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The MiniHEART Miniat re Heartbeat Sim lator Give a favourite soft toy a beating heart! With both soft sound and a real beat, it could relax a baby, puppy or kitten for sleeping, or even help you sleep better yourself. All are possible with the MiniHeart! M any newborns – human babies as well as pets – are unsettled when left alone to sleep. They miss their mum, and it’s lonely and frightening for them. Just being able to cuddle up to the sound of a heartbeat can help with their anxiety. The MiniHeart is a small gizmo that produces a low-level soothing heartbeat sound, mimicking that of a real heart. The beat rate can be adjusted so that it more accurately matches the rate of the heart it is to emulate, while a timer will shut off the heartbeat after a set time. The unit is switched on and off with a toggle switch with the actuating lever only protruding slightly outside the box. This is to prevent any injury to a baby. It is fully enclosed into a plastic case that clips together, and we have added extra screw supports to make sure it stays shut. That way, the two internal AAA cells will not be easily accessed to cause a choking hazard. We recommend enclosing the device into a cloth bag that is sewn or zippered shut. That provides an extra margin of choke hazard safety which is necessary when used with a baby. We should point out that the simulated heartbeat is not a loud sound – it is not meant to be. By J ohn Clarke 24 It is more like the subtle sound of a real beating heart; it needs to be placed close to the ear, and is felt more than heard. Think of it as a tiny heart, but in a rounded rectangular prism shape. A loud heartbeat sound would require a large loudspeaker, properly baffled to produce bass, along with an amplifier with a reasonable amount of power. Neither of these are a feature of our MiniHeart simulator (but could be added externally). Heart sounds When listening to a heartbeat, you will hear two distinct, separate sounds, often called a ‘lub’ and a ‘dub’. These two sounds are produced by the closing of heart valves required to pump blood efficiently. You’ve almost certainly seen the classic heartbeat waveform shown on an electrocardiogram (ECG). These are the electrical signals sent to the heart muscles, and when monitored with electrodes on the skin, are useful for diagnosing heart problems. Electrode readings do not represent the sounds and vibrations made by the heart; heartbeat sounds are heard using a stethoscope. The MiniHeart block diagram is shown in Fig.1. Microcontroller IC1 produces a heartbeat waveform in the form of a pulse-width modulated (PWM) signal. The pulse rate is 31.25kHz, and the pulse width is varied to produce a smoothed lower-frequency waveform after passing through a low-pass filter. This removes the high-frequency signals so that only the heartbeat waveform remains. Fig.2 shows how a PWM signal is used to produce a lower-frequency, smooth waveform. The red waveform is the PWM output from the microcontroller, IC1, while the green waveform is its average value after filtering out the PWM pulse frequency. For convenience, we show a sinewave, although any wave shape could be generated. If the PWM signal has a 50% duty cycle, ie, an equal period of being high and low, then the filtered voltage will sit mid-way between the high and low voltage levels. To produce a higher voltage, the PWM signal duty cycle is altered so that the period while high is longer than the period when low (ie, duty cycle > 50%). Conversely, for a lower voltage, the PWM period is kept low for longer than it is high (duty cycle <50%). The green wave shows the signal that appears after the low-pass filter has removed all of the higher frequencies. Note that this PWM signal is a representation only – in Practical Electronics | January | 2022 FEATURES AND SPECIFICATIONS • • • • • • • Fig.1: this block diagram shows that the MiniHeart is quite simple, using just a microcontroller and a Class-D amplifier chip to produce the sound. A basic RC low-pass filter turns the PWM output of the micro into an analogue signal for the amp, while ferrite beads and capacitors reduce EMI from the Class-D drive to the speaker reality, the frequency of the PWM signal is very much higher (around 700 times higher!) than the sinewave shown and cannot be reproduced to scale on the diagram. Overleaf, we show the various scope waveforms for the MiniHeart. Scope1-Scope3 show the general operation. Scope1 shows a few periods of the PWM signal at around 31kHz (25µs timebase). Scope2 and Scope3 (10ms timebase) are the ‘lub’ and ‘dub’ signals produced after filtering the PWM signal. Scope4 shows a single heartbeat with both