Posts in Electronics (20 found)
Jeff Geerling Yesterday

Converting hot dog plasma video to sound with OpenCV

When you ground a hot dog to an AM radio tower, it generates plasma. While the hot dog's flesh is getting vaporized, a tiny plasma arc moves the air around it back and forth. And because this tower is an AM tower, it uses Amplitude Modulation , where a transmitter changes the amplitude of a carrier wave up and down. Just like a speaker cone moving up and down, the plasma arc from the hot dog turns that modulation into audible sound.

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Daniel Mangum Yesterday

Interesting SPI Routing with iCE40 FPGAs

A few weeks ago I posted about how much fun I was having with the Fomu FPGA development board while travelling. This project from Tim ‘mithro’ Ansell and Sean ‘xobs’ Cross is not new, but remains a favorite of mine because of how portable it is — the entire board can fit in your USB port! The Fomu includes a Lattice Semiconductor iCE40 UltraPlus 5K, which has been a popular FPGA option over the past few years due to the reverse engineered bitstream format and ability to program it with a fully open source toolchain (see updated repository here).

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Jeff Geerling 1 weeks ago

The Arduino Uno Q is a weird hybrid SBC

The Arduino Uno Q is... a weird board. It's the first product born out of Qualcomm's buyout of Arduino . It's like if you married an Intel CPU, and a Raspberry Pi RP2040 microcontroller—oh wait, Radxa's X4 did that . Arduino even tried it before with their old Yún board, which had Linux running on a MIPS CPU, married to an ATmega microcontroller.

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My impressions of the MacBook Pro M4

I have been using a MacBook Pro M4 as my portable computer for the last half a year and wanted to share a few short impressions. As always, I am not a professional laptop reviewer, so in this article you won’t find benchmarks, just subjective thoughts! Back in 2021, I wrote about the MacBook Air M1 , which was the first computer I used that contained Apple’s own ARM-based CPU. Having a silent laptop with long battery life was a game-changer, so I wanted to keep those properties. When the US government announced tariffs, I figured I would replace my 4-year old MacBook Air M1 with a more recent model that should last a few more years. Ultimately, Apple’s prices remained stable, so, in retrospect, I could have stayed with the M1 for a few more years. Oh well. I went to the Apple Store to compare the different options in person. Specifically, I was curious about the display and whether the increased weight and form factor of the MacBook Pro (compared to a MacBook Air) would be acceptable. Another downside of the Pro model is that it comes with a fan, and I really like absolutely quiet computers. Online, I read from other MacBook Pro owners that the fan mostly stays off. In general, I would have preferred to go with a MacBook Air because it has enough compute power for my needs and I like the case better (no ventilation slots), but unfortunately only the MacBook Pro line has the better displays. Why aren’t all displays nano-textured? The employee at the Apple Store presented the trade-off as follows: The nano texture display is great at reducing reflections, at the expense of also making the picture slightly less vibrant. I could immediately see the difference when placing two laptops side by side: The bright Apple Store lights showed up very prominently on the normal display (left), and were almost not visible at all on the nano texture display (right): Personally, I did not perceive a big difference in “vibrancy”, so my choice was clear: I’ll pick the MacBook Pro over the MacBook Air (despite the weight) for the nano texture display! After using the laptop in a number of situations, I am very happy with this choice. In normal scenarios, I notice no reflections at all (where my previous laptop did show reflections!). This includes using the laptop on a train (next to the window), or using the laptop outside in daylight. (When I chose the new laptop, Apple’s M4 chips were current. By now, they have released the first devices with M5 chips.) I decided to go with the MacBook Pro with M4 chip instead of the M4 Pro chip because I don’t need the extra compute, and the M4 needs less cooling — the M4 Pro apparently runs hotter. This increases the chance of the fan staying off. Here are the specs I ended up with: One thing I noticed is that the MacBook Pro M4 sometimes gets warm, even when it is connected to power, but is suspended to RAM (and has been fully charged for hours). I’m not sure why. Luckily, the fan indeed stays silent. I think I might have heard it spin up once in half a year or so? The battery life is amazing! The previous MacBook Air M1 had amazing all-day battery life already, and this MacBook Pro M4 lasts even longer. For example, watching videos on a train ride (with VLC) for 3 hours consumed only 10% of battery life. I generally never even carry the charger. Because of that, Apple’s re-introduction of MagSafe, a magnetic power connector (so you don’t damage the laptop when you trip over it), is nice-to-have but doesn’t really make much of a difference anymore. In fact, it might be better to pack a USB-C cable when traveling, as that makes you more flexible in how you use the charger. I was curious whether the 120 Hz display would make a difference in practice. I mostly notice the increased refresh rate when there are animations, but not, for example, when scrolling. One surprising discovery (but obvious in retrospect) is that even non-animations can become faster. For example, when running a Go web server on , I noticed that navigating between pages by clicking links felt faster on the 120 Hz display! The following illustration shows why that is, using a page load that takes 6ms of processing time. There are three cases (the illustration shows an average case and the worst case): As you can see, the waiting time becomes shorter when going from 60 Hz (one frame every 16.6ms) to 120 Hz (one frame every 8.3ms). So if you’re working with a system that has <8ms response times, you might observe actions completing (up to) twice as fast! I don’t notice going back to 60 Hz displays on computers. However, on phones, where a lot more animations are a key part of the user experience, I think 120 Hz displays are more interesting. My ideal MacBook would probably be a MacBook Air, but with the nano-texture display! :) I still don’t like macOS and would prefer to run Linux on this laptop. But Asahi Linux still needs some work before it’s usable for me (I need external display output, and M4 support). This doesn’t bother me too much, though, as I don’t use this computer for serious work. 14" Liquid Retina XDR Display with nano texture Apple M4 Chip (10 core CPU, 10 core GPU) 32 GB RAM (this is the maximum!), 2 TB SSD (enough for this computer) Best case: Page load finishes just before the next frame is displayed: no delay. Worst case: Page load finishes just after a frame is displayed: one frame of delay. Most page loads are somewhere in between. We’ll have 0.x to 1.0 frames of delay

