A Rant on Bad Datasheets | Voltlog 381

Welcome to this short video where I’m gonna rant about the quality of Chinese electronic component datasheets because for me it’s already the second time I’ve had trouble because of missing or incorrectly presented information.

JTAG Adapter PCB 20pin 2.54mm to 10pin 1.27mm – Voltlog #347

Welcome to a new Voltlog, this will be a rather short project video, I thought I’d start the year with something simple. If you’ve ever used JTAG before, either to program or debug an ARM processor, or something like an ESP32 or maybe to load a bitstream into an FPGA, you’ve likely encountered the ubiquitous 20 pin JTAG connector which is this 2×10 0.1inch spaced connector. It’s a rather large connector, it takes up a lot of space on a PCB, you don’t really need that many pins but you can’t go without it because it’s usually present on the fully featured programmer/debugging tools. Here is an example: this is an ST-Link V2, or to be precise a cheap clone from aliexpress but for the purpose of this discussion it doesn’t matter, it looks the same as the original and it has this 20 pin JTAG connector. 

And to some extent this isn’t really a problem if you are using big development boards like this STM32F4 dev board that I got from Aliexpress. This features the same 20 pin connector for programming so it’s a matter of connecting a simple ribbon cable and you’re up and running. However, most modern boards that you are going to be designing might not have enough space to install such a big connector, you might for example use the simpler 10 pin JTAG connection, cause you don’t even need that many signals, most of the pins are GND anyway on the 20 pin connection. And instead of using 2.54mm pin header you can use something smaller like half the size, 1.27mm, and this can save a lot of space on a board.

Designing PCBs With Castellated Holes | Voltlog #335

Welcome to a new Voltlog, in today’s video we’re going to be talking about castellated holes and how you can create them in your CAD tool. If this term is new to you, it’s pretty simple, you’ve certainly come across them if you’ve ever played with a bluetooth or wifi module because those 99% of the time will use castellated holes, which are these semi-plated holes on the edges of a PCB.  Having these connection points allows these modules to be soldered on top of a main PCB which contains our main circuit.

You might ask yourself why do we use castellated holes, why not use a simple through hole header as a board interconnect or just some simple SMD pads. Well in my opinion the most important reason is the relatively low difficulty for soldering castellated holes. If you think about it, having some SMD pads that go on the bottom of the PCB makes it pretty hard to solder, at least without proper equipment, you need to deposit solder paste on those pads, you need to get it in the right amount and then you need to have perfect alignment of the module on top of the pads which reside on the main PCB. Having these connection points underneath our module makes it very hard to align because you can’t see them. Also debugging such a module is going to be a pain because you won’t have access to all of those connection points.

So this where castellated holes improve, by having the plated half-holes at the side you can solder them even with a simple soldering iron, alignment is pretty easy because you can clearly see the connection points and debugging these is much easier because once again you can access them, you can do measurements with your scope probes or whatever instrument you are using.

Another advantage of having castellated holes on a design is to think of it like a building block, you might improve this building block externally or switch to a new building block that uses the same pinout and you just drop it into your system as a simple upgrade.

And believe it or not but having a module with castellated holes can lower your BOM cost in some cases because let’s say you need to use an RF module which might be 4 layers or a complicated system on module that may be 6 or 8 layer PCB with a powerful processor. Instead of building your entire system on an 8 layer PCB and assembling that complicated BGA chip yourself, you can buy the module ready made, it has castellated holes and you just drop that module into your system which may be a 2 layer mainboard or 4 layer mainboard that costs less.

PCB Solder Trainer | Voltlog #328

Welcome to a new Voltlog, the title probably gave it away already, this video is about a pcb solder trainer that I designed to measure ones soldering skills. This is not a new idea they have been around for a long time and there have been different designs around but you can join me in this video to see how I designed mine. 

I remember how soldering felt back when I was just starting tinkering with electronics, I think I was about 7-8 years old and I had this big communist soldering iron that I got from my father, this was about 100W rated, it had a small flashlight incandescent bulb and it used this thick copper wire as the soldering tip. It was great for soldering big stuff due to the power rating and the ability to transfer that heat efficiently. I remember I was using too much solder, I was making these huge solder blobs.

So back to the board design, let’s take a closer look at what I have in here. On the left we start with 01005 passives, these are 5 resistors in series and at the end of the string there is an LED. The LED has to be bigger because you can’t get them that small so the LED starts at 0402, imperial size. If you get all 5 resistors and the LED soldered right and you apply 5V to this header, the LED should light up and that’s your indication that you’ve at least electrically got everything connected right.

And the size of the components then goes up to 0204, 0402, 0603, 0805, 1203 then we have some resistor networks which I believe are 4×0603. Then we have some SOT23 devices these can be dual diodes also connected in series that will light up an LED.

Depending on the type of LED you choose and it’s forward voltage you can calculate what resistor values you need so the LED will light-up. In general a green LED with 47ohms resistor should work for this and the pads for each of these passive components are the hand solder type which makes them wider so that’s something to help you out.

