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ng Add bli ifi h to your er ! amplifi Pt.2: By Nicholas Vinen 100dB Stereo LED Audio Level/VU Meter Last month, we introduced our new Audio Level/VU Meter which uses 80 high-brightness SMD LEDs to give a colourful dual-bargraph display showing average and/or peak audio levels. It has a number of useful features such as adjustable dynamic range, reference level and LED brightness. This article deals with assembling it and explains how to set it up and use it. A S DESCRIBED in Pt.1 last month, the meter is based on a 32-bit PIC processor driving 88 bright SMD LEDs. It has an analog front end with the processed signals being delivered to the PIC’s analog inputs to be converted to digital format by its internal ADC so that the signals can be analysed by the software. Having gone over the details of its operation, let’s now get straight into building the PCB. Construction The PCB overlay diagram is shown in Fig.4. All parts are fitted to this board, with most being surface mount 76  Silicon Chip devices (SMDs). The exceptions are connectors CON1-CON4 and switches S1 & S2. All of these through-hole components can be mounted off-board (eg, chassis-mounted) and connected via shielded cables (for CON1 & CON2) or twin lead (eg, figure-8, for CON3, S1 & S2) – see below for more details. If doing this, these components are left off the PCB and PC stakes can be fitted to the test points near the DC socket for wire termination. Start construction with the SMD components. It’s best to fit IC1 first, as it has the finest lead pitch of any of the SMDs on this board, although it is not too daunting as the pins have a relatively generous 0.8mm pitch on a 10x10mm package. There are various valid techniques for hand-soldering SMDs as well as other methods involving toaster ovens, frying pans and so on. Our preferred technique (as long-time readers will no doubt be aware) is to first place a small amount of solder on one pad, heat this while sliding the part into place, check its orientation and that all the pins are correctly centred over their pads, then solder the remaining pins before finally refreshing the initial joint. Take your time doing this with IC1 and be careful to ensure that its pin siliconchip.com.au K Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 LED40 LED1 K K CON1 LEFT INPUT 22k 22k 1nF 3.9k 20k 680pF 100pF 1 D2 2 × BAT54S D3 22k 22k IC2 5532 1k 22k 1nF 2.2µF 22k 2.2µF 3.9k 20k CON4 22k 2.2µF 22k 1k 100pF 2.2µF 22k 1k 100pF 1k 22k 1 1k 1 IC3 5532 22k 100nF IC4 5532 1k 22k 1k 22k 2.2µF 47µF 22k 34 IC1 100nF D7 D8 D9 3.9k 3.9k 47µF 3 × BAT54S Top K ZD1 5.6V 1k 1k 40dB K 100nF 23 1k Range 1 12 10Ω 100pF 2.2µF100pF 1k 1k 10µF 10k PIC32MX170F256D-I/PT 100nF 22k ICSP 1 1k 100nF 680pF 100pF CON2 RIGHT INPUT LED80 D4 D6 D5 22k 1k 60dB K LEDs 81-84 1k 80dB 100dB K 1k 1k q10dBV 0dBV 4dBu 7dBV K © 2016 10µF 2.2µF S2 10Ω 33Ω 1W K 100dB Digital Stereo Level/VU Meter 01104161 S1 TP3.3 TPV+ TPG2 1k LEDs 85-88 REG1 MCP1703-3302E/DB K 1k K SILICON CHIP 2.2µF VR1 BRIGHTNESS 10k K REG2 5201 2.2µF 100nF 3 × BAT54S 2.2µF LED41 K D1 2.2µF 1 12k CON3 1.5k POWER 12-15V DC TPBR TPG1 Fig.4: follow this parts layout to build the Stereo Level/VU Meter. Most parts are SMDs and all are fitted to the top side of the board. Microcontroller IC1 has a 0.8mm pin pitch, while the remaining parts have wider-spaced pins. Take care when fitting the LEDs to ensure they have the correct orientation and that they are lined up neatly. 1 dot is in the location indicated on Fig.4 and that all its pins are nicely aligned before soldering more than one. Spreading a thin layer of no-clean flux paste over the pads aids in soldering. Don’t worry too much about bridging the pins – in fact, it’s easier if you simply place some solder on the iron and run it along the side of the IC to solder a whole bunch of pins at once. You can then clean up the bridges by adding a little extra flux paste and then gently applying some solder wick and heating it until the excess solder flows off the IC pins and into the wick. Repeat until all the pins have been properly soldered. It’s a good idea to then inspect the joints under a magnifying lamp after cleaning off any flux residue with pure alcohol or a specialised flux cleaning solution. The remaining ICs, including REG1 and REG2, can be soldered using a similar technique although their pins are far enough apart to be soldered individually. Note that if IC2, IC3, IC4 or REG2 lack a dot to indicate pin 1, you should be able to identify it as being on the chamfered side of the package. For