the ‘lub’ and ‘dub’ waveforms, while Scope5 shows two heartbeats, with the pause between each heartbeat visible. The period between each heartbeat, the frequency of the ‘lub’ and ‘dub’ waveforms and the period between the ‘lub’ and ‘dub’ waveforms have a small amount of randomness added. This is to prevent the heartbeat from sounding too artificial. It simulates the variation in heartbeat rate and timing of a real heart. These waveforms are fed to a tiny Class-D (ie, switching) amplifier that’s usually used in mobile phones and it is designed to be highly efficient. It drives the small loudspeaker in bridge mode, to maximise the power output from the limited 3V DC supply. The loudspeaker is weighted, ie, the speaker cone has a weight attached to it. This is so that low-frequency vibrations will be heard and felt. Circuit details The full circuit is shown in Fig.3. At its heart (!) is a PIC12F617 microcontroller, IC1. Its master clear (MCLR) input, pin 4, is tied to the 3V supply rail via a 10kΩ resistor to provide a power-up reset function. IC1 applies 3V across adjustment trimpot VR1 via its GP5 digital output; this is only brought high when the trimpot position is monitored via IC1’s AN3 analogue input (pin 3). After the GP5 output is brought high, to 3V, the voltage at AN3 is converted to a digital value via IC1’s internal analogue-to-digital converter (ADC). Once the value is read, the GP5 output goes low again (0V) to conserve power. Jumper link JP1 can be placed in one of two positions; position 1 where GP1 is pulled to 0V, or position 2 where GP1 is pulled to the 3V supply. When in position 1, trimpot VR1 adjusts the heartbeat rate. When in position 2, VR1 adjusts the timeout period. The heartbeat rate can be set from 42 to 114 beats per minute (BPM). The timeout can be set between two minutes and four hours. The heartbeat rate can be adjusted while the heartbeat is being generated, but the timeout is only checked at powerup. So after charging timeout value via VR1, power must be switched off and on again for the new timeout to take effect. The heartbeat generation switches off after the set timeout period. This conserves power in case it is left switched on. Note that if JP1 is removed then the pin 6 GP1 input is not held high or low. The voltage can float at a voltage Practical Electronics | January | 2022 • • • • • • Compact size Adjustable volume Adjustable timeout and heart rate Flashing LED synchronised with the heartbeat On/off power switch Power: two AAA cells (nominally 3V), operating down to below 2.5V Current draw: 10mA average during operation, 500nA standby (typical) Timeout: adjustable from two minutes to four hours Heartbeat rate: 42 to 114 BPM Rate randomness: about 15% variation Sound frequency: 45Hz-51Hz (with a 2Hz randomness) Waveform generation method: PWM <at> 31.25kHz Waveform sampling rate: approximately 1kHz anywhere between 0V and 3V. This can lead to high current consumption in IC1, reducing cell life, as digital inputs are supposed to be in one state or the other. So IC1 checks for this condition by changing GP1 to an output and setting it to a high level for 1ms. The 1kΩ resistor charges the 100nF capacitor to 3V. Then GP1 is changed to an input, and the level is checked. If the input voltage remains high, then there is either a jumper in position 2 pulling the input high, or there is no jumper, and the input is held high via the charged 100nF capacitor. This test is repeated with a low output. If the level changed, then JP1 is inserted. To prevent the floating input condition, GP1 is changed to a low (0V) output and left like that, minimising power consumption. Heartbeat generation IC1 uses its internal 8MHz oscillator to generate the 31.25kHz PWM signal at output pin 5. This is fed to a twostage RC low-pass filter. The first stage comprises a 10kΩ resistor and 100nF capacitor to give a −3dB roll-off at 159Hz. The second stage has the same roll-off frequency but uses a 100kΩ resistor with a 10nF capacitor. These components give an impedance which is 10 times that of the first stage filter, minimising the loading on the first stage due to the second stage. The filtered signal is fed to volume control trimpot VR2 and then to the non-inverting input, pin 3, of amplifier IC2 via a 1µF capacitor and 27kΩ resistor. IC2 is a TPA2005D1 Class-D (ie, switching) amplifier in a tiny SMD package, measuring only 3 × 5mm. It is specifically designed for use in mobile phones where its high efficiency is crucial. The block diagram of the TPA2005D1 is shown in Fig.4. RED WAVEFORM = PWM (PULSE WIDTH MODULATION) SIGNAL GREEN WAVEFORM = SYNTHESISED SINEWAVE (AFTER LOW-PASS FILTERING) Fig.2: this shows how a high-frequency pulse-widthmodulated ‘square wave’ can be fed through a low-pass filter to produce a smoothly varying, lower-frequency arbitrary waveform (shown in green). The instantaneous voltage of the green waveform equals the average voltage of the red waveform. In reality, the pulse frequency would be much higher in comparison to the reconstructed waveform. 