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Last month, we described how the S/PDIF Digital Audio Signal Generator works. This month, we describe how to assemble the PC boards, mount them in the case and check that they are working correctly. By NICHOLAS VINEN A High-Quality Digital Audio Signal Generator; Pt.2 T HE DIGITAL AUDIO Signal Generator is built on two PC boards: a main board and a control board. Construction can begin with the assembly of the main PC board. There are two versions, one to suit the Jaycar case (Fig.10) and the other to suit the Altronics case (Fig.11). The Jaycar main board is coded 04203101 while the Altronics board is coded 04203103. Before starting, examine the copper side of the PC board for any defects. It’s also a good idea to place it inside the case, up against the end, in order to check that it fits properly. Verify that the mounting holes line up with the posts in the base of the enclosure. Once you are satisfied that it will fit, start the assembly by installing the seven wire links. You can use 0Ω resis82  Silicon Chip tors for the shorter links and 0.71mm tinned copper wire for the longer ones (or you can use tinned copper wire for the lot). Next, install the 0.25W resistors. It’s best to check the value of each with a DMM before installation, as the colour codes can be hard to read. Follow these with the four diodes (D1-D4) and zener diode ZD1. Pay careful attention to the orientation of these parts. You will have to bend the leads of the 1N5819s close to their bodies for them to fit. The five IC sockets can now be installed. Be sure to line the notches up with those shown on the overlay. Solder two diagonally opposite pins on each to begin with, then check that they are sitting flat on the PC board before soldering the rest. Crystals X1 and X2 are next on the list. Be careful not to get them mixed up. The markings on their cases should match the corresponding frequency values on the PC board overlay. Once these are in place you can install the 10Ω 1W resistor (see panel). If you install it, you can only use rechargeable cells or the plugpack to power the device – you cannot use alkaline or other non-rechargeable batteries. If you do want to use alkaline batteries (or the plugpack), then leave this resistor out. Since the physical size of this resistor can vary, you will need to make sure that it doesn’t interfere with the battery connector. If necessary, install it slightly proud of the PC board so siliconchip.com.au siliconchip.com.au CON6 LEFT ANALOG OUT RIGHT ANALOG OUT 16 560Ω 200Ω Q1 5.1V 47k 100 µF Q5 Q7 100nF CON3 D3 5819 D1 10 µF CON1 CON2 CON1CON2 220Ω 150Ω 390Ω 100nF IC3 74HC393 150nF 100nF 5819 REG1 7805 33pF 13k 150pF 1.5k Q6 1k 100k 100k 100k 10k 10Ω 100nF D2 REG2 LM3940-3.3 X2 68pF 11.2896MHz 100 µF 10 µF 180Ω 100nF 1.0Ω 1.0Ω 10k 10k 10k 10k 10M 620Ω IC1 MC34063 13k 13k 10 µF 13k 11k 150pF 100 µH Q2 IC5 D4 5819 ZD1 LMC6482 13k 150pF 100 µF IC2 74HCU04 10 µF 100nF 33pF 10 µF 5819 10 µF 100nF Q4 BC337 33k 11k 100Ω 10k 1k 15 X1 24.576MHz 100Ω 10k CON4 2 1 15nF S/PDIF OUT 100nF IC4 dsPIC33FJ64GP802 15nF CON7 CON8 10 µF 47 µF Transistors Q1-Q7 can now be fitted. These all come in TO-92 plastic packages but there are three different types, so read the markings carefully. In some cases, the leads may be too close to fit through the mounting holes. If so, use needle nose pliers to bend the two outer legs apart to 45°, close to where they emerge from the case, and then back parallel again further down, so that they will fit in place. The three polarised header connectors (CON1-CON3) go in near the bottom edge of the board. Be sure to orient them as shown and make sure they are sitting flat against the board before soldering their pins. Now fit the capacitors, starting with the MKT and ceramic types. These can go in either way around. Once these are all in, install the electrolytics. Their orientation does matter so make sure they go in the right way around. Be extra careful with the tantalum capacitor, which is also an electrolytic but uses a different electrode material. It’s easily destroyed by reverse polarity. The only reliable way to check the orientation is to look for the “+” sign printed on the epoxy case, above one of the legs. The main board assembly can now be completed by installing the 100µH inductor, the TOSLINK transmitter CON5 10 Ω 1W Installing the transistors TOSLINK OUT Q3 that it sits above the adjacent 1N5819 diode (D2). Follow with the two TO-220 voltage regulators (REG1 & REG2), taking care not to get