REG1, it’s easiest to solder the smaller pins first and then the tab, as the tab will require significant heat (and thus time) to solder. It helps to spread some solder paste on the large pad for the tab before sliding REG1 in place. With the ICs soldered, follow with D1 and ZD1, ensuring that D1’s cathode stripe goes to the left as shown in Fig.4. You can then solder transistors Q1-Q8 and diodes D2-D9 in place, siliconchip.com.au making sure you don’t get them mixed up as they are in identical packages. Follow with all the capacitors and resistors, none of which are polarised. Note that the resistors will have value codes printed on top (eg, 223 = 22kΩ) while the capacitors will be unmarked. The 10µF and 47µF capacitors may be a larger size than the others and larger pads are provided to accommodate these. Similarly, the two 22kΩ resistors in the input divider are larger than the others (in case they have to dissipate more power in a fault condition) and the 33Ω 1W resistor is larger again. Next fit VR1, unless you are going to mount an external brightness pot. Try to avoid getting solder on its metal body as the flat pins cover most of the pads and are quite close to the body. Installing the SMD LEDs In terms of SMDs, that just leaves the LEDs. The first job to do is to check their polarity. All the LEDs we used (which are the same types as we will be supplying) have green cathode dots. However, some LEDs have green anode dots so you should confirm this. To do this, set a DMM on diode test mode and touch the probes to either side of one of the LEDs. If they make good contact and the polarity is correct, the LED will light up. In this case, the red lead is on the anode. If nothing happens, try flipping the LED around (or reversing the leads). You should get it to light up with one polarity, although it’s possible some DMMs will not have enough bias volt- age to light some LEDs (eg, blue). It’s a good idea to do a reasonably neat job of soldering LEDs81-88, centring them on their pads and making sure they are not fitted crookedly, but it’s absolutely critical for LEDs1-80 if you want the bargraphs to look good. The first trick to doing a neat job is to solder all the LEDs at one end and inspect them critically before soldering the other ends. This gives you the possibility of nudging any LEDs which are misaligned compared to the others. Don’t forget that the cathodes for LEDs81-88 face the bottom of the PCB while the cathodes for LEDs1-80 face the top. Reversing the polarity of LEDs en masse is possible but time consuming! We aligned the main bargraph LEDs by hand and while close inspection reveals that a few are slightly askew, this really isn’t obvious when viewing the device during operation. If you want them perfectly aligned, the best solution may be to clamp a straight edge parallel to the top of the PCB so that you can push the LEDs up against it and have them located evenly between the pads and square with them. It would then just be a matter of sliding them until they were centred and soldering the far side. Once they’re all in place, you can remove the straight edge and solder the other ends. Note that SMD LEDs are easier to solder if you’ve first applied a little flux paste to the pad and/or terminal. Don’t overheat the plastic lenses though, they can be burnt quite easily – we strongly adJuly 2016  77 This view shows the prototype PCB assembly. Take extra care when installing the LEDs and ICs to ensure they are all orientated correctly. We used green LEDs for LEDs 1-30 & 41-70, yellow for LEDs 31-34 & 71-24, amber for LEDs 35-38 & 75-78, red for LEDs 39-40 & 79-80 and blue for LEDs 81-88. vise against using a hot air reflow tool in their vicinity. Through-hole parts Assuming you are fitting them, push switches S1 and S2 down fully onto the PCB and solder their leads. Otherwise you could fit PC stakes to their mounting pads, or simply solder wires direct to the PCB. DC connector CON3 should also be pushed down fully before soldering or, as stated earlier, connect supply leads to TPV+ and TPG2. If your microcontroller has been supplied pre-programmed you don’t need to fit CON4. Otherwise, solder it in place in the usual manner. Alternatively, it can later be fitted to the rear of the PCB if necessary. That just leaves RCA sockets CON1 & CON2. If using the RCA sockets supplied by SILICON CHIP, you will need to bend the two side pins out to make them fit the pads (see photos of our prototype). We supply them in a pack of four including white and red; unfortunately, white RCA sockets are hard to find. Alternatively, you