25               SC MINIHEART MiniHeart Heartbeat Simulator HEARTBEAT SIMULATOR   Fig.3: the full MiniHeart Simulator circuit is not much more complicated than the block diagram. Here you can see the detail of the second-order low-pass filter, the AC-coupling capacitors to the inputs of IC2 and the series resistors which set its gain. LED1 responds to the average voltage delivered to the speaker, so it starts to light once sound is being produced. (FB1 and FB2) plus 1nF shunting capacitors to reduce electromagnetic interference (EMI). It has differential inputs to an internal amplifier that drives the PWM section at a switching frequency of 250kHz, set by the internal oscillator. The PWM section then feeds an H-bridge circuit for driving an external loudspeaker. The data sheet for the TPA2005 highlights two interesting points. The first is its high CMRR (common-mode rejection ratio) which supposedly eliminates the need for input coupling capacitors. But this high CMRR only applies if the amplifier is used in balanced mode, with both inputs at the same DC level. In our circuit, we are using it in unbalanced mode, with the inverting input grounded (via the 1µF capacitor), so we need to use two input capacitors. The 27kΩ resistor for the non-inverting input, in conjunction with the internal 150kΩ feedback resistor, sets amplifier gain at about 5.5 times. Since the amplifier is a bridge type, the overall gain is double that, ie, 11 times. The second interesting point is that the TPA2005 can run without an output filter that would usually be required to remove the 250kHz switching signal. That is, provided the output leads are kept short. Even so, we use ferrite beads Power supply Power is from two series AAA cells to provide a nominal 3V supply, switched on or off by power switch S1. A 100µF capacitor bypasses the switched supply with a 1µF ceramic capacitor close to IC2’s supply rails, and a 100nF capacitor at IC1’s supply rails. Diode D1 is included to protect against component damage if the cells are inserted with reversed polarity. In that case, the diode will conduct and limit the negative voltage to the circuit. The disadvantage is that this will quickly drain the cells, but presumably, you would notice that the device is not working and fix it straight away. The alternative protection method, with a diode in series with the supply, drops too much voltage for this application. Even a Schottky type, with its lower forward voltage, would not be suitable and we can’t justify the cost of a MOSFET in this role (which would have a lower voltage drop again). Scope1: this shows just over seven periods of the ~32kHz PWM signal that is produced at pin 5 of IC1. The signal swing is 3V peak-to-peak, and the timebase is 25µs. Scope2: this ‘lub’ signal reproduces a a real heartbeat sound, produced by filtering the PWM waveform, measured at the wiper of VR2. Note the longer timebase used here (10ms/div). 26 Practical Electronics | January | 2022 Indication LED1 lights simultaneously with the lub/ dub sounds and is driven via the VOoutput of IC2. With no signal, this output sits at an average of 1.5V. This is derived by an RC low-pass filter (2.2kΩ/100nF) from the 250kHz square wave signal at pin 8 of IC2. It swings between 0V and 3V with a 50% duty cycle when idle. The LED lights when this voltage rises above the usual LED forward voltage of around 1.8V, and that happens when the duty cycle of the pin 8 output increases above 60%. VDD INTERNAL OSCILLATOR + IN – VO+ – DIFFERENTIAL INPUT PWM SHUTDOWN H-BRIDGE VO– + – TO BATTERY IN + BIAS CIRCUITRY GND Saving power Since the device is powered from AAA TPA2005D1 cells, we need to minimise power usage to conserve cell life. Typically, the Fig.4: the internal block diagram of the TPA2005 Class-D audio amplifier IC. circuit draws an average of 10mA when Its differential inputs go to a balanced analogue amplifier and then onto a producing the heartbeat. However, once PWM modulator which drives a MOSFET H-bridge, and that in turn drives the the timeout period has ended, the current speaker. This provides high efficiency and plenty of power from a low supply needs to drop to a very low level until voltage. As shown, it can drive a speaker in Class-D mode without a filter. the unit is switched off. This is achieved in several ways. First, as already men- liquid flux, if you don’t have