor. This was a very small microphone at 6mm in diameter and 3.5mm deep that fits within the back of the diecast moulding. This proved to be unsatisfactory for this application, although there was nothing wrong with the microphone itself. The main problem was that it would detect far more than was required for a stethoscope including detection of noises from adjacent rooms. The use of an electret microphone, however, is ideal for use as an eavesdropper. See the separate section concerning its use. A piezo element proved to produce a much better result. The piezo element is removed from its plastic transducer housing. To do this, firstly remove the backing siliconchip.com.au 1 PCB, coded 01108111, 65 x 86mm 1 remote control case 135 x 70 x 24mm (Jaycar HB5610 or equivalent) 1 panel label 50 x 114mm 1 9V battery 1 9V battery clip lead 1 low cost stethoscope (Jaycar QM7255) used for parts 1 miniature PC mount SPDT toggle switch (Altronics S 1421 or equivalent) (S1) 1 DP4T switch (Tyco Electronics STS2400PC04) (Element14 Cat.1291137) (S2) 1 10k log potentiometer, 9mm square, PCB mount (VR1) 1 50k linear potentiometer, 9mm square, PCB mount (VR2) 2 knobs to suit potentiometers 2 PC mount 3.5mm stereo sockets (CON1,CON2) 1 3.5mm mono line jack plug 1 DIP8 IC socket (optional) 1 DIP14 IC socket (optional) 1 piezo transducer (Jaycar AB-3440, Altronics S 6140) 1 PAL (Belling Lee) line plug with plastic housing (Jaycar PP0600) (required for the metal crimp shield connector) 4 M3 x 6mm screws 1 M2 x 3mm screw (or a cut down M2 x 8mm screw) 1 6-way DIL pin header 1 2-way pin header with 2.54mm spacing (with jumper shunt) 2 PC stakes 1 60mm length of 10mm diameter heatshrink tubing 1 750mm length of single core shielded cable Semiconductors 1 TL074 quad op amp (IC1) 1 LM386N amplifier (IC2) 1 1N5819 1A Schottky diode (D1) 1 1N4148 switching diode (D2) 1 4.7V 1W zener diode (ZD1) 1 3mm high intensity red LED (LED1) Capacitors 2 470F 16V PC electrolytic 3 100F 16V PC electrolytic 5 10F 16V PC electrolytic 1 820nF MKT polyester 2 220nF MKT polyester 2 100nF MKT polyester 1 56nF MKT polyester 1 47nF MKT polyester 1 18nF MKT polyester 1 15nF MKT polyester 1 4.7nF MKT polyester 3 1nF MKT polyester 1 270pF ceramic 1 68pF ceramic 1 33pF ceramic Mechanic’s Stethoscope – Optional Parts 1 43mm diameter circle of 1mm aluminium 1 M3 x 12mm tapped brass spacer 1 M3 x 6mm countersunk screw 1 brass rod 2mm diameter x 40mm long with a 6mm brass spacer tip or 1 top end from a telescopic antenna Audio Eavesdropper – Optional Parts 1 electret microphone insert (9.5mm diameter) (Jaycar AM-4010 or sim) 1 300mm length of single-core shielded cable 1 3.5mm mono line plug 1 IP68 waterproof gland for 4-8mm diameter cable 1 plastic cylinder 125ID x 157mm long (eg empty 100 x CD container) or similar (see text) 1 timber handle 65 x 115mm (eg, pine off cuts or similar) 2 wood screws (to secure handle) Resistors (0.25W, 1%) 1 1M 1 220k 2 100k 4 10k 1 4.7k 1 2.7k 1 1.8k 3 1k 1 150 2 47 2 10 1 100kmultiturn top adjust trimpot (VR3) Miscellaneous Earphones or headphones, neutral cure silicone sealant, solder. August 2011  27 These three photos show, respectively (from left) the disassembled chest piece with the piezo fitted; the reassembled chestpiece with the cable going off to the amplifier fitted into a short length of the tubing from the original stethoscope and finally a close-up of the “clamp” (actually the wire clamp from a TV cable plug) used to hold it all together. piece from the housing to expose the transducer. The transducer is easily prised out as it is glued to the housing with a soft rubber-based adhesive. Take care not to crack the piezo element. Wires connected to the transducer are removed by melting the solder from the metal disk and piezo element itself. Remove the solder with some solder wick. The piezo element is attached to the chestpiece using an M2 x 3mm screw that is tapped into the chestpiece casting. Drill a 2mm hole in the edge of the transducer but away from the piezo material itself and align the transducer central to the chestpiece housing. Mark out where the mounting hole is required. Drill a 1.5mm hole (1/16”) and screw the M2 screw into the hole. You may need to file a small notch along the M2 screw thread to act as a makeshift thread cutter if the screw does not enter the hole easily. (Of course, if you happen to have an M2 tap, use that!) Once the hole is ‘tapped’, remove the screw. The piezo sensor is placed onto the chestpiece housing with the piezo element facing inward. The core wire of the shielded cable passes through the metal tubing of the chestpiece and is soldered to the centre of the piezo element. The shielding wire is soldered to the end of the metal tube after firstly filing out a small flat landing on the side of the tubing to allow for a solder joint. Secure the transducer with the M2 screw. A smear of neutral cure silicone sealant (eg, roof and gutter