could use different colours (eg, red & black). Regardless, make sure they are pushed down fully and properly perpendicular to the edge of the board before soldering the pins. If you don’t want to use sockets, solder the braid of a length of shielded cable to the central pin and the signal wire to the terminal closer to the top edge of the board. Programming the micro If you don’t have a pre-programmed micro, you will need a PICkit 3 (or equivalent) and the HEX file from the SILICON CHIP website. The Microchip 78  Silicon Chip MPLAB Integrated Programming Environment is a free download from the Microchip website. Enter the chip type, connect to the programmer, then go into advanced mode and under “Power” options, enable “Power Target Circuit from Tool”. You can then go back to “Operate”, click on the “Browse” button next to “Source” and select the HEX file. Plug the PICkit 3 into CON4 on the PCB, with the triangle on the programming tool lined up with the pin 1 indicator on the PCB. Press the “Program” button and after 20 seconds or so it should announce that the chip was successfully programmed and verified. You can then unplug the tool. If programming fails, check that the solder joints on IC1 are OK, along with those on the four capacitors surrounding it. Check also that you have enabled power from the PICkit 3 (at 3.3V or so) and that it has been correctly plugged into CON4 and is not offset or reversed. Chassis-mounting connectors and/or controls If fitting the VU meter assembly inside a power amplifier chassis, you may be able to do without connectors altogether, although they do make installation somewhat more convenient. In this case, CON1 and CON2 can be wired directly to the amplifier outputs. Similarly, CON3 can be omitted and TPV+/TPG2 wired directly to a regulated 12-15V DC supply within the amplifier. Be careful to avoid creating a ground loop involving the signal grounds and power ground connections. Ideally, the power supply should be floating and if necessary, derived from a dedi- cated transformer secondary winding (or separate transformer). We also recommend that you avoid using a DC supply that’s also used to power a preamplifier. That’s because the pulsed current drawn by the VU meter might affect the preamp’s performance. The ideal solution is a small, separate rectifier/filter/regulator based on, say, a 7812 and powered from a separate low-voltage winding on the transformer. It only needs to be able to deliver 150mA. If you can’t use a floating supply, make sure there is no difference in ground potentials between the supply for the VU Meter and the amplifier outputs. Also, if the amplifier outputs are bridged, do not connect the negative output to the inputs of the VU Meter. Instead, wire these inputs to ground and keep in mind that the input signal swing will be half of the amplifier output swing (ie, 3dB lower). Switches S1 and S2 may be mounted off-board if desired, so that they are accessible outside the chassis, although in cases where the inputs are hardwired to amplifier outputs, you probably won’t need access to S2. In this case, the unit will normally be used with a fixed reference level of +7dBV. VR1 can also be mounted off-board so that the brightness adjustment can be easily accessible. Any potentiometer of approximately the same value should be fine. Wire its wiper to TPBR, the bottom of the track (anti-clockwise) to TPG1 and the top of the track (clockwise) to TP3.3. Testing Ideally the unit should be powered for the first time with a current-limitsiliconchip.com.au Table 1: Display Modes Mode Averaging Display LEDs Flashing 1 (default) RMS average bar + peak dot LED81, LED85 2 RMS average bar only LED81, LED86 3 N/A peak bar only LED81, LED87 4 VU-style average bar + peak dot LED82, LED85 5 VU-style average bar only LED82, LED86 ed power supply. If you have a bench supply, set it for 12V with a limit of 200mA. Otherwise, you could use a 15V regulated (or 12V unregulated) plugpack wired with a 47Ω 5W resistor in series. Apply power and check that LED81 (40dB) and LED86 (0dBV) are lit. A quick press of S1 and S2 should cycle the lit LEDs. The current drain should be around 50mA. If using a series resistor, you can check this by measuring the voltage across the resistor (eg, ~2.35V across 47Ω). When LED84 (100dB) is lit, you may find some of the bottom segments of the bargraphs light up. This is normal as the inputs are currently un-terminated. Check the voltage between TP3.3 and TPG2. It should be between 3.28V and 3.32V; a little lower or higher is OK. You may also wish to check the