paste), position IC2 carefully tioned, there is no voltage across VR1 most of the time. over its pads, then tack-solder pin 4 to its pad. Also, after the timeout period expires, microcontroller Check that the IC is still aligned with the PCB pads on IC1 is placed in sleep mode and only draws about 150nA. both sides; remelt the solder if required. If all is OK, solder Amplifier IC2 is also switched off by IC1 taking the GP0 the remaining corner pins and then pins 2, 3, 6 and 7. Use output low, which connects to its SDWN (shutdown) input. solder wick to remove any solder that bridges between IC2 then draws around 500nA. We measured a 500nA cur- the IC pins. rent for the whole heartbeat circuit when in shutdown on IC2 also has a ground pad that needs to be soldered to the our prototype (half a microamp!). The cells should last for PCB. This can be done by feeding solder from the underside their shelf life with such a small current drain. of the PCB, through the hole positioned under the IC. Use minimal solder to prevent the solder from spreading out Construction and shorting to the IC leads. The MiniHeart Simulator is built on a double-sided, platedThe flux you added earlier will help this solder flow onto through PCB coded 01109201 which measures 70 × 73mm the pad on the underside of the IC. and available from the PE PCB Service. It is housed in an Now install the resistors and surface-mount capacitors. 80 × 80 × 20mm vented plastic enclosure. These components are located on both sides of the PCB. The Fig.5 shows the PCB component overlays. Begin by capacitors are usually unmarked except on their packaging. fitting the SMD Class-D amplifier chip, IC2. It requires a The resistors will probably be marked with a small code, very fine soldering iron tip and, ideally, a lit gooseneck as shown in the parts list. The first few digits indicate the or desktop magnifier (a good LED headband magnifier resistance value, followed by the number of extra zeroes in also works well). the last position. So for example, a 1kΩ resistor will have Identify its pin 1 dot under magnification, then orient the code 102 or 1001. That is a 10 followed by two zeros, it as shown in Fig.5, with pin 1 towards the speaker hole. or 100 followed by one zero. For 10kΩ, the code will be Add some flux paste to the middle of the central pad (or 103 or 1002, and so on. Scope3: this is the ‘dub’ signal measured identically to the ‘lub’ signal shown in Scope2. Again, it is a reproduction of a real heartbeat sound. Practical Electronics | January | 2022 Scope4: a single heartbeat sound with both the ‘lub’ and ‘dub’ waveform. You can see their slightly different shapes and amplitudes, and the delay between them. 27 Scope5: two heartbeats as shown in Scope4. With this slower timebase, you can also see the delay between beats. Next, fit diode D1, taking care to orient it correctly. Then mount ferrite beads FB1 and FB2 by first feeding tinned copper wire through the centre hole, then inserting and soldering these to the PCB pads. Keep the wire taught when soldering to prevent the beads from being loose. We used a socket for IC1 in case we ever want to remove it for reprogramming. Take care to orient the socket correctly (notch toward the PCB edge). Trimpots VR1 and VR2 can be mounted now. Take care to place the 10kΩ trimpot in the VR1 position and the 100kΩ trimpot in the VR2 position. Then fit three-way header JP1 with the shorter ends of the pins through the PCB holes. Power switch (S1) is installed in the position shown. The switch we used differs slightly from the one in the parts list in that the actuator is longer on the recommended switch. The positioning of the switch has therefore been moved further from the edge of the PCB. That way, the switch actuator will protrude from the case by the same amount as shown on our prototype. LED1 mounts with the anode (longer lead) in the hole marked ‘A’. Solder it so that the top of the lens is 11mm above the top edge of the PCB For the AAA cell holders, bend the wire terminals so that they stick out the sides of the holder, then bend them up to feed the leads through the holes on the PCB from the underside, and solder them on the top. The cell holders need to be oriented correctly, as shown on the overlay diagram. The base of the cell holders should be positioned so that they sit on the enclosure base when the PCB is seated on the four mounting posts. That means that the bottom of the cell holders will be lower than the bottom edge of the PCB. Next, fit the 100µF capacitor. Insert its leads with the longer lead through the hole marked ‘+’, then lie it over, so the capacitor body is between the LED and AAA cell holder. It must be no higher than 11mm above the