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Touch-Screen Digital Audio Recorder Pt.2 By ANDREW LEVIDO Last month, we introduced our new TouchScreen Digital Audio Recorder and described how it worked. This month, we will give the assembly details, provide some performance graphs and describe how the unit is used. A LL THE PARTS for the TouchScreen Digital Audio Recorder (except the battery) are mounted on a single PCB coded 01105141 (106 x 74mm). Most of these parts and the connectors are surface-mount types which are installed on the top side of the PCB. Only the microphone, touchscreen display and the SD card socket are located on the underside. Don’t be put off by the surfacemount nature of this project. With a little bit of care and patience, any reasonably experienced hobbyist can put this together in a few hours. As with any surface mount work, you will need a good work light, fine tweezers, a flux pen and a roll of solder wick. If you have some form of magnification (or a magnifying lamp), then so much the better. 70  Silicon Chip Fig.6 shows the parts layout on the PCB. Begin with IC2 (LM3658). This is a 10-pin QFN (quad flat no-lead) package with 0.5mm pin spacing and a central thermal pad. Once you have successfully soldered this device in (as described below), you can consider yourself an SMD guru! The technique is as follows: start by applying a small amount of solder to each of the 10 outer pads on the PCB. You want just enough solder to later re-flow under the pins but not so much that the pads are bridged or the chip does not sit flat against the board. Use solder wick to take off any excess if you have to. Next, apply some liquid flux to the pads, then carefully position the chip so that it is centred horizontally and vertically with respect to the pads (take care to ensure that it is orientated correctly). Check that the pins, which are just visible on the sides of the chip, line up with the elongated pads. This is best done with the aid of a magnifying lens (or a microscope if you have access to one). Once it’s in place, hold the chip down using a pair of tweezers and apply your soldering iron to the elongated pads. Heat them just long enough for the solder you applied earlier to melt and re-flow under the pins. Add a small amount of solder during this procedure if necessary. Once you have done this for all pins, flip the board over and melt a blob of solder into the four holes under the thermal pad. You will have to apply quite a bit of heat here, since the large rectangle of copper on the bottom side siliconchip.com.au Microphone bias You must now decide if you want the microphone bias to be available to the external microphone. If you do, install a link in the position marked “BOTH” (near CON2); otherwise install a link in the position marked “INTL”. You can either use a 0Ω (0805) resistor (as we have) or just bridge the pads with a blob of solder. Next, install the USB connector (CON7). First, place it on the board (make sure that the locating pins go into their holes) and solder the four mounting tabs. You will have to apply a fair bit of heat to get the solder to take to these. It’s then just a matter of soldering the five pins. These protrude siliconchip.com.au 1 CON5 INT MIC CON1 ICSP AUDIO RECORDER 100k 4.7M 2.2 µF IC3 PIC32MX695– F512H 2.2 µF 2.2 µF 5.1k 5.1k Q4 01105141 4.7M NTR4170N 100nF 2.2 µF 1 is designed to be a heatsink. This step will not only solder the thermal pad but will also continue the re-flow process for the pads. Alternatively, if you are lucky enough to have a hot-air rework tool, simply apply a little solder paste to the pads, position the chip and re-flow the solder with the hot air. Surface tension will help pull the chip into position if your placement was not perfect. This is a lot faster and less prone to errors than using a soldering iron, so if you do a lot of SMT work it is an investment worth considering. Note, however, that once the chip is in place, you still have to flip the PCB over and melt solder into the holes under the thermal pad. Microcontroller IC3 is the next on the list. Apply a generous amount of flux, then carefully position it on the pads (make sure it’s correctly orientated) and tack solder a couple of pins on opposite corners. Now check that it’s perfectly aligned and adjust if necessary