voltage across the 2.2µF capacitor to the right of REG2; it should be between about 10.8V and 11.5V. If it’s above 11.2V, you may wish to consider shunting the 12kΩ resistor with a 470kΩ resistor (which can be soldered on top) to reduce it, to ensure the regulator won’t enter drop-out with a supply voltage very close to 12V. If using a series resistor to limit current, this will not permit the unit to draw enough current to light up all LEDs and continue to operate normally. So short out the resistor before performing further tests. If you switch off the unit and hold down S1 while applying power, all LEDs will light up. You can use this feature to check that they are all soldered properly and operating normally. If any do not light up, check their soldering and orientation. If you need to remove a LED (eg, if it is faulty), you can do so by alternately heating the two pads until it lifts off. Then add a little flux paste and use solder wick to remove the remaining solder from the pad(s). Assuming all LEDs are working, release S1 to exit LED test mode, then connect a signal source to the unit. You can then check that the bar displays are working normally and respond to presses of S1 and S2 as expected (use the instructions below as a guide). Operating instructions The Stereo LED Level/VU Meter will fire up as soon as it has power and resumes the last used mode. You only need to use the controls to switch modes or to perform calibration. A brief press of S1 will cycle to the next meter scale. The default is 40dB. Pressing S1 will change this to 60dB, then 80dB, then 100dB, then back to 40dB. The decibel level of the top-most segment remains the same, ie, this lights when the input signal reaches the reference level which is 0dBV by default. Pressing S2 cycles through the four available reference level options. Initially, the display shows the average level as a bar, with a dot indicating the peak level. In some cases, the peak dot may coincide with, or be just above, the top of the bar so it will not be visible. Normal program material will typically have a 5-15dB difference between the average and peak, so there will normally be a significant separation. You can change to a different display mode by pressing both S1 and S2 simultaneously, then quickly lifting off both. Refer to Table 1 for a list of the five available modes. To adjust the bar brightness, simply rotate VR1. Note that the specified SMD trimpot does not have an end-stop so if you turn it too far in one direction it will “wrap around”. Note also that the minimum brightness setting gives about 5-10% duty cycle, which may not be all that dim, given how bright modern SMD LEDs are. Further adjustments can be made using switches S1 & S2 to access the various set-up modes described below. The method to access these modes is summarised in Table 2. Noise nulling The input noise level of our proto- Are Your S ILICON C HIP Issues Getting Dog-Eared? Are your copies getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? REAL VALUE AT $16.95 * PLUS P & P Keep your copies of SILICON CHIP secure & organised 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. siliconchip.com.au July 2016  79 Table 2: Summary Of Set-Up Modes Setting Action LED test Hold down S1 while powering on Relative LED brightness adjustment Hold down S2 while powering on Set noise null levels Hold down S1 after power on for at least 0.5s Cancel noise null Hold down S2 after power on for at least 0.5s Change average/peak hold period Hold down S1 after power on, press S2 once (LED81 flashes), release S1 0dB calibration with reference signals Hold down S1 after power on, press S2 twice (LED82 flashes), release S1 0dB calibration without reference signals Hold down S1 after power on, press S2 three times (LED83 flashes), release S1 type unit is around -100dBV although this depends on how the inputs are terminated, the LED brightness setting, how quiet the power supply is, etc. With the unit set to 100dB dynamic range, a 7dBV reference level and average-only display, both bars should be totally unlit. However, if the meter is set to 100dB dynamic range and you select a lower reference level or enable peak metering, some of the segments will be lit all of the time, even with no signal. If your signal source produces some noise, and most do, it will likely increase the no-signal reading and may even light one or more segments on the less sensitive ranges if it is particularly noisy. In either case, you can null out the noise to give a blank display with no signal by simply hooking the signal source up, switching both units on and, with no audio output, holding down S1 for a little over half a second. LED84 (“100dB”) will flash and the bars should drop to zero. If they don’t, try again. Now introduce a signal and verify that the meters still light up as expected. This works by storing the average and peak level measured when S1 is held down and these readings are subtracted from future measurements. If you want to cancel it and go back to showing the raw (unadjusted) reading, simply hold down S2 for at least half a second. LED88 (“7dBV”) will flash and the display will go back to how it was before. 