them mixed up. In each case, bend the leads down through 90° about 5mm from the body using a pair of needle-nose pliers. That done, mount the device on the board, line up the tab mounting hole and secure it using an M3 x 6mm machine screw, nut and star washer (the latter goes under the head of the bolt). Finally, solder the leads to their respective pads and trim away the excess. Do not solder the leads before you have bolted the devices down, otherwise you could crack the copper tracks as the screw is tightened. The next step is to install the IDC (insulation displacement connector) socket (CON4). It should be installed with its notched side towards the bottom (see layout diagram). Solder pins 1 & 16 first and make sure the socket is sitting flush against the board before soldering the rest. 33pF Fig.10: follow this diagram to build the main PC board for the Jaycar case. Make sure that all polarised parts are correctly oriented and install the 10Ω 1W resistor only if you intend using rechargeable cells (see panel). and the three RCA sockets. When mounting the TOSLINK transmitter, push its two plastic posts down through the holes in the board until they snap in, then check that it is sitting flat and parallel with the edge of the PC board before soldering its pins. Be sure to use a white RCA socket for the left analog output, red for the right analog output and black for the S/PDIF output. You may have to press the sockets down hard into the board to get their plastic posts to sit properly. Note that they do not go all the way down through the board but rather sit in the holes. Ensure that the RCA socket bases are sitting parallel with the PC board before soldering them in place. It is best to check them from the perspective of the sides and end of the PC board as they can be mounted askew in either plane. That completes the main assembly Choosing The Trickle Charge Resistor As mentioned in Pt.1, Nickel Metal Hydride (NiMH) rechargeable cells can be used to power the unit and the circuit includes a 10Ω 1W resistor to trickle charge them whenever the plugpack is connected. This resistor value is suitable for 2000mAh cells and provides just under 100mA to the cells once they are fully charged This equates to a charge rate of C/20 for 2000mAh cells, although it will be appreciably higher than this when the cells are flat. If you use lower capacity cells, then you need to increase the value of the resistor accordingly. For example, 800mAh cells require a 27Ω 1W resistor, while 600mAh cells require a 33Ω 1W resistor. Note that you should install this resistor only if you intend using NiMH or Nicad cells in the device. Do not install it if you intend using alkaline (or any other non-rechargeable) cells. April 2010  83 9-10V DC INPUT SOCKET TOSLINK OUT CON5 CON6 LEFT ANALOG OUT RIGHT ANALOG OUT 15nF 2 1 CON4 5819 D1 10 µF CON1 CON2 CON1CON2 150nF 220Ω 150Ω 390Ω 100nF 100nF D2 REG2 LM3940-3.3 X2 68pF 11.2896MHz Q7 100nF D3 10M 620Ω IC2 74HCU04 100nF 5819 REG1 7805 33pF 13k 150pF Q5 CON3 1k Q1 Q2 47k 100 µF Q6 10 µF 47 µF 200Ω 5.1V 1.5k 33k 11k 560Ω 100k 100k 100k 10k 10Ω 5819 100 µF 10 µF 180Ω 100nF 1.0Ω 1.0Ω 10k 10k 10k 10k 10 Ω 1W IC1 MC34063 13k 13k 10 µF 13k 11k 150pF 100 µH Q3 IC5 D4 5819 ZD1 LMC6482 13k 150pF 100 µF IC4 dsPIC33FJ64GP802 10 µF 100nF IC3 74HC393 10 µF 33pF 100nF _ TO CON1 Q4 BC337 15 100Ω 10k 10 µF + 16 X1 24.576MHz 100Ω 10k S/PDIF OUT 100nF 1k 15nF CON7 CON8 33pF Fig.11: this is the alternative main board layout to suit the Altronics case. The circuit layout is almost identical to the Jaycar version but the mounting holes and output sockets are in slightly different locations. The diagram at top right shows how to wire the DC socket (both versions). Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o o No.   1   3   1   2   4   2   7   1   2   1   1   1   1   1   1   1   2   1   1   2 84  Silicon Chip Value 10MΩ 100kΩ 47kΩ 33kΩ 13kΩ 11kΩ 10kΩ 1.5kΩ 1kΩ 620Ω 560Ω 390Ω 220Ω 200Ω 180Ω 150Ω 100Ω 10Ω 5.6Ω 1Ω 4-Band Code (1%) brown black blue brown brown black yellow brown yellow violet orange brown orange orange orange brown brown orange orange brown brown brown orange brown brown black orange brown brown green red brown brown black red brown blue red brown brown green blue brown brown orange white brown brown red red brown brown red black brown brown brown grey brown brown brown green brown brown brown black brown brown brown black black brown green blue gold brown brown black gold gold 5-Band Code (1%) brown black black green brown brown black black orange brown