before soldering the remaining pins. Don’t worry if you end up with a few solder bridges – you can clean these up later with solder wick. The CODEC (IC1) can now be installed. With its 0.65mm pin spacing, it will seem positively huge compared to the last two chips. Use the same technique as for the microcontroller. Regulator REG1 is the easiest of the lot. Once it’s in, install the four SOT-23 Mosfets and then the passive SMDs (ie, the resistors and capacitors). These are 0805 size and so are fairly straightforward to solder by hand. Take care here because the capacitors are not marked and it’s easy to mix them up. X1 12MHz Q1 NTR4170N 22pF 2.2 µF 5.1k MIC IN CON7 USB 100nF BOTH INTL 2.2 µF MIC BIAS 10nF REG1 Q2 NTR4170N 10k 1M IC1 5.1k 1M TLV320AIC23 100k IC2 2.2 µF 100nF Q3 NTR4170N 10Ω 2.2 µF 100k 100nF 2.2 µF100k 1M LM3658 100k 470Ω100k 1 100k 100k 100k 100k 100nF 1M X2 22pF CON3 LINE OUT LED1 ACTIVITY 2.2 µF 22pF CON2 22pF 2.2 µF 22pF 2.2 µF MCP1725 2.2 µF LINE IN 22pF 5.1k 5.1k 100 µF 100 µF A CON4 1M TEST PHONES OUT 100 µF 32kHz (TOP OF PCB) BATTERY CLIP LEADS INT MIC + CON6 LCD CONNECTION CABLE FOLDED UNDER TO ATTACH TO PCB HERE STRIPS OF DOUBLE-SIDED ADHESIVE TAPE ATTACHING LCD DISPLAY TO UNDERSIDE OF PCB QVGA 64K COLOUR TFT LCD DISPLAY WITH LED BACKLIGHTING SD CARD SOCKET (UNDERSIDE OF PCB) Fig.6: install the parts on the PCB as shown on these layout diagrams. The three SMD ICs and regulator REG1 go in first (see text), followed by the passive SMDs (resistors & capacitors) and then the larger through-hole parts. The LCD, SD card socket and internal microphone are mounted on the underside of the PCB, as shown in the bottom layout. just enough from back of the connector to get at with a soldering iron. Follow with the four 3.5mm audio jacks (CON1-CON4), then fit the through-hole electrolytic capacitors and the two crystals (X1 & X2). We used one of the capacitor lead offcuts to make a small strap to hold the 32kHz (X2) crystal in place. It’s a good idea to also solder two more lead offcuts to the pads on the back of the electret microphone insert. This makes it easy to install later on. The final step for this side of the board is to mount the ICSP header (CON5) if you intend to program the micro yourself. There’s no need to mount the test header – it was used during development as a handy way to get access to the data traffic between the micro and the SD card. SD card socket & LCD As stated earlier, the SD card socket and the touch-screen LCD are mounted on the underside of the PCB (see Fig.6). Start with the SD socket (CON6), as it’s difficult to get to some of its pins once the LCD is installed. This socket shares its locating holes with two of July 2014  71 These views show the assembled PCB. The touch-screen LCD is installed by first soldering its ribbon cable to its PCB pads, then folding the LCD over this cable and securing it using strips of double-sided adhesive tape (see Fig.6). the audio jacks. There are four pads to solder around the outside and 11 on the inside. Mounting the LCD involves soldering its flexible ribbon cable to corresponding pads on the underside of the PCB. The two 1mm holes at either edge of the ribbon are used to line everything up. Take a minute or two to familiarise yourself with the correct orientation of the display before soldering any pads; it should be positioned such that it can fold back over the ribbon cable to face up as shown in Fig.6. To install it, first peel the cover paper off the strip of double-sided sticky tape on the ribbon (not the strips on the back of the display). That done, line up the two 1mm holes in the ribbon with their corresponding holes in the PCB (we used a couple of 1mm drill bits to make this easy), then press down on the ribbon to fix it onto the PCB with the tape (TOP OF CASE) B A 58 7.8 16 31 3.7 30.5 B 22 35 12.5 24 C C L 11.5 58 x 44mm CUTOUT FOR LCD DISPLAY/TOUCH PANEL 8.5 8.5 3.0 B 22 A B 5.3 (FRONT PANEL) HOLES A: 3.0mm DIA. HOLES B: 4.5mm DIA. HOLE C: 3.75mm x 8.0/6.5mm ALL DIMENSIONS IN MILLIMETRES Fig.7: follow this diagram to make the display cutout in the case lid and to drill and cut the holes in the end panel. The display cutout can be made by drilling a series of holes around the inside perimeter, knocking out the centre piece and filing the job to a smooth finish. Hole ‘A’ at top left is for the microphone. 