sealant) is applied around the outside of the transducer to form an air seal to the chestpiece housing. A short (60mm) length of the tubing from the low-cost stethoscope is cut and slid over the shielded cable and onto the metal tubing of the chestpiece. The tubing is crimped to the shielded cable wire – we used the crimp section of a PAL (Belling Lee connector) placed over the tubing. This is squeezed down over the tubing to grip the shielded cable in place within the tubing. A 20mm length of 10mm diameter heatshrink tubing is shrunk down over the section to hold the crimp fingers closed. The diaphragm and annulus can now be reattached to the chestpiece housing by screwing this back together. RESISTOR COLOUR CODES 1 1 1 1 1 1 1 1 1 1 1 No. 1 1 2 1 1 1 1 3 1 2 2 Value 1MΩ 220kΩ 100kΩ 10kΩ 4.7kΩ 2.7kΩ 1.8kΩ 1kΩ 150Ω 47Ω 10Ω 28  Silicon Chip 4-Band Code (1%) brown black green brown red red yellow brown brown black yellow brown brown black orange brown yellow purple red brown red purple red brown brown grey red brown brown black red brown brown green brown brown yellow purple black brown brown black black brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown black black orange brown brown black black red brown yellow purple black brown brown red purple black brown brown brown grey black brown brown brown black black brown brown brown green black black brown yellow purple black gold brown brown black black gold brown The opposite end of the shielded cable is terminated to a 3.5mm mono jack plug. Mechanic’s stethoscope attachment For the mechanic’s attachment, we used a 43mm diameter disk of 1mm aluminium to replace the flexible diaphragm of the chestpiece. This means that the annulus is unscrewed and the flexible diaphragm removed by pressing it out with using your fingers. A rod attaches through the centre of this aluminium disk to provide contact with the machinery. We supported our rod using an M3 tapped brass spacer secured to the disk with an M3 x 6mm screw. To this spacer is soldered a brass rod with a tipped end. We used the end from a discarded telescopic antenna for the rod and soldered this to the 12mm spacer. The rod is 60mm long but it could be longer than that if you need it to be. An alternative tip could be made from a length of 2mm diameter brass rod and a 6mm long brass standoff. These parts are then soldered together. The aluminium disk is held in place Capacitor Codes Value 820nF 220nF 100nF 56nF 47nF 18nF 15nF 4.7nF 1nF 270pF 68pF 33pF F Value IEC Code EIA Code 0.82F 820n 824 0.22F 220n 224 0.1F 100n 104 0.056F 56n 563 0.047F 47n 473 0.018F 18n 183 0.015F 15n 153 0.0047F 4n7 472 0.001F 1n 102 270p 271 68p 68 33p 33 siliconchip.com.au using the anulus in the same way as the diaphragm. Testing Testing can be done with the 9V battery connected. Apply power and check that the power LED momentarily lights brightly when switched on and then dims. Wind the VR1 volume control fully anticlockwise and set the tone control to mid position. This will prevent IC2 from producing large signal levels with no input connected. This allows DC voltages to be tested without being masked by a large AC signal. For a 9V battery supply, we measured 8.7V at the cathode of D1, 7.7V at pin 4 of IC1 and 8.7V at pin 6 of IC2 with the multimeter’s negative probe connected to the casing of one of the 3.5mm jack sockets. A half supply voltage of around 4.3V should be at pins 1, 7, 8, and 14 of IC1 and at pin 5 of IC2. Make sure the jumper link (LK1) for the electret microphone bias is not inserted for the piezo element of the chestpiece. Connect the chestpiece and headphones or earpieces to the Stethoscope. Set the volume about mid way and adjust VR3 for a suitable level of volume when monitoring the heart beat on the left side of your chest. Rotate VR3 clockwise for more gain and anticlockwise for less gain. Check that the tone can be adjusted to boost and cut the selected band of frequencies. This will be evident on the low setting as the heart beat thump is boosted or cut. On the high band more hissing sound will be produced on boost but reduced on cut. For an idea of what various body sounds make, log onto www.easyauscultation.com. You can then try and find those sounds using your SILICON CHIP Electronic Stethoscope. Using the stethoscope as an audio eavesdropper The stethoscope can be used to monitor sounds from a distance using an electret microphone mounted in an open ended container instead of the piezo element within the chestpiece. With this setup you can listen to bird or animal calls (or virtually anything else) at a distance. The container provides directional sound response, where sound enters the open ended container to be received by the microphone. siliconchip.com.au Designing your own single-band equaliser While most users will be satisfied with the four frequency bands selected for the equaliser, there may be some who require different bands. To satisfy this, we have included a method to design your own equaliser section. Fig.9 shows the typical bandwidth of an equaliser section under boost. The centre of the band is designated F0 while the frequencies where the response is 3dB down from the F0 level are shown as F1 and F2. To design for a particular frequency band you can use the equation: AMPLITUDE 1.00 –3dB 0.707 0 F1 F0 F2 FREQUENCY Fig.9: typical bandwidth of an equaliser section under boost. 