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By Nicholas Vinen The Bad Vibes Infrasound Snooper Back in March 2013 we published the Infrasound Detector for low frequency measurements. Now you can “listen” to low frequency vibrations with our Infrasound Snooper. It frequency shifts and amplitude modulates a frequency range of about 1Hz to 20Hz by about five or six octaves so that you can listen directly to wind turbines or elephants, crocodiles and other animals that communicate with infrasound. O UR INFRASOUND SNOOPER uses Digital Signal Processing (DSP) techniques in a PIC32MX170 microcontroller, an electret microphone, a DAC chip, a TL074 quad op amp and very little else, to drive a pair of headphones. High levels of infrasound can have a negative impact on your health but you might not even know when you are being exposed to low frequency vibrations unless they excite harmonics by rattling window panes and similar, because they’re otherwise inaudible. In January this year, a study by acous36  Silicon Chip tics expert Steven Cooper of Bridgewater Acoustics attracted a great deal of controversy over its findings which support the notion that infrasound from wind turbines can cause negative health impacts on people some distance away. At the SILICON CHIP offices, some of our staff recently experienced ill effects, including headaches and nausea, when a ground compacting machine was operating on a nearby building site. It evidently set up all sorts of standing waves in our building, as it moved around the construction site. Some “nodes” in our building were quite unpleasant places to be. So if you are living or working near potential sources of infrasound and are suffering from some of the potential symptoms, our Infrasound Snooper can certainly help. Our Infrasound Detector (SILICON CHIP, March 2013) allows you to measure the amplitude and frequency of infrasonic sound waves but the results can be somewhat difficult to interpret since you cannot hear the phenomenon. The Infrasound Snooper lets you assess the amplitude and frequency of siliconchip.com.au Scope1: amplified infrasound output from IC2c (green) and the modulated signal to the headphones (yellow) for a lowfrequency impulse of about 10Hz. The mode is AM+FM with low-frequency boost and you can see the output freq­ uency shifting for the positive/negative infrasound signal as well as the delay from the low-frequency boosting filter. Scope2: a similar impulse at a longer timebase than Scope1 (50ms/div rather than 20ms/div). The mode is AM+FM without low-frequency boost and thus the output waveform modulation corresponds very closely to the green input signal excursions. As before, positive excursions produce higher modulated frequencies than negative excursions. the waves but importantly, you can also hear the details – whether they are short, repetitive bursts, continuous waves or somewhere in between. Our Infrasound Snooper is housed in a small plastic box and uses a doublesided PCB (code 04104151) measuring 104 x 60.5mm. An electret microphone is mounted at one end of the case and a rotary switch on the lid offers a number of different listening modes. Circuit description Fig.1 shows the circuit details. Infrasonic sound waves are sensed with the electret microphone (MIC1) or an external microphone plugged into CON4. A 6.8kΩ pull-up resistor from the 5V regulated rail provides the electret’s operating current. The electret signal is AC-coupled via a 1µF capacitor to the non-inverting input of op amp IC2b, one section of a TL074 quad JFET-input op amp. In conjunction with the 1MΩ resistor, this capacitor forms a low pass filter with a -3dB corner frequency at 0.2Hz. Thus signals above 0.5Hz pass through with little or no attenuation. The 5V rail is used as a convenient DC bias point, to bring the signal within IC2’s supply rails, ie, roughly half-way between 0V and VCC which is typically 8.7V. IC2b operates as a simple buffer, feeding the following third-order active low-pass filter based around op amp stage IC2a which has a gain