0dB calibration If you want the unit to handle signals above 2.33V RMS, you will need 80  Silicon Chip to change the input divider. But if you want to make a small adjustment, eg, to set a reference level other than one of the four existing options, or to compensate for resistor error (including differences between the two channels), you can do that using the software’s calibration feature. A new reference level can be set for each channel in each of the four available “slots” corresponding to LEDs8588. When you set a new reference level, it overrides the pre-existing level for that slot. Before you set a new reference level, use S2 to select the slot in which you want to store the new level(s). The easiest method is to feed a signal into both channels at the level you want for full scale, then switch the unit on with S1 held down. Before releasing S1, press S2 twice. LED86 will flash a few times. The signal level for both channels will be used as the new 0dB reference level for the currently selected slot. Alternatively, if you do not have a signal generator that can produce the appropriate levels, you can adjust the reference level for a slot incrementally, in 0.1dB steps between -20dBV and +7.3dBV. Instead of pressing S2 twice before releasing S1, press it three times. LED87 flashes instead. Now, the left channel display (top-most bar) will be shown as usual but the right channel display will instead show the currently selected reference level. This is achieved by lighting up a 12LED section of the bar which moves up and down by one LED for each 1dB change in reference level. At the minimum setting of -20.0dBV, this bar will start at the bottom-most LED, so you can figure out the whole number of decibels by counting the number of LEDs before the bar starts. At the maximum setting of +7.4dBV, the bar will stop one LED from the top. The fractional number of decibels is indicated by switching off one LED within the bar. If the number ends in .0, the second LED will be off. If it’s .1, the third LED will be off, and so on until it’s .9 in which case the secondfrom-last LED will be off. This may sound complicated but once you see it in action, you should find it pretty easy to figure out. A quick press of S1 will reduce the selected reference level by 0.1dB while a quick press of S2 will increase it by the same amount. Because the left-channel bar operates normally, you can observe the effect of changing the reference level on the display, and adjust it for a particular level for a particular signal should you wish. Hold down S1 for at least half a second in order to set the level for the other channel. You can switch back and forth, adjusting the levels as required. When you’ve finished, hold down S2 for at least half a second and the changes will be saved. The unit will return to its normal display. If you want to abort changing the reference level, simply pull power from the unit. There’s one extra function available in this mode: if you press S1 and S2 simultaneously (briefly), it will copy the level setting from the other channel to the currently selected channel. This makes it easy to set both channels to the same (or a similar) reference level. Changing the averaging/ peak hold period When the unit is in VU mode (modes 4 & 5 shown in Table 1), the unit performs RMS averaging on each block of 1024 samples and then uses a ballistic simulation of a moving needle to provide the required 300ms settling time to 99% and 1-1.5% overshoot for a VU meter. But in the other modes, the average value is calculated by averaging one or more of the RMS amplitude results from the 1024 sample blocks. By changing the number of values averaged, you can change the response time. The minimum is one block, representing around 25ms of signal, and the maximum is 40 blocks, ie, around one second’s worth of data. Similarly, the peak value is calculated as the maximum peak value of between one siliconchip.com.au The PCB assembly can be housed in a laser-cut