yellow violet black red brown orange orange black red brown brown orange black red brown brown brown black red brown brown black black red brown brown green black brown brown brown black black brown brown blue red black black brown green blue black black brown orange white black black brown red red black black brown red black black black brown brown grey black black brown brown green black black brown brown black black black brown brown black black gold brown green blue black silver brown brown black black silver brown siliconchip.com.au Table 2: Capacitor Codes Value 150nF 100nF 15nF 150pF 68pF 33pF µF Value 0.15µF 0.1µF 0.015µF NA NA NA IEC Code EIA Code 150n 154 100n 104   15n 153 150p 151   68p   68   33p   33 but leave the ICs out of their sockets for the time being. Building the control board Fig.12 shows the parts layout for the control board. Start by installing the seven 1N4148 small signal diodes (D5-D11). They all face in the same direction. That done, install the IDC socket with the orientation shown, then install the 5.6Ω resistor and the single 100nF MKT capacitor. The way in which the LCD is mount­ ed depends on which case you are using. Don’t remove the protective plastic from the top of the LCD yet. (1) Jaycar case: if you are using the Jaycar case, begin by fitting M3 x 6mm machine screws through the holes on the control board (ie, either side of the LCD position), with the head on the copper side. Next, thread an M3 nut onto each screw until it is tight and screw an M3 x 9mm tapped Nylon spacer down on top. The LCD connects to the PC board via male and female 16-pin headers. For the time being, just loosely insert the long pins of the male header into the female header. (2) Altronics case: for the Altronics case, first fit an M3 x 6mm screw through the holes on either side of the LCD position (head on the copper side), then screw on an M3 x 9mm Nylon spacer (ie, no nut). That done, use a pair of pliers to pull the pins out of the plastic spacer of the male pin header. These must then be fully inserted, one at a time, into the holes of the female header. When you are finished, you can discard the leftover plastic spacer. Mounting the LCD You are now ready to mount the LCD. Begin by placing the female header’s pins into the row of 16 holes on the PC board, then sit the LCD on top. The upwards-facing pins should siliconchip.com.au This is the view inside the Jaycar case after the main board, power switch (top right), DC socket (top left) and cell holders have been installed. The cell holders are connected in series. fit into the corresponding row of holes on the LCD board. You then secure the LCD module using M3 x 6mm machine screws which go through the LCD module and into the Nylon spacers. It’s now just a matter of soldering the pins on the underside of the control board and on the top of the LCD module. There are 32 in all, so don’t miss any and be careful to avoid shorts between them. time to install the seven tactile switch buttons. Their bases are rectangular, so you can’t install them the wrong way. To install each switch, first insert its angled pins through the holes and push it down so that it sits flat against the PC board. Check that the shaft is as close to vertical as possible, then solder all four pins. Be careful that the buttons don’t move when you turn the assembly over to solder them. Pushbutton switches Completing the control board Now that the LCD is in position, it’s Having fitted the switches, the conApril 2010  85 CON9 16 15 2 1 16X2 LCD MODULE Z-7013 (B/L) 5.6 ALTRONICS 14 13 12 11 10 9 8 7 6 5 4 3 2 1 16 15 100nF LEFT CH MUTE S2 S1 D6 D8 D9 4148 RIGHT CH MUTE UP LEFT 4148 4148 RIGHT SELECT S5 S4 S3 S6 4148 4148 4148 4148 D7 D5 D11 D10 DOWN S7 Fig.12: follow this parts layout diagram and the photo at left to build the control board. Note that the mounting arrangements for the LCD depend on the case you will be using – see text. trol board can be completed by fitting its mounting screws and spacers. Begin by inserting an M3 x 15mm machine screw through each of the four corner mounting holes (with the head on the copper side), then thread an M3 x 9mm tapped Nylon spacer over each screw and tighten it down. When that’s done, each screw should protrude about 4mm beyond its spacer. The next step depends on the case you are using. If you have the Jaycar case, simply screw an M3 x 12mm Nylon spacer down over each exposed screw. Alternatively, for the Altronics case, fit two M3 star washers over each screw, then screw down another M3 x 9mm tapped