Input impedance plottet as a function of trace impedance for trace lengths of 1/10, 1/16 and 1/20 of a wavelength. (Credit: Baltic Labs)

When Does Impedance Matching A PCB Trace Become Unavoidable?

August 27, 2023 by Maya Posch 21 Comments

A common joke in electronics is that every piece of wire and PCB trace is an antenna, with the only difference being whether this was intentional or not. In practical terms, low-frequency wiring is generally considered to be ‘safe’, while higher frequency circuits require special considerations, including impedance (Z) matching. Where the cut-off is between these two types of circuits is not entirely clear, however, with various rules-of-thumb in existence, as [Sebastian] over at Baltic Lab explains.

A popular rule is that no impedance matching between the trace and load is necessary if the critical length of a PCB trace (l_crit) is 1/10th of the wavelength (λ). Yet is this rule of thumb correct? Running through a number of calculations it’s obvious that the only case where the length of the PCB trace doesn’t matter is when trace and load impedance are matched.

According to these calculations, the 1/10 rule is not a great pick if your target is a mismatch loss of less than 0.1 dB, with 1/16 being a better rule. Making traces wider on the PCB can be advisable here, but ultimately you have to know what is best for your design, as each project has its own requirements. Even when the calculations look good, that’s no excuse to skip the measurement on the physical board, especially with how variable the dielectric constant of FR4 PCB material can be between different manufacturers and batches.

Heading image: Input impedance plotted as a function of trace impedance for trace lengths of 1/10, 1/16, and 1/20 of a wavelength. (Credit: Baltic Labs)

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Flexure PCB Actuators Made Before Your Very Eyes

August 24, 2023 by Jenny List 10 Comments

When we see something from [Carl Bugeja], we expect to see flexible PCBs and magnets being pushed to do unexpected things. His latest video in which he designs a set of PCB actuators using flexure joints certainly doesn’t fail to please.

His intent is to create a simple actuator in which a magnet is placed over a coil, and moves upward within the confines of he flexure which surrounds it. And rather than try individual designs one after the other he’s created a huge all-in-one test array of different flexure actuators, each having a slightly different design and construction to whichever one is next to it. There are plenty of magnet flips as he tests them, and using this approach he’s quickly able to eliminate the designs which work less well.

To give an idea how these actuators might be best used, he tried them in a few applications. Their lifting force is relatively tiny, but he found them possibly suitable for a haptic feedback device. Of particular interest is that as the structure is a PCB it’s relatively straightforward to run a line to the magnet and turn it into a touch sensor. The idea of an all in one sensor and haptic feedback component is rather appealing, we think.

If you’ve not seen Carl’s work before, we’ve encountered him many times over the years.

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Tiny Spheres Hiding In Your Display

August 20, 2023 by Adam Fabio 6 Comments

Liquid crystal and Organic LED displays have revolutionized portable computing. They’re also made of glass. Which presents a problem: How do we get electrical signals from fiberglass circuit boards to the glass displays? The answer is double-sided adhesive tape. But we’re not talking about packing tape here. As [Breakingtaps] explains, this tape has a trick up its sleeve.

The magic is that the tape conducts only in the vertical plane. Even more so, any two conducting sections of the tape are insulated from each other. How does it do that? Magic ~~beans~~ balls, of course!

The tape and adhesive are insulators. Embedded in the adhesive are tiny spheres. The spheres are made of plastic and coated with metal. When the tape (also known as ACF or Anisotropic Conductive Film) is pressed between a PCB with conductors and glass, a few spheres are squished down between the layers. Electrical signals pass between the squished spheres, allowing an image to be displayed on the glass screen. The final step uses heat and pressure to bond the adhesive and cure it. You can also get the material in paste form if you don’t like the tape.

The system works so well that it can be used for connections from a silicon chip directly to the glass. This is how many display controllers are mounted right to the module — definitely an improvement on the rubber strips used on LCDs of the past.

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No Need To Buy A Woodchipper – Build One!

August 8, 2023 by Dave Rowntree 32 Comments

Polish YouTuber WorkshopFromScratch finally got fed up with tripping over piles of garden detritus and decided to have a go at building a woodchipper (Video, embedded below). Since they had a ‘small’ 1.5kW gearmotor just lying idle (as you do) it was an obvious fit for a machine that needs torque rather than supersonic speed. The video is a fabulous 20-minute journey through the workshop showing just about every conceivable metalworking tool being used at some point.