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Jeff Geerling 2 weeks ago

Why do some radio towers blink?

One day on my drive home, I saw three towers. One of them had a bunch of blinking white lights, another one had red lights that kind of faded in and out, and the third one, well, it wasn't doing anything. I'm lucky to have a radio engineer for a dad, so Dad: why do some towers blink? Joe: Well, blinking I would call like the way you described it, "flashing", "white light", or "strobe". All these lights are to aid pilots and air traffic. helicopters, fighter planes, regular jets. So that's the purpose of it. Jeff: Well that one tower that I saw had red lights that faded in and out, but I even think there's a freestanding tower just north of here that has red and white on top.

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Maurycy 2 weeks ago

Some hot rocks:

I recently went on a rock collecting trip, but apart from the usual — quartz, K feldspar crystals, garnet, etc — I found some slightly radioactive rocks: All of these were found using my prospecting scintillator , but I took measurements with a Radiacode 102 — a very common hobbyist detector — so that other people can compare readings. Despite being small, it is still a gamma scintillator, so the count rates are much higher then any G-M tube. None of these are crazy hot, but they were all collected off the surface: I didn’t bring any good digging equipment on the trip. (Really should have considering how my detector is able to pick up deeply buried specimens) The biggest hazard with my rocks is dropping them on your toes. Even if you were to grind them up and inhale the dust, the host rock is much more of a danger then the radioactivity. I’ve personally been in multiple residential and office buildings that are more radioactive then my specimens because of the stone that was used to construct them. Also, if you have any “Anti-Radiation” or “Bio Energy” or “Quantum Energy” wellness products: they are quite the opposite. (and many are spicier then my rocks.) … or how about some nice decorative glass ? It glows