Nylon spacer on top. When you have finished this step, the spacer ends should be slightly above the level of the LCD. You can now remove the protective plastic coating from the LCD and place the control board to one side while you run some basic tests. Testing the main board It’s a good idea to test the main board before going any further. You can either use a bench supply (set at 9.5V with a current limit of 150mA) or a 7.5-10V DC plugpack with an ammeter in series. If you are going to use an unregu86  Silicon Chip lated plugpack, 7.5V may be the best choice since it will deliver a higher voltage due to the relatively light load – probably at least 9V. Check with a voltmeter if you are unsure. If the noload voltage output is above 9V and the current rating is at least 500mA it should be fine. Place a shorting link (or “jumper”) across the switch pin header (without it, the circuit will not turn on). Also, make sure none of the ICs are installed in the sockets. Next, apply power via the external DC header (CON1) – not the battery header – and observe the current reading. It should be less than 10mA. If that checks out, measure the voltage at the output of each TO-220 regulator using your DMM. In each case, place the black probe on pin 2 or the tab and the red probe on pin 3. You should get readings of 6.8V ± 10% for REG1 and 3.3V ±5% for REG2. If any reading is wrong, switch off immediately and check that all parts have been installed correctly. Now measure the voltage between pins 6 & 4 of the socket for IC1 (MC34063). It should be close to 5.0V. If it is below 4.7V or above 5.2V, then check the voltage across ZD1. Because low-voltage zener diodes have a relatively high impedance, you will find it is well below its rated voltage of 5.1V. We want it to be around 4.3V. If yours does not read between 4.0V and 4.5V then that will be the reason for IC1’s voltage reading being out of range. In that case, you will need to try a different zener diode with a different voltage rating or try one from a different manufacturer. The most likely types to be suitable are 4.7V and 5.1V zeners but unfortunately there is no easy way to tell without measuring it. Once the supply voltage is within the acceptable range, turn the power off and install IC1 (MC34063), ensuring its orientation matches the socket. If you are using a bench supply, set it to 7.0V, reapply power and again check that the current is less than 10mA. Now measure the voltage between pins 8 & 4 of IC5’s socket. It should be 5.0V ± 5%. If not, there is a problem with the MC34063 IC or the surrounding components. Turn the power off again and install IC4 (dsPIC33). Make sure it has been programmed with the appropriate software and that it is installed with the correct orientation. Also, install the three remaining ICs – IC2, IC3 and IC5. Don’t get the 74HC04 and 74HC393 ICs mixed up as they have the same number of pins. siliconchip.com.au This is the view inside the prototype using the Jaycar case. The main board mounts in the base, while the control board is installed on the lid and the two connected via a ribbon cable and IDC connectors. The photo below right shows the digital and analog outputs at the top of the case. Now reapply power and check that the current is below 150mA. In fact, it should be close to 100mA. If you have a frequency counter, measure the frequency at pins 6 & 8 of IC3 (74HC393) relative to pin 7 (ground). Pin 6 should read 705.6kHz and pin 8 should read 1.536MHz. If not, check the crystal oscillators and the circuitry surrounding IC3 for mistakes. If the pin 6 reading is correct but the pin 8 reading is not, there could be a problem with IC4’s (dsPIC33) oscillator circuit. Check its power supply. Testing the outputs The next step is to test the analog outputs. When powered up for the first time, both channels should output a full scale (1V RMS) 1kHz sinewave after a couple of seconds. You can test them by connecting them to an oscilloscope or to an audio amplifier. If you use an amplifier, make sure its volume is turned well down before applying power to the signal generator. If they do not work properly, check the circuitry around IC5 (LMC6482). To test the digital outputs (S/PDIF & TOSLINK), connect them to a DAC siliconchip.com.au or to an amplifier with digital inputs, again being careful with the volume. If neither output works, the dsPIC33 may not be programmed correctly or it may not be functioning due to incorrect parts placement or an incorrect power supply. Testing the control board Now that the