72  Silicon Chip siliconchip.com.au Parts List The ribbon cable can now be soldered to the pads. Work quickly here, as the ribbon will melt if you apply too much heat. We found it best to press the ribbon down onto the PCB with a probe, then solder three or four pads at a time, moving progressively along the ribbon. Once the soldering is complete, fold the display over the ribbon and secure it to the PCB using the two strips of double-sided adhesive tape on the back of the panel. During this process, make sure that the LCD panel’s four plastic posts (one at each corner) drop into their locating holes in the PCB. Now fit the electret microphone insert (ie, with the wire leads soldered to it) to the PCB, observing the polarity. Don’t solder it yet though; instead, position the PCB in the top half of the case and temporarily fix it there with a couple of mounting screws. That done, push the microphone down until its face is flush with the inside of the case, then solder its leads and trim off any excess. The PCB assembly can now be completed by soldering the battery clip leads to the battery terminals (marked B+ & B-). Be sure to loop these leads through the strain relief holes as shown in Fig.6. Preparing the case Now turn your attention to the case. First, carefully mark out the LCD cutout and the microphone hole on the siliconchip.com.au 1 double-sided PCB, code 01105141, 106 x 74mm 1 black hand-held ABS case with battery compartment, 89 x 147 x 25mm (Altronics H8986) 1 12MHz HC-49/US SMD crystal (X1) (Altronics V2267, element14 1842280, Digi-Key 535-10218-1-ND) 1 32.768kHz watch crystal (X2) 1 3mm orange or red LED (LED1) 1 QVGA RGB LCD touch-screen with LED back-light and controller (Altronics Z7080) 4 3.5mm switched SMD stereo jack sockets (CON1-CON4) (Digi-Key CP-3524SJCT-ND) 1 5-pin header (CON5) (optional, for in-circuit programming) 1 push-push SD card socket (CON6) (Altronics P5720 or equivalent) 1 SMD mini-B type USB connector (CON7) (Altronics P1308, element14 1507528, Digi-Key 151-1206-1-ND) 1 electret microphone insert (MIC1) (Jaycar AM4008) 4 No.4 x 6mm self-tapping screws 1 short length light duty red hookup wire 1 short length light duty black hook-up wire 1 3.6V Lithium-ion AA cell 1 AA cell sized piece of nonconductive foam 1 SD/SDHC/SDXC card 1 USB type A to mini type B cable 1 short length double-sided tape lid as shown in Fig.7. That done, cut out the rectangular hole for the LCD and clean up the edges using a fine file. We chamfered the edges of the cut-out to improve its appearance, as shown in the photos of the unit. Next, drill the 3mm-diameter hole for the microphone, then mark out and drill the end panel according as shown in Fig.7. Alternatively, you can simply attach the end-panel diagram to the panel and use it as a drilling template. The cut-outs for the USB socket and the SD card socket can be made by drilling a series of very small holes, joining them up and then filing the job to shape. Once completed, test fit the panel to the PCB – you may need to fettle the socket openings to get everything to line up nicely. Be careful when drilling and cutting Semiconductors 1 TLV320AIC23 96kHz audio CODEC IC (IC1) (element14 1575048, Digi-Key 296-26817-1ND) 1 LM3658SD Li-ion battery charger IC (IC2) (element14 1312584 or Digi-Key LM3658SD/NOPBCT-ND) 1 PIC32MX695F512H SMD microcontroller programmed with 0110514A.HEX (IC3) 1 MCP1725-3002E/SN LDO 3.0V regulator (REG1) (element14 1851958 or Digi-Key MCP17253002E/SN-ND) 4 NTR4170N SMD N-channel SOT-23 Mosfets (Q1-Q4) (element14 1887064 or Digi-Key NTR4170NT1GOSCT-ND) Capacitors (all SMD 2012 size [0805 imperial] unless specified) 3 100µF 6.3V electrolytic 13 2.2µF 16V X7R ceramic 5 100nF 50V X7R ceramic 1 10nF 50V X7R ceramic 6 22pF 50V C0G/NP0 ceramic Resistors (all SMD 2012 size [0805 imperial]) 2 4.7MΩ 6 5.1kΩ 5 1MΩ 1 470Ω 10 100kΩ 1 10Ω 1 10kΩ 1 0Ω Where to buy a kit: a kit of parts will be available for $139.95 from Altronics (Cat. K 5530). It will have SMDs IC1IC3 pre-soldered to the PCB. out this end panel. There are a lot of connectors in a very small space, so there is almost “more hole” than panel. Once everything is correct, fit the front panel to the PCB and then insert the assembly into the top section of the case. Secure the board with four short self-tapping