ScopeShunt Visualising The Current Waveform With Your Oscilloscope | Voltlog #310

We usually use an oscilloscope for visualizing a voltage over time but sometimes it’s also useful to visualize the current waveform over time. The right way to do it is to get a current probe which can sense the current and convert that to a voltage that the oscilloscope can display however such devices are pretty expensive, they can be around $1000 even for an entry level one like the Rigol RP1001C which is only rated up to 300KHz bandwidth.

But we can improvise something for a much lower cost and it should allow us to visualize the current waveform on the oscilloscope. You’ve probably seen me use a shunt resistor when testing power supply to take a look at the current waveform. Because as you know passing a current through a resistor will generate a voltage drop.

That voltage drop is directly proportional with the passing current and with a round value resistor we can have an easy to use transformation ratio between voltage and current. All we have to do is o introduce this resistor inline between our power supply and the device under test

For example if I have a 1ohm resistor, we have a 1:1 ration, for each mA passed through that resistor we will have  1mV of voltage drop that our oscilloscope can display. Such a circuit will of course have it’s limitations, for example it won’t work very well when testing low voltage low power devices because our resistor will introduce a burden voltage, which will drop our supply voltage to the device under test. This is also not an isolated measurement so it might not be safe when connected with higher voltage circuits.

But there are still a lot of scenarios where you could use this successfully on the electronics workbench so it might be worth building something like this. I want to make this nicer by building it inside an enclosure with the required bnc connector for connecting to the oscilloscope and 4mm banana plugs for passing the current through. I picked this small aluminium enclosure which would be enough to house the resistor, actually the resistors, because there are several advantages to using multiple resistors in parallel.

Alternative to this simple shunt resistor measuring method include the Joulescope which is a fully featured dc energy measurement test instrument with incredibly wide dynamic range that allows you to capture the smallest currents next to a jump to a higher current. I reviewed the Joulescope in Voltlog #211.

Voltlog #287 – Switching From Eagle To Kicad

Welcome to a new Voltlog, today I’m gonna talk a bit about my process of switching from EagleCad to Kicad. So i’ve been an EagleCAD user for years, I’ve practically learned how to do pcb layout in eagle cad so there is that emotional attachment to a piece of software because it’s what I used while developing this skill. And to be honest there wasn’t any better alternative years ago, Eagle was the first decent piece of cad software to offer a freeware license and it quickly became popular for hobbyists. It didn’t have all the bells and whistles of the expensive software like Altium or Candece but it did the job while being user friendly.

Since it quickly became popular for makers and hobbyists it also meant there was an abundance of support on the forums as well as many user generated scripts and libraries freely available. It was similar to the popularity Arduino got but it was never open source and if you needed some extra features like 4 layer layout or pcb sizes larger than 8x10cm you needed to pay for a license.

Voltlog #249 – Making Some USB Serial Converter Boards With CH340E (part 1)

Welcome to a new video, today I’m building a bunch of usb to serial converter boards because if you are into electronics and microcontrollers you will for sure need a bunch of usb to serial converters to connect your boards to a computer for example.

The idea for building these boards started when I found the CH340E converter chip on aliexpress, I like several things about this chip, it was small because it comes in MSOP10 package, it was cheap at about $0.40 a piece and it requires minimal external circuitry, in fact it only needs an external bypass cap.

Voltlog #235 – I Made A PCB Business Card

Welcome to a new Voltlog, today we’re going to be talking about the possibility of designing a pcb business card. So if you’re an electronics engineer who offers consulting services or even just an enthusiast and you would like to create a nice original and suggestive business card, stick around.

The idea of making a pcb business card is not new and there are many previous examples of pcb business cards, some people build them passive, others like to integrate a small battery some LEDs, maybe a microcontroller with NFC or a USB interface and generate some interaction with the user.

Maybe 7-8 years ago I first wanted to try this idea but back then, it was quite expensive to get PCBs professionally manufactured in China, first they were charging a setup fee which was usually $150 and then you would have the cost of the PCB which for 10 pcs 10 by 10cm was usually about 2-3$ per piece and then you would have the shipping cost which starting from something like $50 for the first kg via DHL. And all of this was for the standard 1.6mm FR4 with green solder mask and tin finish. If you wanted matte black soldermask and gold finish, there was an additional cost.

Voltlog #230 – Guidelines For Board Level Temperature Sensor Layout & Placement

Welcome to a new Voltlog, today I’m gonna show you some techniques that you can use when doing temperature measurements. Typically you will want to measure either the ambient temperature or the system or component temperature. Depending on one of these goals you are going to follow different design rules to achieve that. And to better illustrate the problems, I have these 3 pcbs which have exactly the same electrical circuit but with different layouts on the pcb.

The boards consist of an esp8266 and a digital temperature sensor and you might expect that since all 3 boards are placed so close together, they should all indicate the same temperature but that is not the case as we can see on this graph we are getting 3 different temperatures and only one of them is close to the actual ambient temperature measured with another thermometer, so why is that happening? Well the answer lies in the layout of the PCB and that includes component placement, copper planes and various other elements on the PCB.