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Want a 200kHz LCD Scope for about $40.00? One tiny catch: First you have to build it! You get the complete kit, as shown here, with the instruction manual. Inset below is the clear acrylic case, ready for assembly. Jim Rowe looks at the “Banggood” DSO138 LCD Scope Kit We’ve looked at some very cheap modules from China in recent months. Here’s one that more-or-less fits into the same category – except that for the princely sum of $40 (or less!), you get a real, working Digital Sampling Oscilloscope kit. It’s from Banggood and you really do get a good bang for your buck! S ixty or so years ago, the only way that most people could acquire even a basic oscilloscope was to build it yourself, using components salvaged from war-surplus equipment. Even then, by-and-large, most were only “audio” scopes with, perhaps, 10kHz bandwidth. Commercial scopes were so expensive that they could only be afforded by large manufacturers and research labs. That was back in the valve era and things soon began to change for the better when the solid-state revolution got under way. Before long the cost of commercial scopes started to fall, while at the same time their performance climbed steadily, especially in terms of analog bandwidth. But the real breakthrough came with digital sampling scopes and particularly when cheap LCDs (liquid-crystal displays) started to replace the costly CRTs (cathode-ray tubes) which formed the heart of all the early scopes. This eliminated the need for an expensive high voltage power siliconchip.com.au supply and also enabled many useful features to be added while the cost of scopes continued to plummet. So nowadays you can buy a wide selection of digital sampling oscilloscopes or “DSOs” at quite reasonable prices. Handheld, single channel, battery-operated units with 10MHz analog bandwidth are available for less than $370, while 4-channel 100MHz bandwidth benchtop models cost less than $800. You can even get a 2-channel 300MHz MSO (mixed signal oscilloscope) for around $3000. What if you only need a scope occasionally, and don’t need a lot of bandwidth – for example, if you’re mainly working on audio equipment? This would make it hard to justify an outlay of even $370. But this kit is around 1/10th of that amount! It’s known as the DSO138 and recently has become very popular all around the world. It’s available from online retailer Banggood, which has its main office and warehouse April 2017  53 Completed and working – here displaying its own 1kHz calibration square wave. We purchased the optional clear plastic case – the kit is also available without case for about $30.00, including postage from Banggood in China. in Guangzhou, China. The firm has over 1000 employees with offices in nearby Shenzhen and Hong Kong as well as Hangzhou and Yiwu, plus offices in the UK and USA. The electronics part of the kit is manufactured by the firm JYE Tech Ltd, based in Guangxi, China. The manufacturer of the laser-cut acrylic sheet parts which are used to make up its custom acrylic case (as shown in the photo of the completed unit above) is not specified but is presumably also in China. If you look at the Banggood website (www.banggood. com), you’ll find that the DSO138 kit is available in two forms: one with all of the SMD components already soldered in place on the main PCB, leaving only the throughhole components for you to fit and solder, and one with just the SMD CPU pre-soldered. We’re reviewing the version with all SMDs pre-soldered. It’s known as the 13803K (product ID 1051616), and is currently available for only AU$33.17, with acrylic case and with free “standard shipping” to Australia. The other version is known as the 13804K, and is currently available from Banggood for AU$41.30, also with case and free delivery to Australia (product ID 1051617). So it not only costs more, but you have more work to do building it up. You can see why we chose the 13803K version to review! By the way, both versions are also available without the matching assemble-it-yourself acrylic case. But if you 54  Silicon Chip want to buy it later, or you need to replace it, it’s available separately from Banggood for only AU$8.60 (at press time) including delivery (product ID 1034768). The electronics Apart from the DSO138’s 2.4-inch colour TFT LCD screen (320 x 240 pixel resolution), which is mounted on a small PCB of its own, all the rest of the kit’s electronics mount on a single PCB measuring 117 x 76mm. And as noted above, the PCB in the 13803K kit has all of the SMD parts already fitted: nineteen 0805 resistors, two ICs and an LM1117-3.3 LDO regulator in a TO-263 package. Everything else in the kit is in the form of through-hole components and connectors etc for you to fit yourself. The two ICs are a TL084 quad op amp, used for processing the analog input signal and generating trigger pulses, and the STM32F103C8 CPU which does everything else. The STM32F103Cx is in a QFP-48 package and is a 72MHz, 32-bit ARM Cortex-M3 processor, with the following features: 64KB of flash memory, 20KB of SRAM, two 12-bit/1MHz ADCs providing up to 16 analog input channels, seven different timers, a full-speed USB 2.0 interface, two I2C interfaces, two SPI interfaces (18Mb/s), three USART interfaces and a 7-channel DMA controller. So it’s quite capable of doing all of the sampling, display and other work needed to perform the functions of a basic audio DSO – with the right firmware, of course. siliconchip.com.au A close-up of the LCD screen before mounting in the case, showing a 10kHz square wave. No-one is pretending that it’s perfect – obviously not as good as you’d find on a multihundred (or multi-thousand!) dollar DSO . . . but for around $40.00, the DSO138’s performance is surprisingly good! Assembling the PCB is fairly straightforward because JYE Tech has provided the kit with a double-sided A4 instruction sheet with 22 numbered assembly step boxes – each one accompanied by a small but clear colour illustration. Each step also has a checklist, allowing you to tick each component’s box as you fit it. The instruction sheet has quite a few helpful hints, like advising you to check the value of each resistor with your DMM before soldering it into the PCB. This is good advice, because the coloured bands on the tiny 1/8W resistors are hard to see even with a magnifying glass. There’s also a guide to checking the voltages on the DSO138’s main PCB following assembly, attaching the LCD board and then giving it a basic functional checkout. And there’s a troubleshooting flowchart, an explanation of the self-test mode built into the firmware and a picture of the main PCB showing the location of all important test points. By the way, a PDF file of the instructions can be downloaded