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iDiallo 2 weeks ago

Why We Don't Have Flying Cars

Imagine this: You walk up to your driveway where your car is parked. You reach for the handle that automatically senses your presence, confirms your identity, and opens to welcome you in. You sit down, the controls appear in front of you, and your seatbelt secures itself around your waist. Instead of driving forward onto the pavement, you take off. You soar into the skies like an eagle and fly to your destination. This is what technology promises: freedom, power, and something undeniably cool. The part we fail to imagine is what happens when your engine sputters before takeoff. What happens when you reach the sky and there are thousands of other vehicles in the air, all trying to remain in those artificial lanes? How do we deal with traffic? Which directions are we safely allowed to go? And how high? We have flying cars today. They're called helicopters. In understanding the helicopter, we understand why our dream remains a dream. There's nothing romantic about helicopters. They're deafeningly loud and incredibly expensive to buy and maintain. They require highly skilled pilots, are dangerously vulnerable to engine failure, and present a logistical nightmare of three-dimensional traffic control. I can't even picture what a million of them buzzing between skyscrapers would look like. Chaos, noise pollution, and a new form of gridlock in the sky. Even with smaller drones, as the technology evolves and becomes familiar, cities are creating regulations around them, sucking all the fun and freedom out in favor of safety and security. This leads me to believe that the whole idea of flying cars and drones is more about freedom than practicality. And unregulated freedom is impossible. This isn't limited to flying cars. The initial, pure idea is always intoxicating. But the moment we build a prototype, we're forced to confront the messy reality. In 1993, a Japanese man brought a video phone to demo for my father as a new tech to adopt in our embassy. I was only a child, but I remember the screen lighting up with a video feed of the man sitting right next to my father. I could only imagine the possibilities. It was something I thought only existed in sci-fi movies. If this was possible, teleportation couldn't be too far away. In my imagined future, we'd sit at a table with life-like projections of colleagues from across the globe, feeling as if we were in the same room. It would be the end of business travel, a world without borders. But now that the technology is ubiquitous, the term "Zoom fatigue" is trending. It's ironic when I get on a call and see that 95% of my colleagues have their cameras turned off. In movies, communication was spontaneous. You press a button, your colleauge appears as a hologram, and you converse. In reality, there's a calendar invite, a link, and the awkward "you're on mute!" dance. It's a scheduled performance, not an organic interaction. And then there are people who have perfect lighting, high-speed internet, and a quiet home office. And those who don't. Video calls have made us realize the importance of physical space and connection. Facebook's metaverse didn't resolve this. Imagine having a device that holds all of human knowledge at the click of a button. For generations, this was the ultimate dream of librarians and educators. It would create a society of enlightened, informed citizens. And we got the smartphone. Despite being a marvel of technology, the library of the world at your fingertips, it hasn't ushered us into utopia. The attention economy it brought along has turned it into a slot machine designed to hijack our dopamine cycles. You may have Wikipedia open in one tab, but right next to it is TikTok. The medium has reshaped the message from "seek knowledge" to "consume content." While you have access to information, misinformation is just as rampant. The constant stimulation kills moments of quiet reflection, which are often the birthplace of creativity and deep thought. In The Machine Stops by E.M. Forster, every desire can be delivered by pulling a lever on the machine. Whether it's food, a device, or toilet paper. The machine delivers everything. With Amazon, we've created a pretty similar scenario. I ordered replacement wheels for my trash bin one evening, expecting them to arrive after a couple of days. The very next morning, they were waiting at my doorstep. Amazing. But this isn't magical. Behind it are real human workers who labor without benefits, job security, or predictable income. They have an algorithmic boss that can be more demanding than a human one. That promise of instant delivery has created a shadow workforce of people dealing with traffic, poor weather, and difficult customers, all while racing against a timer. The convenience for the user is built on the stress of the driver. The dream of a meal from anywhere didn't account for the reality of our cities now being clogged with double-parked delivery scooters and a constant stream of gig workers. Every technological dream follows the same pattern. The initial vision is pure, focusing only on the benefit. The freedom, the convenience, the power. But reality is always a compromise, a negotiation with physics, economics, and most importantly, human psychology and society. We wanted flying cars. We understood the problems. And we got helicopters with a mountain of regulations instead. That's probably for the best. The lesson isn't to stop dreaming or stop innovating. It's to dream with our eyes open. When we imagine the future, we need to ask not just "what will this enable?" but also "what will this cost?" Not in dollars, but in human terms. In stress, inequality, unintended consequences, and the things we'll lose along the way. We're great at imagining benefits and terrible at predicting costs. And until we get better at the second part, every flying car we build will remain grounded by the weight of what we failed to consider.