of two siliconchip.com.au Features & Specifications •  Converts infrasonic sound waves into audible waves via frequency shift modulation •  Minimal delay between detection of infrasound and audible response; essentially real-time •  Output volume proportional to infrasonic wave amplitude •  Output pitch deviation indicates infrasonic wave polarity •  Optional digital filter to compensate for typical low-frequency microphone roll-off •  Quick response time allows listener to determine nature of infrasound (pulsed, continuous, etc) as well as frequency and amplitude •  Operating input frequency range: approximately 1-20Hz •  Power supply: 9V battery, ~60mA current drain (9-15V DC plugpack can also be used) •  Five modes: AM+FM with or without microphone response compensation, AM only with or without microphone response compensation, FM only (fixed amplitude) June 2015  37 CON1 D1 1N400 4 22Ω 6-12V DC/AC POWER A 1 ON/OFF 2 3 S1b K 4 D2 1N 5819 A + Vcc K 6 100k 5 9V BATTERY Vcc +5V +5V VR2 10k 22k +3.3V IC2: TL074 6.8k 1M 470Ω 100nF 470nF 1 µF 5 4 IC2b 6 7 2.2M 470nF 22k 22k 2 22k INPUT 22pF 9 6.2k 3 IC2a 1 10 IC2c 47k 1 µF 8 6.8k 11 MODE 1 SELECT 2 3 2.2M 470nF + MIC1 CON4 4 S1a ELECTRET 6 5 SWITCH S1 SETTINGS 1: 2: 3: 4: 5: 6: OFF AM+FM+BOOST AM+FM AM+BOOST AM FM CON3 ICSP 10k 1 2 3 4 5 SC 20 1 5 INFRASOUND SNOOPER Fig.1: the complete circuit diagram. The infrasound is picked up by an electret microphone & then buffered, filtered & amplified by IC2b-IC2a before being fed to microcontroller IC1. IC1 digitises the signal & carries out the necessary signal processing before feeding it to DAC IC3. IC3 then feeds gain stage IC2d which in turn drives the output socket (CON5). (set by the pair of 22kΩ resistors at its pin 2). The filter is a Butterworth type which is pretty much flat from DC up to 20Hz, with gain rapidly falling off at higher frequencies. This is important since we need to apply a fair bit of gain to the infrasonic signals to scale them to an appropriate level for the microcontroller’s ADC (~1V RMS). Op amp IC2c provides the requisite gain which is variable using VR2. So the gain ranges from a minimum of 6x (47kΩ ÷ (10kΩ + 470Ω) + 1) to a maximum of around 100x (47kΩ ÷ (470Ω + W) + 1, where W is VR2’s wiper resistance). Thus VR2 acts as the unit’s sensitivity adjustment. 38  Silicon Chip The signal must then have its DC bias shifted to suit the PIC32MX microcontroller’s ADC, which runs from a 3.3V regulated rail. Thus it is AC-coupled with a 1µF capacitor and biased with a pair of 2.2MΩ resistors forming a voltage divider between the 3.3V rail and ground. This sets the DC level at pin 2 of IC1 at around 1.65V. The 6.8kΩ resistor protects IC1 from high voltages from IC2 during powerup, power-down and high signal excursions. IC1 digitises the signal and then applies some DSP-based filtering to correct for low-frequency roll-off due to the two coupling stages and the microphone’s response. To create an audible signal, the infrasound signal is rectified and then used to amplitude modulate a sinewave at about 200Hz. Some frequency modulation is normally also applied to this waveform, based on the pre-rectification signal. This allows the polarity of infrasonic excursions to be distinguished, based on the difference in resulting signal frequency. This modulated signal appears in digital (I2S) format at pins 5, 7 & 25 of IC1. The serial audio data is produced at pin 5 (RB1) which is mapped to one of the two internal SPI peripherals so that the data stream is uninterrupted. siliconchip.com.au REG1 78L05 Vcc +5V OUT IN REG2 MCP1700-3.3/TO GND GND IN 100 µF 100nF 16V 25V 78L05 GND 100 µF 220 µF +3.3V OUT IN OUT 16V 33k MC P1700 IN GND OUT +5V +3.3V LEDS 10Ω 470Ω MMC MMC 13 3 2 VDD AVDD RA1/AN1/VREF– SOSCO/RA4 RA0 /AN 0 /VREF+ PGED1/AN2/RB 0 10 9 6 AN 10 /RB1 4 SOSCI/RB4 AN11/RB13 RA3/CLKO AN12/RB12 RA2/CLKI PGEC2/RB11 RB2/AN4 IC1 PIC32MX170PIC3 2 MX170F256B PGED2/RB10 TD0/RB9 TCK/RB8 TDI/RB7 1 14 15 AN5/RB3 12 LOW BATTERY/ OPERATE PGEC1/AN3/RB1 VCAP PGEC3/RB6 λ LED2 CLIP K D1, D2 A 4 K 26 25 24 