screws, then fit two of the supplied battery terminals into the bottom half of the case. Solder the battery wires to these terminals, making sure that the positive lead goes to the positive terminal and the negative lead goes to the (spring-loaded) negative terminbal. We used a piece of foam to fill the open space in the left-over battery position, so the cell cannot move. Testing & Troubleshooting Now for the smoke test! The stepJuly 2014  73 READY PATH & FILENAME SD: DIR1 / DIR2 FILENAME.WAV FILE NAVIGATION + FILE + –6.0dB 00 : 15 : 36 FILENAME.WAV + + FILE SKIP TO NEXT/ PREVIOUS FILE – – PLAYING PHONES VOLUME (PRESS OR SWIPE UP OR DOWN) & MUTE RECORD/PLAY COUNTER & FILENAME (SWIPE LEFT/ RIGHT TO SKIP BACK/ FORWARD) –6.0dB + + –3.0dB 48kbps BACKLT – – – PLAY ONE/ ALL IN DIRECTORY CONFIG & USB MODE ABOUT RECORD & PLAYBACK CONTROLS 14:37:16 HIGH TIME, DATE & BATTERY STATUS 2014–05–16 SET RECORD SAMPLE RATE + INPUT SOURCE & GAIN – – CONFIGURE SET TIME & DATE 14:37:16 HIGH A SET BACKLIGHT LEVEL STATUS VIEW ABOUT SCREEN OK 14:37:16 2014–05–16 HIGH 2014–05–16 RETURN TO MAIN SCREEN C B Fig.8: the icons (or buttons) displayed on the touch-screen LCD are context sensitive and may be greyed out if not relevant to the current function. There are three main status screens: READY, PLAYING and CONFIGURE. by-step procedure is as follows: (1) Connect a bench power supply set to deliver 4V to the battery terminals (ie, without the battery installed). Be sure to get the polarity correct and if your supply has current limiting, set this to around 500mA. (2) Apply power and check that the LCD lights up and displays the various menus and icons. Of course, this assumes that you purchased a preprogrammed micro. If the micro didn’t come pre-programmed, now is the time to program it. (3) Once you have a working micro, insert an SD card and check that you 1 can move through the directories, play audio files and make a recording using the internal microphone. (4) If that checks out, remove the power supply, insert a battery and check that it begins to charge when the unit is connected to a USB port on a PC. (5) Press the USB button and verify that the audio recorder appears as an external drive on the PC. If this all checks out, you are ready to start using the Touch-Screen Digital Audio Recorder. If you do have problems, work logically to isolate the cause. If the display is black, for example, check the 3V rail and 12MHz Touchscreen Recorder THD vs Frequency 05/20/14 11:52:38 +10 05/20/14 11:35:13 +6 Headphone Out (8Ω -10dB) 0.2 +4 Headphone Out (16Ω -10dB) 0.1 +2 Headphone Out (32Ω 0dB) Amplitude Variation (dBr) Total Harmonic Distortion + Noise (%) Touchscreen Recorder Frequency Response +8 0.5 0.05 0.02 Headphone Out (32Ω -10dB) Line Out 0.01 Headphone Out (100kΩ 0dB) 0.005 Headphone Out (100kΩ 0dB) -0 Line Out Headphone Out (32Ω 0dB) -2 -4 Headphone Out (8Ω 0dB) -6 Headphone Out (16Ω 0dB) -8 -10 -12 -14 -16 Signal-to-noise ratio, both outputs <at> 0dB, backlight full on/off: 92dB unweighted, 96dB A-weighted 0.002 0.001 clock to make sure these are OK, since the micro can’t run without them. If the 32kHz oscillator is working, then you can be sure the micro itself is OK, so look for problems with Mosfets Q1 & Q2 and for soldering issues on the LCD ribbon cable and the corresponding pins on the micro. If the display works but you can’t read an SD card, check the components and soldering in that part of the circuit. Make sure that the SD card is formatted correctly and try an alternative card if there are still problems. Similarly, if there is a problem with the audio, check the circuitry around IC1. 20 50 100 200 500 1k 2k 5k 10k -18 20k Frequency (Hz) Fig.9: distortion vs frequency plots for the recorder. Note that since the headphone output performance (at 0dB) into a high impedance is better than the line output, there’s little reason to use the line output. We had to use a 20kHz low-pass filter for these measurements so distortion above 10kHz is understated; the dotted lines indicate our guess as to the real performance. 