Checkout out the thickness of my blades!

One interesting point is the bottom roller, which sits between a pair of removable guides, which should help the thing self-feed without jamming. Whether that was necessary is not for us to judge, but it can’t hurt. The frame looks like it was constructed from at least 1/4″ thick steel, which is expensive if you don’t happen to have a supply to hand. There’s lots to see, everything from thin sheet metalworking, which was plasma cut, constructing the feed and exhaust guides, to box sections being skilfully welded at some interesting angles to make a cart to move the thing. They tell us the blades were constructed from some seriously thick slabs of C45 grade steel, but currently are not hardened. This is planned for the future, but we suspect not something that is easily achieved in the home workshop!

If this channel is familiar, then you might remember the earlier stump grinder they built. If you are drowning in sawdust, but have a log burner, then you’ll appreciate this sawdust briquette machine.

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The Egyptian Coin Box ‘Trick’

August 7, 2023 by Dave Rowntree 9 Comments

[James Stanley] likes to spend time making puzzles and gadgets for escape rooms, and decided for a change to try their hand at a bit of magic. The idea was to construct a ‘magic box’, in which a coin can be placed in one of a number of slots, and then be able to remotely be able to determine the slot by means unseen. Obviously, this is an electronics hack, with a neat package of sensor and radio comms hidden inside a stack of CNC-milled wood. Coin locations are transmitted via Bluetooth to a Bangle.js smartwatch, which vibrates according to the slot occupied, allowing [James] to predict where the coin was placed. Continue reading “The Egyptian Coin Box ‘Trick’” →

RFID Emulator + E-paper Badge Can Be Programmed With Sound

July 22, 2023 by Donald Papp 9 Comments

In a way, an e-paper display makes an excellent foundation for a reprogrammable RFID card. The display only needs power during a refresh, and 125 kHz RFID tags are passive in the sense that the power for the RFID transaction comes from the reader itself. [Georgi Gerganov] has put those together in the GGtag, an open-source project for a 3.52″ e-paper badge with a trick or two up its sleeve.

One clever function is that it is programmable with sound, a feature built off another project of [Georgi]’s called ggwave, a data-to-sound (and vice-versa) framework that has been ported to just about every hardware platform one cares to imagine — including mobile phones — and can reliably send data through the air.

Transmitting data over sound is limited in throughput but has a number of advantages, not least of which is the huge range of compatible devices. There’s a web-based tool for programming the GGtag with sound available at ggtag.io that will give you a preview and let you hear how it works. The data encoding method gives transmissions a charming beep-boop quality that’s a bit reminiscent of an analog modem handshake. GGtag can also be programmed over USB serial, a faster (but somewhat less exciting) option.

The project’s GitHub repository contains GGtag’s code and technical details, and the CrowdSupply project is in the works for anyone who would prefer to buy one once they become available.

AC-DC Converter Is Reliable, Safe, And Efficient

July 12, 2023 by Bryan Cockfield 25 Comments

When first starting an electronics project, it’s not uncommon to dive right in to getting the core parts of the project working. Breadboarding the project usually involves working with a benchtop power supply of some sort, but when it comes to finalizing the project the actual power supply is often glossed over. It’s not a glamorous part of a project or the part most of us want to be working with, but it’s critical to making sure projects don’t turn up with mysterious issues in the future. We can look to some others’ work to simplify this part of our projects, though, like this power supply from [hesam.moshiri].

The power supply is designed around a switch-mode topology known as a flyback converter. Flyback converters work by storing electrical energy in the magnetic field of a transformer when it is switched on, and then delivering that energy to the circuit when it is switched off. By manipulating the switching frequency and turns ratios of the transformer, the circuit can have an arbitrary output voltage. In this case, it is designed to take 220V AC and convert it to 8V DC. It uses a simplified controller chip to decrease complexity and parts count, maintains galvanic isolation for safety, and is built to be as stable as possible within its 24W power limitation to eliminate any potential issues downstream.

For anyone trying to track down electrical gremlins in a project, it’s not a bad idea to take a long look at the power supply first. Any noise or unwanted behavior here is likely to cause effects especially in projects involving sensors, ADC or DAC, or other low-voltage or sensitive components. The schematic and bill of materials are available for this one as well, so anyone’s next project could use this and even make slight adjustments to change the output voltage if needed. And, if this is your first introduction to switched-mode power supplies, check out this in-depth look at the similar buck converter circuit to better understand what’s going on behind the scenes on these devices.

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