main board is working, it is time to connect the control board. First, you will need to make a ribbon cable. One option is to use an IDC crimping tool such as the Altronics T1540 or Jaycar TH1941 but if you do not have one, a vice can do the job of squeezing the two sections together. One 16-pin IDC connector should be attached to each end of the ribbon cable, on opposite sides and with the plastic tabs facing out from the middle, as shown in Fig.13. Don’t forget to feed the cable through the top of the connector first before looping it around to the blades below (the DAC project showed this the wrong way around – my mistake – although if you aren’t putting a lot of stress on the cables it doesn’t really matter). Once you have made the cable, it’s a good idea to plug it into both boards and use a DMM set to continuity mode to check that all the corresponding pins on the two PC boards are electrically connected. If you haven’t crimped the cable with sufficient force, some of the blades may not pierce the insulation properly and those wires will read as open circuit. If any lines are open circuit you will need to crimp the connectors harder, or make up a new cable. Once you are sure that the cable is OK, leave it connected to both boards and reapply power. If you are using a bench supply, you should increase the current limit setting to 300mA, as the LCD backlight will draw additional current. As soon as power is applied, the LCD backlight should turn on and some text should be visible. The current should be in the range 120-150mA. Initially, the display contrast will probably be too high but that’s because we’ve erred on the side of caution to cater for any April 2010  87 150mm 16-WAY IDC SOCKET 16-WAY IDC SOCKET (170mm LENGTH OF 16-WAY IDC RIBBON CABLE) CABLE EDGE STRIPE Fig.13: here’s how to make up the IDC header cable that connects the two PC boards. Be sure to orient the header sockets exactly as shown. You should also leave about 15mm free at each end so that the cable can be looped back and clamped with the locking bar. variations between the panels. If you don’t see anything on the display, check the cable continuity again, as well as the components and solder joints on the control board. It’s also worth checking the components in the contrast control circuitry on the main board (ie, the circuitry associated with transistor Q5). If all the hardware seems OK, then you may have a faulty microcontroller or LCD module. Once it is working, try pressing the “Up”, “Down” and “Select” buttons and check that the display changes each time you do. Now turn the power off and then on again and try the “Left” and “Right” buttons. You should see a cursor appear on the display that you can move around. Finally, press the “Left Mute” and “Right Mute” buttons and check that the display changes when you do. If your boards pass all these test, they are working properly. Adjustments & calibration There are few tweaks that have to be made before the unit is installed in its case. However, be careful not to let the bottom of the control board short against the main board while you do this. First, you should adjust the LCD contrast to its optimum setting. To do this, turn the unit off and then on again, then press the following sequence of buttons: Select, Up, Up, Left. The display will show the current brightness and contrast settings and you can now use the Up and Down buttons to adjust the contrast. Once you have found a good setting, press Select, Up, Right. The display will now read “3.3V Cal.: 3.300V”. When it does, carefully measure the 88  Silicon Chip output of the 3.3V regulator (REG2) – ie, black probe on pin 2 or the tab and the red probe on pin 3. Once you have taken the reading, use the Left/Right buttons to move the cursor and the Up/Down buttons to change the digits on the display until it is as close as possible to the measured voltage. Finally, press: Select, Down, Down, Left, Up. The display should read “Saved”, indicating that the settings have been saved to the dsPIC33’s flash memory. Performing this calibration routine maximises the accuracy of the microcontroller’s ADC readings, as they are measured relative to the 3.3V supply voltage. Once calibration is complete, remove the shorting jumper from the switch header. Preparing the case The main PC board is designed to fit into a sealed polycarbonate enclosure with a transparent lid – either the Jaycar HB-6218 (171 × 121 × 55mm) or the Altronics H-0330 (186 × 146 × 75mm) In each case, the transparent lid saves you the effort