74  Silicon Chip -20 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.10: the frequency response is flat for either output up to 18kHz into a high-impedance load. However, the lowfrequency roll-off is significant when driving headphones (typical 16Ω headphones/ear-buds have a -3dB point of 100Hz). Note that bass performance could be improved by increasing the value of the two 100μF electrolytic coupling capacitors to 470μF. siliconchip.com.au If the activity LED pulses steadily while playing and recording, you can be reasonably sure that the problem lies with the CODEC or its surrounding circuitry. Any problems are most likely to be caused by incorrectly positioned parts, solder bridges or missed/faulty solder joints. Using the recorder The Touch-Screen Digital Audio Recorder is fairly intuitive to use. Most of the time, the user interacts with a single screen like that shown in Fig.8A. The “buttons” are context sensitive and may be greyed out if they are not relevant for some reason. For example, as shown in Fig.8A, the Stop button will be greyed if there is nothing playing or recording. A status bar is shown at the top of the screen and immediately below that are two lines of text (eg, to indicate the directory path and filename). In addition, the current time and date are displayed at the bottom of the screen, along with the battery charge/ discharge status. The READY status in Fig.8A indicates that an SD card has been successfully mounted. The file navigation keys can then be used move through the directories and files on the card. The file “rocker” moves through the entries in the current directory. The current entry (eg, a filename) is displayed on the lower of the two text lines and the path is shown on the upper line. If the current entry is a directory, the down arrow allows a sub-directory to be selected. Similarly, the up arrow button allow you to move up to the parent directory, if you are currently below the root directory. Immediately below the file navigation area are buttons to jump to the configuration screen and to put the recorder into USB mode. MSD mode allow the files on the recorder to be read on a PC via the USB port. To the right of this are the headphone volume control and the output mute button. The bottom row of buttons control the record and playback functions. Pressing the record button creates a new file and begins a recording, while pressing the Play button opens and plays the selected file. Once playing, the screen changes slightly to that shown in Fig.8B (the record screen is similar). At the same time, the status changes to PLAYING and a counter replaces siliconchip.com.au The PCB is secured to integral mounting posts inside the case using four self-tapping screws. the path name. In addition, the file navigation and USB mode buttons are disabled and the play/record controls change as shown. The action of the Pause button is fairly obvious – it toggles between pause and resume on alternate presses. The Skip button stops playing the current file and skips to the next one in the directory. When recording, the Skip button closes the current file and immediately opens another and carries on recording into that. Fig.8C shows the configuration screen. It’s entered by pressing the Configuration button on the main screen. At the top left is the line input gain rocker, while immediately below that is the input source selector. Pressing this repeatedly rolls through the three options: line input, mic input and mic input with 20dB extra gain. At top centre is the record sample rate selector and to its right, the backlight level control. Any change to the sample rate applies from the next recording. The About button displays the software version information (as you might expect) while the OK button returns control to the main screen. Either side of the OK button are buttons that allow setting of the time and date. If an error occurs at any time, a message is displayed at the top of the screen along with an Acknowledge button to dismiss it. The error message gives the reason for the error and a code indicating which part of the software was responsible. If a catastrophic error occurs, the activity LED will flash an error code. These error codes correspond to the processor exception codes except for a double flash that indicates the event queue has overflowed. These errors should never happen under normal circumstances and a full reset is the only way to recover. You can explore the rest of the user interface yourself and even attempt some software modifications of your SC own if you are so inclined. July 2014  75