100nF 23 MMC 22 21 18 1 17 16 2 7 3 MCLR PGED3/RB5 K A A λ LED1 K 28 AN9/RB15 11 A 100nF 100nF 1k BitCLK W Sel DATA 5 Vdd IC3 TDA1 5 43 GND 4 AoutR VrefO AoutL 8 7 6 12 13 5 IC2d 100 µF 16V 680Ω 4.7nF 20 14 VR3 1k VSS 19 VSS 8 This data is clocked by a signal from pin 25 (RB14/SCK1). The left/right “word” clock is produced at pin 7 (RB3), also by the SPI peripheral, using its audio framing feature. These three signals pass to IC3, a TDA1543 16-bit oversampling DAC. We’ve used this chip for a number of reasons: it’s available in an 8-pin DIL package which is easy to solder; it runs from a single 5V rail; it’s quite cheap; it’s easy to interface to and its audio performance is respectable. Its outputs at pins 6 & 8 are current sink stages and since we only need a mono signal, they are simply connected together, filtered (to remove siliconchip.com.au OUTPUT 2 1 5 CON5 10 µF AVSS 27 4 3 6.3V TANT. OR SMD CERAMIC the digital aliasing artefacts) and converted to a voltage by remaining op amp IC2d. The 680Ω resistor sets the output voltage swing. IC2d’s pin 12 non-inverting input is connected to the 2.2V reference voltage which sets the DC level of the resulting signal. The Vref (pin 7) of IC3 has a dual purpose; the current drawn from this pin is internally amplified and added to the current sink by the left & right output pins. However, in this case, the circuit works best with no extra current sink, hence there is no load on the Vref pin. The DC in the output of IC2d is blocked by a 100µF electrolytic capaci- tor and biased to ground by the track of VR3, the volume potentiometer. The headphones or earphones are connected to its wiper via CON5 with no extra buffering. This is a relatively crude system but it works well enough. The main purpose is to allow the user to reduce the output to a comfortable level when used in conjunction with sensitive earphones. Power supply The Infrasound Snooper is designed to run off a 9V battery but a 9-15V DC plugpack could also be used. The supply current therefore flows through one of two reverse-polarity protection June 2015  39 1 µF CON4 S MIC1 VR2 10k Clip 1k LED1 LED2 A Batt VR3 1k INPUT + 10 µF PIC32MX170F256B IC1 1 100nF ICSP 2.2M 2.2M + 10Ω 33k 6.8k 22Ω 100 µF 470Ω 5819 D1 REG2 + 100nF CON3 100nF REG1 470Ω 1M 4004 Power/Mode A T 10k D2 220 µF + 47k 100 µF + 6.8k R S1 + 100nF Snooper IC3 100nF 22pF 470nF 9V 0V 100k C 2015 TDA1543 4.7nF 680Ω 22k 22k 22k 6.2k 22k + Infrasonic CON1 1 µF IC2 TL074 470nF ELECTRET MIC INSERT 470nF 04104151 9V BATTERY R 100 µF CON5 S T OUTPUT (BLUE OUTLINES REPRESENT COMPONENTS NOT USED IN THIS PROJECT) Fig.2: follow this parts layout diagram to build the PCB. Take care to ensure that all polarised parts are correctly orientated and use a socket for microcontroller IC1. Sockets are optional for IC2 & IC3. diodes, D1 for the plugpack or D2 for the battery. D2 is a Schottky diode, to minimise voltage drop and therefore extend battery life. Rotary switch S1 acts as both the power and mode switch. One pole connects the power supply directly to IC2 as well as to the input of REG1. This regulator provides the 5V rail for DAC IC3, the electret supply and for signal biasing in the input filter. It also feeds REG2, a 3.3V low-dropout regulator which powers microcontroller IC1. The other pole of S1 is connected to pins 6, 9, 10 & 11 of IC1 which are configured as inputs with internal pull-up currents enabled. Thus IC1 can sense which position S1 is in by determining which of these inputs is pulled low. If none are then the switch must be in the second position, as the circuit is not powered in the first position. IC1 monitors the battery voltage via a 4:1 divider (100kΩ/33kΩ), digitising the resulting voltage at its AN1 analog input (pin 3). If the battery voltage is low (<7V), it illuminates the low-battery LED (LED2) via its pin 12 output (RA4). The 470Ω currentlimiting resistor sets the LED current to around 2-3mA. Similarly, IC1 can light LED1 if there is an input signal overload, using its pin 4 output. The red LED is a little more efficient so operates at a lower current, with a 1kΩ current-limiting resistor resulting in around 1-1.5mA flowing. 