of having to cut a neat rectangular hole for the LCD to be visible. These polycarbonate enclosures are also quite sturdy. The main board mounts on posts which are moulded into the bottom of the box. It is necessary to drill or cut holes for the outputs (three for the RCA sockets and one for the TOSLINK transmitter), a hole for the power switch and one for the DC connector. If you are building the project from a kit, then it’s likely that the case will be supplied pre-drilled. If not, then you will have to drill the holes yourself. Fig.14 shows the drilling details for the Jaycar case and this can be photocopied and used as a drilling template. Alternatively, the equivalent diagram for the Altronics case can be downloaded from the SILICON CHIP website (note: Altronics will be supplying a kit with a pre-drilled case). Once the template is in place, it is a good idea to temporarily place the main board inside the box and check that the sockets line up correctly with the indicated hole positions. When you are sure it is correct, remove the PC board and drill a small pilot hole in the centre of each RCA socket position. Also drill a small hole inside each corner of the TOSLINK connector outline (make sure that these do not go outside its outline). By the way, there is a simple way to accurately drill holes in the plastic. At each location where you want to drill a hole, press the sharp point of a hobby knife there and rotate it several times, until you have made a small divot in the plastic. This will guide the drill bit and prevent if from slipping. Even if you are using a drill press, this simple technique will help to initially guide the bit. Having drilled the pilot holes, remove the template and place the PC board back inside the box. Slide it up against the pilot holes and check that they are correctly aligned. You can do this by inserting a piece of wire into each hole and checking that it passes through the centre of the corresponding socket. If any holes are misaligned, then now is the time to correct the situation. When they are correctly lined up, use a stepped drill bit or a series of increasingly larger bits to enlarge the RCA socket holes. A tapered reamer siliconchip.com.au can then be used to get the size just right (about 10mm). Making the cut-out Main Board Code The rectangular cut-out for the TOSLINK transmitter is made by first drilling a series of small holes around the inside perimeter, using the four corner holes you drilled earlier as a guide. It’s then just a matter of knocking out the centre piece and filing the job to a smooth finish. During this process, you can test fit the PC board to determine which sides need further filing. Continue this process until the connector is a neat fit. Note that because of the thickness of the box, we’ve had to put the TOSLINK connector closer to the edge of the PC board than it is supposed to be. This means that the wider rear portion has to fit through the cut-out too. So, if it looks like it should fit but it won’t go all the way in, it is probably the larger rear portion which is getting stuck. When you are finished, the PC board should slide right up against the end of the case and the mounting holes on the board should line up with the posts. The TOSLINK transmitter face should sit flush (or nearly so) with the outside wall of the case. The parts list last month listed the main board as 04203101. This is correct for the Jaycar version only. The alternative Altronics version is coded 04203103. 49 A A 30 30 28 28 72.5 B B B 15 B 15 B A 15 B 15 B A Power switch & socket Before finally installing the main board, you also have to drill the holes for the power switch and DC socket. The recommended switch type is a 20mm round rocker type but you can use a different type if you like (eg, a sub-miniature toggle switch, as used in our second prototype). The main thing to keep in mind is that the internal portion of the switch needs to clear the main PC board and its components. In the Jaycar case, that’s done by mounting the switch on the side, between the PC board mounting holes. The DC power socket is mounted on the side opposite to the power switch (see photos). Once you have marked their positions, remove the PC board and drill two pilot holes. That done, enlarge the holes to the correct sizes using a tapered reamer – 20mm for the rocker switch and 7.5mm for the DC socket. If you use the same switch we did, it will also be necessary to file a small notch in the top of the mounting hole. This is because the switch has a