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Automata All the Way Down

Fabs and EDA companies collaborate to provide the abstraction of synchronous digital logic to hardware designers. A hardware design comprises: A set of state elements (e.g., registers and on-chip memories), which retain values from one clock cycle to another A transfer function , which maps the values of all state elements at clock cycle N to new values of all state elements at clock cycle The transfer function cannot be too fancy. It can be large but cannot be defined with unbounded loops/recursion. The pragmatic reason for this restriction is that the function is implemented with physical gates on a chip, and each gate can only do one useful thing per clock cycle. You cannot loop the output of a circuit element back to itself without delaying the value by at least one clock cycle (via a state element). It feels to me like there is a deeper reason why this restriction must exist. Many people dabbled with synchronous digital logic in college. If you did, you probably designed a processor, which provides the stored program computer abstraction to software developers. And here comes the inception: you can think of a computer program as a transfer function. In this twisted mindset, the stored program computer abstraction enables software engineers to define transfer functions . For example, the following pseudo-assembly program: Can be thought of as the following transfer function: In the stored program computer abstraction, state elements are the architectural registers plus the contents of memory. As with synchronous digital logic, there are limits on what the transfer function can do. The switch statement can have many cases, but the body of each case block is defined by one instruction. Alternatively, you can define the transfer function at the basic block level (one case per basic block, many instructions inside of each case). Programming in assembly is a pain, so higher level languages were developed to make us less crazy. And here we go again, someone could write an interpreter for C. A user of this interpreter works at the C level of abstraction. Following along with our previous pattern, a C program comprises: A set of state elements (variables, both global and local) A transfer function For example, the following C function: Can be thought of with as the following transfer function: Think of and as intrinsics used to implement function calls. The key building blocks of the transfer function are state ments. It is easy to just store the term “statement” into your brain without thinking of where the term comes from. A state ment is a thing which can alter state . This transformation of an imperative program into a transfer function seems strange, but some PL folks do it all the time. In particular, the transfer function view is how small step operational semantics are defined. And of course this can keep going. One could write a Python interpreter in C, which allows development at a higher level of abstraction. But even at that level of abstraction, programs are defined in terms of state elements (variables) and a transfer function (statements). The term Turing Tax was originally meant to describe the performance loss associated with working at the stored-program computer level of abstraction instead of the synchronous digital logic level of abstraction. This idea can be generalized. At a particular level of abstraction, code defines the transfer function while data is held in the state elements. A particular set of bits can simultaneously be described as code at one level of abstraction, while defined as data at a lower level. This code/data duality is intimately related to the Turing Tax. The Turing Tax collector is constantly looking for bags of bits which can be interpreted as either code or data, and he collects his tax each time he finds such a situation. An analogous circumstance arises in hardware design. Some signals can be viewed as either part of the data path or the control path, depending on what level of abstraction one is viewing the hardware from. A compiler is one trick to avoid the Turing Tax by translating code (i.e., a transfer function) from a higher level of abstraction to a lower level. We all felt awkward when I wrote “interpreter for C” earlier, and now we can feel better about it. JIT compilers for Python are one way to avoid the Turing Tax. Another example is an HLS compiler which avoids the Turing Tax between the stored-program computer abstraction layer and the synchronous digital logic layer. No, this section isn’t about your Fitbit. Let’s call each evaluation of a transfer function a step . These steps occur at each level of abstraction. Let’s define the ultimate performance goal that we care about to be the number of steps required to execute a computation at the synchronous digital logic level of abstraction. The trouble with these layers of abstraction is that typically a step at a higher layer of abstraction requires multiple steps at a lower layer. For example, the multi-cycle processor implementation you learned about in a Patterson and Hennessy textbook could require 5 clock cycles to execute each instruction (instruction fetch, register fetch, execute, memory, register write back). Interpreters have the same behavior: one Python statement may be implemented with many C statements. Now imagine the following house of cards: A Python interpreter which requires an average of 4 C statements to implement 1 Python statement A C compiler which requires an average of 3 machine instructions to implement 1 C statement A processor which requires an average of 5 clock cycles to execute 1 machine instruction When the Turing property tax assessor sees this house, they tax each level of the house . In this system, an average Python statement requires (4 x 3 x 5) 60 clock cycles! Much engineering work goes into avoiding this problem (pipelined and superscalar processors, multi-threading, JIT compilation, SIMD). Partial evaluation is another way to avoid the Turing Tax. Partial evaluation transforms data into code . There must be some other method of creating abstractions which is more efficient. Self-modifying code is rarely used in the real world (outside of JIT compilers). Self-modifying code seems crazy to reason about but potentially could offer large performance gains. Partial evaluation is also rarely used but has a large potential. Subscribe now A set of state elements (e.g., registers and on-chip memories), which retain values from one clock cycle to another A transfer function , which maps the values of all state elements at clock cycle N to new values of all state elements at clock cycle A set of state elements (variables, both global and local) A transfer function Code vs Data At a particular level of abstraction, code defines the transfer function while data is held in the state elements. A particular set of bits can simultaneously be described as code at one level of abstraction, while defined as data at a lower level. This code/data duality is intimately related to the Turing Tax. The Turing Tax collector is constantly looking for bags of bits which can be interpreted as either code or data, and he collects his tax each time he finds such a situation. An analogous circumstance arises in hardware design. Some signals can be viewed as either part of the data path or the control path, depending on what level of abstraction one is viewing the hardware from. Compilers vs Interpreters A compiler is one trick to avoid the Turing Tax by translating code (i.e., a transfer function) from a higher level of abstraction to a lower level. We all felt awkward when I wrote “interpreter for C” earlier, and now we can feel better about it. JIT compilers for Python are one way to avoid the Turing Tax. Another example is an HLS compiler which avoids the Turing Tax between the stored-program computer abstraction layer and the synchronous digital logic layer. Step Counting (Multiple Taxation) No, this section isn’t about your Fitbit. Let’s call each evaluation of a transfer function a step . These steps occur at each level of abstraction. Let’s define the ultimate performance goal that we care about to be the number of steps required to execute a computation at the synchronous digital logic level of abstraction. The trouble with these layers of abstraction is that typically a step at a higher layer of abstraction requires multiple steps at a lower layer. For example, the multi-cycle processor implementation you learned about in a Patterson and Hennessy textbook could require 5 clock cycles to execute each instruction (instruction fetch, register fetch, execute, memory, register write back). Interpreters have the same behavior: one Python statement may be implemented with many C statements. Now imagine the following house of cards: A Python interpreter which requires an average of 4 C statements to implement 1 Python statement A C compiler which requires an average of 3 machine instructions to implement 1 C statement A processor which requires an average of 5 clock cycles to execute 1 machine instruction