40  Silicon Chip CON3 is a programming header for IC1 (if required) with a 10kΩ pullup resistor on its MCLR pin (pin 1) preventing unexpected reset events. IC1’s analog supply at pin 28 is lowpass filtered with a 10Ω resistor and 100nF bypass capacitor, while a 10µF capacitor at pin 20 is required for its internal core regulator. Construction All the parts except for the electret microphone are mounted on a doublesided PCB coded 04104151 (104 x 60.5mm). This can be clipped into a standard UB3 jiffy box. Fig.2 shows the parts layout on the PCB. Start by fitting the fixed resistors. Table 1 shows the resistor colour codes, although it’s better to check the values using a DMM. Note that since the same PCB was used for the Low Frequency Distortion Analyser, there are a number of component positions which are not populated (including some resistor locations). Diodes D1 & D2 can go in next, noting that D1 is a 1N4004 while D2 is a 1N5819. Be sure to orientate them correctly, with their striped cathode ends towards the bottom edge of the PCB. Follow with the IC socket(s). It’s a good idea to use a socket for microcontroller IC1 but they are not really necessary for IC2 & IC3. Instead, IC2 & IC3 can be soldered directly to the PCB for greater long-term reliability. Either way, make sure that the pin 1 notch/ dot of each IC goes towards the top of the board. This is especially critical if soldering the ICs in without sockets since you can’t easily remove them once they’re in! The two jack sockets are next on the list, followed by the ceramic and MKT capacitors. REG1 & REG2 can then go in but be careful not to get them mixed up as they look similar. Their leads will need to be cranked out using needle nose pliers to suit the pad spacing on the PCB. Now solder the DC socket in place, followed by the electrolytic capacitors. Be sure to orientate the electros correctly, with the longer (positive) leads towards the top edge of the PCB (see Fig.2). If using a tantalum type rather than an SMD ceramic for the 10µF capacitor, it too is polarised and can go in now. Now fit the two 9mm potentiometers. They’re different values so don’t get them mixed up (the 1kΩ pot may be marked “102” and the 10kΩ pot “103”). The polarised 3-pin header (for the microphone) can then be fitted with its keyway tab orientated as shown. The battery snap is next. Pass its leads through the two strain relief holes before soldering its leads to their respective pads on the top of the PCB as shown in Fig.2. You can then pull the leads back through the holes to reduce the slack. Note that they will probably be a tight fit, to provide the siliconchip.com.au   Table 2: Capacitor Codes Value 1µF 470nF 100nF 4.7nF 22pF IEC Code EIA Code   1u0  105   470n   474   100n   104   4n7  472   22p   22 of the cable, then carefully solder and crimp the leads at one end to the header crimp pins. That done, the crimp pins can be slid into the header (the tang goes into the narrow channel) until they lock into position. The next step is to determine which lead on the electret microphone is the positive and which is the negative. This may be marked but if not, use your DMM (set to ohms) to determine which lead is connected to the case – this is the negative (ground) lead. Next, slip 5mm-lengths of 3mmdiameter heatshrink over the insulation at the end of the cable leads, then solder these two leads to the microphone. Make sure that the positive lead from the header goes to the electret positive (the positive side is marked on the PCB, adjacent to CON4). Once the two leads have been soldered, slip the heatshrink sleeves over the solder connections and shrink them down to provide strain relief (see photo). This view shows the completed PCB assembly. Note how the battery snap leads are looped through strain relief holes before being soldered to the top of the PCB. necessary strain relief. The two 3.5mm switched jack sockets (CON4 & CON5) can now be mounted. Check