1 L=    42 C1 F02 This is to obtain a value for the inductance L, using selected values for C1 and F0. The equation is just a rearrangement of the standard F0 =    1     2  L C1 Knowing the inductance enables us to calculate the required value for CL. Use the equation     C = L L      R1 R2 For our circuit we used a 1.8kresistance for R1 and 220kresistance for R2. The Q of the circuit determines the two frequencies either side of F0 where the signal drops off in level by 3dB. You can calculate the Q using this equation: Q= 2 F0 L R1 The Q is also found using the equation Q=     F0 F2-F1 although the equations to find F1 and F2 are more difficult. A useful calculator to find F1 and F2 is at: www.sengpielaudio.com/calculator-cutoffFrequencies.htm The tables below show the components used in the stethoscope, the inductance, Q and F1 and F2 for the four bands. You can use these values to practice calculating the values for L and C2. F0 63Hz 250Hz 1kHz 4kHz   Required    F0 63Hz 250Hz 1kHz 4kHz CL 18nF 4.7nF 1nF 270pF R1 1.8k 1.8k 1.8k 1.8k Calculated    F0 65Hz 248Hz 1068Hz 3.974kHz R2 220k 220k 220k 220k C1     820nF 220nF 56nF 15nF Q      F1     F2     L 1.62 1.61 1.49 1.53 48Hz 188Hz 768Hz 2.882kHz 88Hz 337Hz 1485Hz 5.479kHz 7.13H 1.86H 396mH 107mH August 2011  29 into the Electronic Stethoscope and note that for this application (or any other using the electret microphone), the electret bias needs to be selected by inserting LK1. A parabola? For maximum concentration of sound at the microphone, a parabolic dish should be used with the microphone mounted at the focal point. We’ve used a number of different parabolic or near-parabolic shaped dishes in the past. Design of the pickup using a ‘parabolic’ shaped dish and electret microphone is shown in the Ultrasonic Eavesdropper article from August 2006 by Jim Rowe. While that design was to receive ultrasonic sounds and convert them to the normal audio band, the pickup arrangement is the same for the audio sound band. One alternative parabola which we haven’t tried (but should be near perfect!) is a metal cooking wok, available quite cheaply from oriental food suppliers. To find the focal point, shine a single-point light source (eg, a LED) into the wok along its centre line. As you move it in and out, at one point the light will appear to “fill the dish” – that’s the focal point. (See “Ask Silicon Chip” November 1994, page 93). We’ll leave the mechanical arrangement for mounting the microphone up to you. SC To eavesdrop on birds and animals, we made this “sound gun” from an old CD stack pack – but just about any cylinder would do. The idea is to prevent sound entering from the sides. Ideally, for maximum sound pickup the shape should be a parabola with the mic insert at the focus but in practice we found it really doesn’t make a great deal of difference. Sounds coming from the side and rear of the container are reduced in level before reaching the microphone. Our CD pack sound gun Construction is straightforward and is shown the photo and in Fig.8. It’s not at all critical – you can use whatever is available. The closed end of the cylinder is drilled out to accept the IP68 gland. This gland neatly houses the electret while the clamp end secures the shielded cable. Strip the ends of the shielded cable and solder the wires of one end to the microphone insert connection pads. The shielded wires connected to the insert’s case pad and the inner wire to the other pad. The opposite wire end passes through the gland and is clamped down with the electret inserted into the open gland end. The wire is then terminated to a 3.5mm mono jack plug. A handle was fashioned from an off cut of timber (we reshaped the decorative top section of a cyprus pine picket) and secured this to the side of the cylinder with self tapping screws. The shape of the handle is not criti30  Silicon Chip cal so long as it is comfortable to hold. The handle can be finished with olive oil rubbed into the timber before wiping off the excess. The unit is now ready to test. Plug CABLE GLAND OPEN END OF CYLINDER TOWARDS SOUND SOURCE ELECTRET MICROPHONE INSERT SCREWS CYLINDER Fig.8: our “sound gun” fashioned from an old blank CD bulk case and a wooden handle. In this case, the electret microphone is used rather than the piezo – but make sure that LK1 is in place on the PCB to provide bias voltage for the electret. It won’t work otherwise! HANDLE SINGLE CORED SHIELDED CABLE TO 3.5mm JACK PLUG siliconchip.com.au