clear acrylic case which is available from the SILICON CHIP Online Shop. The PCB, programmed microcontroller and other parts are also available from the Online Shop. and 40 blocks worth of data. You can change both values. Changing the peak calculation period will also affect the VU-style mode if the peak is shown. To adjust these settings, simply hold down S1 while applying power, then press S2 once. LED85 should flash and you can then release S1. The averaging window size is shown by which of LEDs1-40 is lit; LED1 indicates averaging over one sample block, LED2 over two, etc. Similarly, LEDs41-80 show the peak period. Initially, one LED will be flashing in the top row. Press S1 to reduce the averaging window size by one sample block or S2 to increase it. Hold down S1 for at least half a second to switch to the other row, to adjust the peak calculation period, and use S1/S2 to reduce/increase it. When finished, hold down S2 to save the settings and return to normal operation. To abort the changes, simply pull power to the unit. LED brightness adjustment If you’re using different colour LEDs which are reasonably well matched in terms of brightness, the display should look good without any further adjustment. However, if you’re particularly fussy or using different LEDs which are not so well matched, you may find that some are noticeably brighter than others. We have incorporated a feature to allow you to dim a subset of the LEDs in the display in order to match the brightness. There are a few limitations siliconchip.com.au (explained below) but this method generally works quite well. To access this setting, hold down S2 while powering the unit up, wait for at least half a second, then release it. Only LED1 and LED2 will be lit. They will be driven at maximum duty cycle, to allow you to compare the brightness of the two LEDs. Short presses of S1 and S2 change which pair of LEDs are lit, to the left and to the right respectively. Use these to light up the first pair of LEDs which have a significant difference in brightness. You can then rotate VR1 to adjust their relative brightness until they appear to be matched. Use S1/S2 to move along until you find another pair of LEDs with mismatched brightness and adjust those too. Continue until you reach the final pair of LEDs for the left channel, LEDs39 & 40. At this point, pressing S2 will illuminate the entire top bargraph and the LED brightness will be adjusted based on the settings you have made so far. You can now use VR1 to adjust the overall brightness of the bar. Note that if you have made more than one adjustment, because they are cumulative, you may find that the brightness matching is not perfect. You can now press S1 and make further adjustments before returning to the “preview” mode. Continue until you are satisfied with the result, then use S2 to switch to the bottom bargraph and use the same procedure to match the brightness of its LEDs. Once you have selected a pair of LEDs and rotated VR1, the brightness offset for that pair remains adjusted. To clear this adjustment, select the pair of LEDs, then hold down S1 for at least half a second. They will be reset to their original state. Holding down S2 for at least half a second resets all LEDs to their default states and allows you to start the adjustment procedure from the beginning. When you are satisfied with the result, press S1 and S2 together briefly and release them. The changes will be saved and the unit will return to its normal operating mode. Changes are stored in flash memory so the unit will apply them each time it is powered on. To make further changes to the relative LED brightness you will need to remove power and repeat the procedure. To disable this feature, re-enter the adjustment mode and hold down S2, then save the changes. Limitations The limitations are as follows. Firstly, any relative brightness adjustment will reduce the overall maximum brightness of the display. Secondly, the software supports up to four different brightness levels within each bank of 10 LEDs. Making adjustments that would require more than this will have undefined consequences. Also, making relative adjustments that are too extreme may result in a flickering display. Finally, the signal-to-noise ratio of the unit and its ability to register very July 2016  81 lid left rear base right front Fig.5: cutting template for the custom-made case. It’s cut from a 208 x 190 x 3mm sheet of clear acrylic (polymethyl methacrylate, Perspex, Lucite, Plexiglas, etc). The red lines show the internal hole cut-outs. The three extra small pieces