that they sit flush against the PCB before soldering their pins. CON3, the ICSP header, can then go in but can be omitted if you’re using a pre-programmed microcontroller. Rotary switch S1 is mounted after first cutting its shaft so that it’s 30mm long, as measured from the top surface of the main body. This can be done using a hacksaw and the end of the shaft then cleaned up with a file to remove any burrs. It must be installed with its polarity-indicating plastic post orientated as shown on Fig.2 (ie, at the three o’clock position). Again, make µF Value 1µF 0.47µF 0.1µF .0047µF  NA sure it’s pushed down flat against the board before soldering its pins. Finally, solder the two LEDs in place. The longer leads are the anodes and go into the pads indicated with “A” on Fig.2. Tack solder these in place at full lead length; you can adjust the height and solder them properly once the box has been prepared. Microphone cable The next job is to make up a cable to connect the microphone. That’s done using a 70mm length of light-duty Fig.8 cable which is terminated at one end in a 2-way polarised header. Begin by removing about 3mm of insulation from the leads at each end Testing If using sockets, plug in the ICs, with their pin 1 dot or notch aligned as shown in Fig.2. If IC1 hasn’t already been programmed (you can buy a pro- Table 1: Resistor Colour Codes   o o o o o o o o o o o o o o o siliconchip.com.au No.   2   1   1   1   1   4   1   2   1   1   1   2   1   1 Value 2.2MΩ 1MΩ 100kΩ 47kΩ 33kΩ 22kΩ 10kΩ 6.8kΩ 6.2kΩ 1kΩ 680Ω 470Ω 22Ω 10Ω 4-Band Code (1%) red red green brown brown black green brown brown black yellow brown yellow violet orange brown orange orange orange brown red red orange brown brown black orange brown blue grey red brown blue red red brown brown black red brown blue grey brown brown yellow violet brown brown red red black brown brown black black brown 5-Band Code (1%) red red black yellow brown brown black black yellow brown brown black black orange brown yellow violet black red brown orange orange black red brown red red black red brown brown black black red brown blue grey black brown brown blue red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown red red black gold brown brown black black gold brown June 2015  41 (UB3 BOX LID) A CL 25.75 5 5 B B CL HOLE SIZES: HOLE A 6.5mm DIAM, HOLES B 3.0mm DIAM HOLES C 6.0mm DIAM, HOLES D 8.0mm DIAM 19 22 D 22 D C C 24 13.5 13.5 24 (FRONT SIDE OF UB3 BOX) ALL DIMENSIONS IN MILLIMETRES SILICON CHIP AM+Boost AM+FM AM+FM+Boost AM FM Off BAD VIBES Infrasound Snooper Overload Ext. Mic Gain On/Low Battery Vol. Output Fig.4: this front panel artwork can be copied and used direct or a PDF version can be downloaded from the SILICON CHIP website & printed onto photo paper or onto Datapol/Dataflex label paper. grammed micro from the SILICON CHIP Online Shop), do it now via CON3. External power can be supplied from the programmer (eg, a PICkit 3). Once all the ICs are in place, you can test the unit as follows: (1) Rotate S1 to the off position (fully anti-clockwise), then connect the battery. 42  Silicon Chip (2) Rotate S1 one step clockwise and check that the yellow LED flashes briefly, then periodically. (3) Turn VR2 & VR3 all the way down and connect a pair of headphones or earphones to the unit. (4) Turn VR2 & VR3 up slowly and blow on the microphone insert. After turning the pots up sufficiently, you Fig.3 (left): use this full-size template to drill the holes in the lid and front side of the UB3 case. should hear the modulated signal from the low frequency components of this sound. With the gain up high, if you blow hard enough, the overload (red) LED may light. (5) Switch S1 to the other positions and check that the sound produced by the unit changes. (6) Switch the unit off and remove the battery. If it doesn’t work as expected, carefully inspect the solder joints under magnification. Also check that the components are all in their correct positions and that the polarised parts (diodes, ICs, electrolytic capacitors etc) are orientated