top edge of the PCB. That will allow the lid to fit. The two PC stakes for the loudspeaker connections can now be installed with the shorter end inserted into the PCB from the top side. At this stage, don’t plug in the PIC microprocessor (IC1). You need to program the firmware (0110920A.hex) which can be loaded from the January 2022 page of the PE website. Housing Press the side clips into the case lid to release it from the baseplate. Locating flanges insert into one edge of the lid also secure it in place. The PCB is designed to be mounted onto the integral standoffs on the base of the case. There is only one correct orientation, and that is with the two notches along the top edge of the PCB fitting into the case lid locating flanges on the base plate. The PCB is secured with small self-tapping screws into the standoffs. We attach two 9mm-long M3 tapped spacers to the PCB to allow the lid to be screwed down. This is in addition to the side clips on the cover that hold it in place. Two screws then go into the standoffs from the outside of the lid. Attach these spacers by feeding short machine screws through the underside of the PCB into the two corner holes, then tighten the tapped spacers onto the screw shafts. Parts List – MiniHeart Heartbeat Simulator 1 double-sided, plated-through PCB coded 01109201, 70 x 73mm, available from the PE PCB Service 1 Hammond 1151V4 vented enclosure, 80 x 80 x 20mm [Jaycar HB6118] 2 AAA PCB-mount cell holders 2 AAA alkaline cells 1 40mm diameter Mylar cone loudspeaker [Jaycar AS3004] 1 PCB-mount SPDT toggle switch (S1) [Altronics S1421] 1 8-pin DIL IC socket 2 ferrite beads, 4mm diameter and 5mm long (FB1,FB2) [Altronics L5250A, Jaycar LF1250] 1 3-way header, 2.54mm pitch with jumper shunt (JP1) 2 9mm-long M3 tapped spacers 2 M3 x 6mm panhead machine screws 4 No.4 self-tapping screws 2 M3 x 6mm nylon machine screws (countersunk head preferred) 1 M8 marine-grade 316 stainless non-magnetic steel nut (6.35mm tall) 1 40mm length of 0.7mm diameter tinned copper wire (for FB1 and FB2) 1 100mm length of light-gauge hookup wire (or 2-way ribbon cable or figure-8) 1 small tube of neutral-cure silicone sealant (eg, roof and gutter silicone) 28 Semiconductors 1 PIC12F617-I/P microcontroller programmed with 0110920A.hex (IC1) 1 TPA2005D1DGNRQ1 1.4W mono filter-free Class-D amplifier (IC2) 1 1N5404 3A diode (D1) 1 3mm high-brightness red LED (LED1) Capacitors 1 100µF 16V PC electrolytic 3 1µF 6.3V SMD M3216/1206 X7R# ceramic 4 100nF 50V SMD M3216/1206 X7R ceramic 1 10nF 50V SMD M3216/1206 X7R ceramic 2 1nF 50V SMD M3216/1206 X7R ceramic Resistors (all 1% SMD M3216/1206) 1 100kΩ (code 1003 or 104) 2 27kΩ (code 2702 or 273) 2 10kΩ (code 1002 or 103) 1 2.2kΩ (code 2201 or 222) 1 1kΩ (code 1001 or 102) 1 10kΩ mini horizontal trimpot (VR1) 1 100kΩ mini horizontal trimpot (VR2) # a Y5V type was found to work in our prototype but X5R or X7R is a better choice Practical Electronics | January | 2022 Fig.5: these (and the matching photos below), show where components are mounted on both sides of the PCB. It’s generally best to fit all the SMDs to the top side (and possibly also the bottom side) before moving on to the through-hole components due to their small size and low height. Note how the speaker is oriented so that its terminals fit through the provided board cut-out, and also how the cell holder wires are bent to fit the PCB pads, fed in through the underside and soldered on top. IC1 is a normal 8-pin DIP . . . but IC2 (a TPA2005D1DGNRQ1) is tiny (it’s shown below about life size). A word of warning: don’t sneeze or turn a fan on if you ever want to see it again! The template (Fig.6) shows the position of the two holes required for the securing screws. It also shows the locations for the LED hole and the two trimpot adjustment access holes. The holes for the trimpots are optional; you can omit them if you’re happy to open the case if you need to make any adjustments. The lid panel artwork (Fig.7) is also available for download from he January 2022 page of the PE website Testing Place a shorting link in JP1’s position 1 and connect two wires, about 80mm long, to the two PC stakes under the PCB in readiness to solder to the miniature 8Ω speaker. We used two wires stripped from rainbow cable; mini figure-8 would also work well as well as separate hookup wires. The loudspeaker mounts on top of the PCB with the speaker terminals in the cut-out area. The wires connect to the speaker terminals from the underside of the PCB. For the moment, the speaker will be loose. Insert the two AAA cells and switch on the power. Check there is about 3V between pins 1 and 8 of IC1’s socket. Disconnect power and insert the programmed PIC in its socket, making