from the JYE Tech website (www.jyetech. com), so you can preview it before buying the kit and you can also download another copy if you lose or damage the original. There’s a second instruction sheet in the kit, intended to familiarise you with the DSO’s various controls and their use. In addition, there’s a section on using the built-in 1kHz squarewave signal to adjust the frequency compensation of Here are the main board (top) and LCD board (bottom, ready to be connected together and mounted in the acrylic case. With the SMD “bits” already soldered in place, it took just a few hours to assemble and get going. siliconchip.com.au April 2017  55 its input divider and any divider probe you connect to its input. There’s also a specification panel, plus a full schematic of the scope on the back of this second sheet. In short, the PCB has been carefully designed to be easy to build and get going. The JYE Tech website also has a four-page booklet you can download, explaining how to upgrade the firmware in its CPU, an 8-page booklet explaining how to use the library of functions built into its firmware and a single sheet showing an overlay diagram of the main PCB. Assembling the box Since the PCB assembly is so straightforward, you might expect the case would also come with clear instructions and that it would be easy to put together. But it isn’t quite that easy. All you actually get are the nine laser-cut pieces of acrylic sheet plus a small plastic bag with some control switch extension pieces moulded in red plastic, and some M2.5 and M3 machine screws and nuts. The acrylic pieces are covered in protective paper sheet on both sides but there is no information on how to put it together. Perhaps this is supposed to be self-evident but after a while I gave up and went to the Banggood website to look for clues. I subsequently found several links to YouTube clips showing the assembly of the DSO138 case. (See https://youtu.be/9vtHZP2_KAU). By playing the clip quite a few times – and pausing here and there as well – I was finally able to get the sequence right. (There are several other clips – simply go to YouTube and search for “DSO138 case”). There was one further little complication with regard to the red plastic control switch extension pieces. These allow you to operate the small slider switches and pushbuttons on the PCB when it’s mounted in the case. The extension pieces for the three slider switches were easy to identify, because they are T-shaped with a small locating slot moulded into the top centre of the “T” (which actually becomes the bottom of the extension). But the extension pieces for the five pushbuttons were harder to work out. Here the DSO138 is displaying a 10kHz sawtooth wave . . . 56  Silicon Chip Taken from Banggood’s website, this shows the assembled case without any content, to show you how all the outer pieces of the box fit together. They seemed to be I-shaped with a bump at both ends, and there seemed to be only three of them in the kit instead of the five I was expecting. Had someone made a mistake? No, because I eventually realised that each “I” piece was actually two small “T” pieces moulded together with a fine central groove which allowed them to be snapped apart. After this I didn’t have any problems, and it all went together nicely as shown in the photo. Performance Putting the completed DSO138 through its paces was a pleasant surprise. The analog bandwidth measured -1dB at 150kHz, -2dB at 175kHz and -3dB at very close to the 200kHz stated in the specification. So a 10kHz square wave signal displays quite nicely, . . . while here it’s a 10kHz triangle wave. siliconchip.com.au confirming that the DSO138 is practical as a basic scope for audio testing. The maximum real-time sampling rate is 1MS/s, with a vertical resolution of 12 bits and a record length of 1024 points. The input sensitivity range spans from 5V/division down to 10mV/division, while the input impedance is 1MΩ shunted by approximately 20pF – pretty much standard. The rated maximum input voltage is 50V peak (100V peak-to-peak), so for measurements in higher-voltage circuits you’d need to use it with a 10:1 divider probe. The timebase range is from 10s/division down to 500s/ division – more than adequate for an audio scope. There are three selectable triggering modes: Auto, Normal and Single(shot) and the trigger level is fixed at 50% but this should again be acceptable for primarily audio use. Incidentally, I don’t know if you’ll be able to see this in the photos, but although the active part of the DSO138’s 2.4-inch TFT LCD screen is fairly small (49 x 37mm), its resolution of 320 x 240 pixels with 262,144 colours results in a very sharp and well-defined display. I should also mention that the DSO138 is designed to run from a nominal 9V DC supply but since its current drain is around 120mA, it isn’t feasible to use a standard 216-type 9V alkaline battery. The simplest options are a 9V DC regulated plugpack or a pack of six AA or C size alkaline cells in series. However, JYE Tech also have a very small step-up DCDC converter module, the JYE140, which can be used to provide the DSO138 with 9V DC derived from a standard low-cost 5V DC plugpack or Li-Ion battery. Based on an MC34063 converter chip, the JYE140 has an output current siliconchip.com.au capability of 150mA, an output ripple of around 100mV at full loading and its output can be plugged directly into the rear of the DSO138. If you are interested in this option, the JYE140 converter can also be ordered online from the Banggood website for AU$5.15 (product ID 1000089), again with free delivery to Australia. At that price, you certainly wouldn’t bother to build it yourself! The final verdict Overall, I’m happy to give the JYE Tech/Banggood DSO138 scope kit a rating of 4.5 stars out of five. The electronics side of the kit is easy to put together and seems well-designed. Features like the inbuilt 1kHz square wave probe calibration signal output and the self-test function testify to this, and makes the kit well above average given its low price. The DIY acrylic box is quite good too, once you have figured out how to put it together. And the performance of the completed DSO138 is quite good enough to qualify it as a useful tool for audio testing and troubleshooting. The DSO138 kit represents outstanding value for money. It’s almost worth buying just for the fun of putting it all together and trying it out, even if you’re going to give it away! Where from? As mentioned earlier in the article, our DSO138 Scope came direct from the Banggood online store (www. banggood.com). The prices quoted were what we paid; however with the Aussie dollar fluctuating as it does, the price you pay could be slightly higher or lower. SC A April pril 2017  57 2017  57