correctly. Case preparation If fitting the PCB into a UB3 jiffy box, you will need to drill four holes in the side of the case for the microphone input and headphone output sockets, plus the gain and volume adjustment knobs. The bottom section of Fig.3 shows the relevant drilling template – this can be copied (or downloaded from the SILICON CHIP website and printed out) and temporarily stuck to the side of the case (eg, using doublesided tape). Note that the top edge of the template must be aligned with the top edge of the box and centred horizontally. The holes must be accurately placed. siliconchip.com.au If you want to be able to run the unit from a plugpack, you will also need to drill a 5.5mm hole in the other side, to allow access to the connector. The same template can be used; simply drill the hole for the power jack centred on the same location as that used for the volume control pot on the opposite side. If in doubt, check the location of the power socket on the board before drilling. Fitting the microphone Above: the PCB is a snapfit inside the case, while the battery sits on a piece of non-conductive foam (see text). Start by drilling pilot holes (eg, 3mm) in each location and then enlarge them using larger drill bits, a stepped drill bit or a tapered reamer. Clean up any burrs, then remove the nuts from the two jack connectors, screw the nuts and washers all the way onto the potentiometers and check that the connectors and pots fit through the holes. A hole also has to drilled in the lefthand end of the case for the electret microphone. The hole should be positioned about 16mm down from the top of the case and centred horizontally. Start by drilling a small pilot hole, then carefully ream the hole out until the microphone is a tight fit. Once the mic fits, adjust it so that its face is flush with the outside of the case. It can then be secured inside the case using a small amount of neutral-cure silicone adhesive and the assembly placed aside to cure while the case lid is drilled. Front panel drilling Three holes are required in the case lid, for the two LEDs and switch S1. The drilling template is at the top of Fig.3 and it’s just a matter of drilling the holes to size and checking that the LEDs and switch shaft fit. Parts List 1 double-sided PCB, coded 04104151, 104 x 60.5mm 1 UB3 jiffy box (optional) 1 10kΩ 9mm single-gang potentiometer (VR2) 1 1kΩ 9mm single-gang potentiometer (VR3) 1 28-pin narrow DIL IC socket 1 14-pin DIL IC socket (optional) 1 8-pin DIL IC socket (optional) 1 piece non-conductive foam, approximately 65 x 40 x 8mm 1 PCB-mount DC socket (CON1) 2 3.5mm switched jack sockets (CON4,CON5) 1 2-pole 6-position rotary switch (S1) 1 medium-sized knob, to suit S1 2 small knobs, to suit VR2 & VR3 1 9V battery snap (BAT1) 1 9V alkaline battery (BAT1) 1 pair headphones or earphones siliconchip.com.au 1 5-pin header, 2.54mm pitch (CON3) (optional – see text) 1 PCB-mount electret microphone insert (Jaycar AM4011) 1 3-pin polarised header, 2.54mm pitch (CON6) 1 3-way polarised header plug 1 70mm-length light duty figure-8 cable 1 10mm length 3mm-diameter heatshrink Semiconductors 1 PIC32MX170F256B-I/SP 32-bit microcontroller programmed with 0420415A.HEX (IC1) 1 TL074 quad JFET-input op amp (IC2) 1 TDA1543 oversampling DAC (IC3) 1 78L05 5V regulator (REG1) 1 MCP1700-3.3/TO 250mA 3.3V LDO regulator (REG2) The next step is to make and attach the panel label (Fig.4). This can be copied or downloaded and printed onto photo paper and affixed to the panel using silicone adhesive. Alternatively it can be printed onto Datapol/Data­flex label paper and stuck onto the lid. The three holes are then cut out using a sharp hobby knife. Final assembly Assuming that the silicone around the microphone has cured, the PCB can now be installed in the case. It’s just a matter of angling the front of the board down so that the sockets and pot shafts go into their respective holes, then pushing down on the back of the board until it snaps into the integral side-rails. If it won’t go in, you may need