are used to space the board off the base, while the two slots in the top side allow the mounting tabs in the PCB to fit through (although they should normally be removed, see text). brief signal peaks may be slightly impacted by this feature. Laser-cut case For those building the Level/VU Meter as a stand-alone unit, we have designed a clear acrylic case. It consists of six pieces that are glued and screwed together and is just a little bit larger than the PCB itself, giving a compact assembly. The cutting details are shown above in Fig.5 and one of the photographs shows the result. All parts except for the lid should be glued using a specialised, solventtype plastic adhesive. We used a tube of SciGrip “Weld On” 16 fast set clear, medium-bodied solvent cement. This is available from Plastix [Sydney (02) 9599 2499 or Northern Beaches (02) 9939 0555]. Note that the PCB has two trapezoidal tabs at the top with mounting holes. These tabs are not required if using the laser-cut case and can be cut off using a fine-bladed hacksaw or similar tool (the sides of the tabs are squared to make this task easier). Note that you can still get the PCB into the case with the tabs intact (as shown in our photos) but it looks a lit82  Silicon Chip tle odd and makes it much more difficult to remove the PCB later if that should be necessary. The first step in assembling the case is to attach the PCB to the base. You can identify this as it is the large piece with two round holes and one rectangular slot. A small T-shaped piece of plastic is supplied and this is glued into the rectangular slot after removing the protective film from both pieces. This small piece forms a support for the top part of the PCB. Two small square pieces with holes in the middle are also supplied. Remove their protective film and place them over the holes in the base. Feed a 10mm machine screw up through each pair of holes. You can then drop the PCB down on top, with the two screws passing through the mounting holes at the bottom of the board. That done, place a pair of 3mm ID shakeproof washers on each screw shaft and then screw an M3 x 12mm tapped Nylon spacer loosely onto each, holding the PCB in place. Make sure the square supports are orientated parallel to the edge of the board, then tighten the spacers up. Now mock-assemble the case, with the protective film still on the remaining pieces, to ensure everything fits. You can temporarily fit the top panel to the two spacers using M3 x 6mm machine screws but don’t do them up too tightly as they may prevent the sides from going on. Push the other four pieces into place and make sure every­ thing fits. If it does, remove the top panel and take the protective coating off both sides, then screw it back on. It’s a good idea to keep a clean, disposable cloth on hand while gluing the case, to wipe off any excess glue quickly before it starts to set. Try to avoid getting the glue on any of the external faces of the case since it can cause hazing. It’s now basically just a matter of removing the protective film from the rear, front and lefthand (input side) pieces and gluing them in turn to the base panel and to each other. Coat all the mating surfaces with the solvent glue before pushing the panel into place and ensure it can’t move until the glue hardens after 5-10 minutes. Full strength is achieved after 24 hours. If you need to get the PCB out of the case, it will be necessary to slide it out, pulling the RCA socket barrels out of the holes in the lefthand side of the case. To allow this, the righthand side piece should be glued not to the rest of the case but to the DC socket. This will hold it in place but allow it to slide out with the board should you need to remove it (assuming you have cut off the top tabs). Like the rest of the pieces, its protective film should be removed before it’s glued. Other case options The Level/VU Meter could also be fitted into a case with a clear lid, such as the Altronics H0332A Sealed ABS Enclosure (220 x 165 x 60mm), although this may be more expensive. It does have the advantage of being sealed against moisture and dust but you would have to use suitable sealed connectors for input and power to keep the IP65 rating for the finished unit. In this case, you would simply need to fit tapped spacers to the four mounting holes on the PCB and either screw and seal or glue these to the base of the case. Alternatively, the PCB assembly can be fitted into an amplifier chassis, behind a clear window on the front of the unit, and attached via those same SC four mounting holes. siliconchip.com.au