tab to stop it rotating. It doesn’t take long to siliconchip.com.au (BOX LID) CL ALL DIMENSIONS IN MILLIMETRES HOLES A: 3.175mm DIA (1/8") HOLES B: 8mm DIA. HOLES C: 10mm DIA. 27.75 19 13 C C 16 23 C 11 11.5 5.5 (UPPER END OF BOX) Fig.14: this diagram shows the drilling details for the Jaycar case, while a similar diagram for the Altronics case can be downloaded as a PDF file from the SILICON CHIP website and printed out. The relevant diagram can be attached to the case and used as a drilling template. April 2010  89 of enlarging the mounting holes to 3mm, while the switch holes should be carefully enlarged using a series of slightly larger drill bits to 8mm. If you prefer, you can use M3 x 10mm countersunk screws (Altronics H3127A) to attach the control board to the lid. If so, you will need to countersink the mounting holes. Alternatively, you can use ordinary pan-head bolts. Installing the control board This second prototype of the Digital Audio Signal Generator has been built into the Altronics case. Note that the pushbutton switch functions are screen printed on the control PC board (both versions). file away enough material and when you are finished, the switch should snap into the panel. Alternatively, for the Altronics case, the switch and DC socket are mounted on the end panel, on either side of the output sockets. The locations of these holes are indicated on the drilling template (download it from the SILICON CHIP website). Installing the main board Now you can slide the main PC board into place and secure it using four M3 x 6mm machine screws. If necessary, temporarily remove the power switch to do this, then reinstall it once the board is bolted down. With the Jaycar case, two of the posts are pre-tapped with metal inserts but the other two are not tapped at all. It takes a large driver and a great deal of force to force an M3 machine screw into these untapped posts (as we did), 90  Silicon Chip so you may prefer to use two small self-tapping screws instead. The Altronics case comes with four self-tapping screws to suit its untapped posts. We’ve provided extra holes in the PC board for the additional posts even though four are enough to hold the board rigidly in place. To install the control board, first press a button cap down over the end of each switch shaft. Make sure that they are all firmly attached, although full engagement is easiest once the board is in place. You should also attach the ribbon cable to the control board at this point, since it’s almost impossible to do it once the board is bolted to the lid. Leave the other end unplugged for now. Once that’s done, it’s just a matter of fitting the control board into place and securing it using four M3 x 10mm machine screws. Note that it may be necessary to slightly loosen the mounting spacers on the board to get them to line up with the mounting holes. They can then be re-tightened once the mounting screws are installed. When the board is secured in place, press down firmly on each button cap to ensure it is fully engaged with its switch shaft. When that is done, they should protrude through the lid by 1mm or so. Front & top panel labels If you buy a kit, it will probably come with the case screen printed. If not, you can download labels to suit your case from the SILICON CHIP website. The switch functions are screenprinted on the control PC board. Drilling the lid Battery holder Fig.14 also shows the drilling details for the transparent lid (Jaycar version). There are 11 holes in all – four to mount the control board and seven for the pushbutton switches. The mounting holes (marked “A”) are all 3mm in diameter while the switch holes (“B”) are 8mm diameter. As before, you should first attach the template and then use a sharp scriber or hobby knife to mark the centre of each hole. The template can then be removed and small pilot holes (say 1.5mm) drilled. It’s then just a matter The signal generator is designed to run from a plugpack or from four 1.5V cells, typically alkaline or NiMH. We used ultra-low self-discharge NiMH cells in our prototypes, so that they don’t go flat if the device is not used for some time. Note that if you elect to use alkaline cells, then the 1W charging resistor must not be installed on the main PC board (see panel). Unfortunately, side-by-side 4 x AAcell holders are not easy to obtain, although 4 x AAA-cell holders are siliconchip.com.au common. Of course, you can use AAA cells but battery life will be less than half that of AAs. The best approach is to use a pair of side-by-side