to enlarge the holes slightly. Now trial fit the lid. If the LED heights are wrong, you will need to remove the PCB and adjust them accordingly. Once they fit properly, re-solder their leads and re-install the board in the case. The potentiometer nuts can then be wound forwards until they’re against the inside face of the case. Next, rotate S1 to off (fully anticlockwise), then connect the battery and place it on top of the PCB with a piece of non-conductive foam sandwiched in between. This will prevent shorts and also stop the battery from rattling around inside the case. Finally, screw the lid in place, then 1 red 3mm LED (LED1) 1 yellow/orange 3mm LED (LED2) 1 1N4004 1A diode (D1) 1 1N5819 1A Schottky diode (D2) Capacitors 1 220µF 25V electrolytic 3 100µF 16V electrolytic 1 10µF 6V tantalum or SMD ceramic (1210/1206/0805) 2 1µF 50V multi-layer ceramic 3 470nF 63V/100V MKT 5 100nF 50V multi-layer ceramic 1 4.7nF 63V/100V MKT 1 22pF disc ceramic Resistors (0.25W, 1%) 2 2.2MΩ 2 6.8kΩ 1 1MΩ 1 6.2kΩ 1 100kΩ 1 1kΩ 1 47kΩ 1 680Ω 1 33kΩ 2 470Ω 4 22kΩ 1 22Ω 1 10kΩ 1 10Ω June 2015  43 Scope3: amplitude modulation-only mode. The output freq­uency is fixed at around 185Hz and only its amplitude varies, increasing for either polarity of infra­sound pressure wave excursion. Each waveform shown here is at maximum sensitivity and this is how the unit should be used unless it is overloading due to intense infrasound. attach the knobs for S1, VR2 & VR3 (the jack socket nuts aren’t required). Using it Typically, you would use the device with the gain somewhere near maximum and the volume adjusted to a level which is not excessive for the headphones or earphones being used. Due to the way the volume control works, this is likely to be somewhere near maximum too, although lower settings may be necessary for “in-ear” ear-phones. Sealed headphones or in-ear phones have the advantage that you can more easily determine the level of infrasound emitted from sources which also produce audible frequencies. Scope4: frequency modulation mode. The signal amplitude is constant and high (generally the output volume should be turned down in this mode) and only the frequency changes in response to infrasonic waves picked up by the microphone. As before, the frequency increases for one polarity of wave and decreases for the other. That’s because they will do a better job of blocking those audible frequencies out and allow you to more clearly hear the output of the device. An example of this would be a door slamming shut. This can generate quite a significant infrasonic pulse but it may be difficult to hear the unit’s response against the audible noise of the door slamming. Other sources which can be used to test the unit include large air-conditioning units, passing trucks and large idling engines. When using one of the frequency modulation (FM) modes, it is possible to determine the polarity of an infrasonic pulse. One polarity will produce a sound which increases in frequency while the other will produce a sound that decreases in frequency. Regular pulses from the same source will normally have a consistent polarity. Typically, a compression wave will precede an expansion wave. Finally, note that a wind shield may be necessary for the microphone if the unit is used outdoors. As with the March 2013 Infrasound Detector, the windshield from a dynamic microphone could be used. Alternatively, a separate external electret microphone (plugged into CON4) could be used instead of the inbuilt electret. Just make sure is has the required high sensitivity, a good low-frequency response and is able to operate from the ~0.5mA bias current SC supplied by the unit. Are Your S ILICON C HIP Issues Getting Dog-Eared? $16.9 REAL VALUE AT Are your SILICON CHIP copies getting damaged or dogeared just lying around in a cupboard or on a shelf? 5* PLUS P &P Keep them safe, secure and always available with these handy binders Order now from www.siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for overseas prices. 44  Silicon Chip siliconchip.com.au