sure it is oriented correctly (the notch toward the edge of the PCB). Reapply power and the speaker should start to move in response to the ‘lub dub’ sound. If not, make sure that VR2 is adjusted at least partly clockwise. Adjust further clockwise for more sound. Practical Electronics | January | 2022 Note that the sound will have an approximate 1kHz background tone. That’s because, even though this tone is filtered out in the circuitry, the speaker is much more efficient at producing 1kHz compared to the approximately 47Hz ‘lub dub’ sounds. Also note that you won’t really hear the ‘lub dub’ sound, but you will feel it if you place a finger at the centre of the loudspeaker cone. The loudspeaker cone needs to be weighted to make the heartbeat audible and to prevent the reproduction of higher frequency tones. To do this, we use an M8 stainless steel (non-magnetic) nut as a weight on the speaker cone. A nonmagnetic nut must be used; otherwise, the speaker cone would be pressed against the magnet of the speaker by the nut. We get away with this because the speaker cone is made from Mylar and so it is quite strong. This means that the central speaker coil is still centred within the magnet gap even with extra mass. To attach the nut, apply a smear of neutral-cure silicone sealant (roof and gutter silicone is ideal) to one side of the nut and affix centrally on the speaker cone. Additional silicone is required to fill the inside of the nut, making sure it is filled down to the cone. Keep the silicone flush with the top face of the nut. Also apply a thin layer around the speaker cone. It’s also a good idea to secure the ferrite beads (FB1 and FB2) using some of the silicone to hold them to the PCB. Only a small amount is necessary. This will prevent them from rattling and adding obscure sounds to the heartbeat. 29 i n i M e h T HEART Fig.7: the ‘front panel’ artwork, which has a hole provided for the LED. You can download a PDF of this artwork from the January 2022 page of the PE website.   SILICON CHIP Fig.6: this drilling diagram shows the locations of the 3mm LED hole, two 3mm lid attachment holes (along the bottom) and optional holes to access the adjustment trimpots without having to remove the lid. Likewise, the loudspeaker is secured to the PCB with some silicone around the central magnet, where it fits into the PCB hole. Note that the speaker needs to be positioned correctly, with the wire entry points positioned over the PCB cut-out and with the back of the speaker magnet resting on the base of the case. The PCB should be temporarily positioned on the integral standoffs in the case while the silicone cures. This way, the speaker will be at the correct height above the PCB. Your best bet since MAPLIN Chock-a-Block with Stock www.siliconchip.com.au Using it Adjust the timeout period so that the heartbeat sound lasts for the length of time you require. This is done with JP1 in position 2. To do this, move JP1 into position 2 with the power off and set the required time. Full clockwise adjustment of VR1 gives a 4-hour timeout. The mid position is two hours and mid-way between fully anticlockwise and mid-way is about one hour. Set the timeout and then switch on the power. The timeout period will be recorded. Any further adjustment of VR1 with the power on will be ignored. It is only the setting of VR1 at power-up when JP1 is in position 2 that is recorded. The setting is stored in non-volatile Flash memory and remembered for use next time. When jumper 1 is in position 1, the heartbeat rate can be adjusted. This can be changed with power on, from 42 to 114 beats per minute. The setting is also stored in Flash memory, and the last setting will be used should the unit be powered up with JP1 in position 2. The volume is set using VR2. However, the drive to the loudspeaker will become distorted if VR2 is rotated too far clockwise, so a position less than halfway clockwise should be used. Reproduced by arrangement with SILICON CHIP magazine 2021. www.siliconchip.com.au Visit: www.cricklewoodelectronics.com O r ph one our friendly kn owledgeable staff on 0 2 0 8 4 5 2 0 1 6 1 Components • Audio • Video • Connectors • Cables Arduino • Test Equipment etc, etc V i s i t ou r S h op , C a ll or B u y on li n e a t : www. cr i ck lewoodelect r on i cs . com 0 2 0 8 4 5 2 0 1 6 1 30 V i s i t ou r s h op a t : 4 0 - 4 2 C r i ck lewood B r oa dwa y L on don N W 2 3 E T This view shows how the PCB is secured to the case lid but more importantly, shows the ‘damper’ glued to the mica speaker diaphragm (in this case, a stainless steel nut). Don’t be tempted to use a mild steel nut: they’re magnetic and will not work in this role. Practical Electronics | January | 2022