double AA-cell holders wired in series. These can be secured to the base of the box using two strips of double-sided tape each or they can be secured using countersink screws. It’s best to attach the leads before installing the holders. Attach a red wire to the positive terminal and a black wire to the negative terminal. If you are joining multiple holders in series, do that now. Once everything is in place, connect the leads from the holders to the main PC board as shown in the wiring diagram. If you are using the 2-pin polarised headers, it’s best to crimp and then lightly solder the wires to the connector pins before pushing them into the plastic block. There is slightly more room for the battery holder in the Altronics box, so we used a 4 × AA holder with an integrated switch and lid. We did not install the lid since it would complicate access to the batteries should they require removal. Note that because the switch is on the opposite side to the lid, it was necessary to use thicker foam-cored double-sided tape to attach it. The holder has integrated leads, so it is only necessary to attach them to the header connector before plugging it in. Wiring the DC socket We have specified a 2.1mm DC socket since this is the most common type for plugpacks. However, a 2.5mm type is also available if that’s what your plugpack’s connector requires. The polarity of a DC connector isn’t always obvious, so it’s best to check the plugpack itself using a DMM. To do this, connect the plugpack to the mains and then place the DMM’s red probe into the hole on the connector and touch the black probe to the outer metal ring. If you get a positive voltage, then your plugpack is centre (tip) positive; otherwise it is centre negative. For a centre-positive plugpack, connect the leads to the DC socket as shown on Fig.11, ie, red lead to the centre pin’s solder tab and the black lead to the adjacent tab. Conversely, for a centre-negative plugpack, reverse the red and black wires. Once you’ve soldered the leads to the DC socket, the free ends can be siliconchip.com.au The Altronics version has the power switch and DC power socket mounted at one end of the case, along with the analog and digital output sockets. terminated in another 2-pin polarised header. Be sure to make the leads long enough to reach CON1. Power switch Almost any type of on/off switch can be used. The recommended switch is an SPST type but it doesn’t matter if it is DPST or DPDT. Note that because the switch goes after the 7805 regulator on the main board, a small amount of power (at least 3mA) will be drawn from the plugpack even if the generator is switched off. This is so that the battery can trickle charge if you are not using the unit. If your switch has spade terminals, crimp two 4.8mm female spade connectors onto appropriate lengths of wire and then attach the other ends to the remaining polarised header connector (it doesn’t matter which way around they go). The spade connectors can then be fitted to the switch terminals and the connector plugged into the main board. If the switch has solder tabs instead of spade terminals, just solder the leads directly to it. Finishing it Now for the final steps. First, ensure that the power switch is off, then install the battery cells. That done, plug the ribbon cable into the main board and fasten down the lid of the case. Because the switch header connector on the main board only just clears the underside of the control board, you may need to fold the ribbon cable slightly to the left, so that it doesn’t get sandwiched between them. If it does, the lid won’t sit properly and screwing it down could bend the board. Also, check that there is no uninsulated copper where the leads exit the polarised header connectors. If there is, it could short to the underside of the control PC board. If there is some exposed wire, you will need to insulate it with electrical tape or heatshrink tubing. Finally, it’s a good idea to use the neoprene seal provided with the case, even though it is no longer water-tight thanks to the various holes. However, the seal will help keep the lid on tight. That’s it – construction is complete. Switch the unit on and make sure it works as expected. If not, remove the lid, unplug the ribbon cable from the main board, and check that the cells have been installed correctly and that the power switch is wired correctly. That’s all we have space for this month. It Pt.3, we will explain how to use the various modes and describe SC the various features in detail. April 2010  91