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Jeff Geerling 3 weeks ago

VCF Midwest was even better than I expected

Earlier this year, I inherited a bunch of old Macs and computer parts, including the PowerBook 520 pictured above. And, for the past three years I've been trying to visit VCF Midwest up in Chicago, where there's this odd blend of old computers, radio, broadcast gear... Honestly, it's hard to pin down exactly what it is. And I also had no idea how overwhelming the two-day event would be. Overwhelmingly awesome , that is.

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./techtipsy 3 weeks ago

Comparing the power consumption of a 30 year old refrigerator to a brand new one

Our apartment came with a refrigerator. It was alright, it made things cold, it kept them cold. It was also incredibly noisy, and no matter how much I fiddled with its settings, the compressor was always running and any ice cream left in the deep freeze part got rock solid. 1 When I hooked up one of my smart plugs to it, I soon learned why: one of the two compressors was running all the time. This lead to a huge block of ice forming on the back of the main compartment, and the deep freeze section icing up really quickly. I suspect that the thermostat may have been busted and contributed to the issue, but after trying to repair a dishwasher, getting cut about 10 times on my hands and losing, I had zero interest in attempting another home appliance repair on my own. The refrigerator was the UPO Jääkarhu ( jääkarhu means polar bear in Finnish), and the manual that the previous owner had still kept around had July 1995 on it, meaning that the refrigerator was about the same age as I am: 30 years old. Not bad at all for a home appliance! I shopped around for a new refrigerator and got a decent one that’s about the same size, except newer. I won’t mention the brand here because they didn’t pay me anything and this post really isn’t a refrigerator review, but it was in the low-to-midrange class, sporting a “no frost” feature, and could be bought for about 369 EUR in Estonia in the summer of 2025. Based on some napkin math, I assumed that within a few years, the electricity savings will cover the upfront cost of buying the new refrigerator, assuming that it doesn’t break down. After letting it run for a while, I had some data! Turns out that the old one consumed 3.7x more electricity compared to the new one. Here are some typical daily power consumption numbers: The difference is more noticeable if we zoom out a bit. Moving from ~78 kWh to ~21 kWh consumed each month is nice. Around the time we replaced the refrigerator, we also got a working dishwasher, and with those two combined I saw a solid 10-20% decrease in the overall power usage of the whole apartment. We went from using 334 kWh in June to 268 kWh in July, 298 kWh in August and 279 kWh in September. Remember that napkin math I made earlier? If we assume about 57 kWh savings per month, and an average electricity price of 17 cents per kWh (based on actual rates during August 2025), it will take about 38 months or a bit over 3 years for the new refrigerator to pay off in the most pessimistic scenario. The pay-off will likely be larger if we account for energy prices usually rising during winter. Don’t worry about the old refrigerator, we gave it away to a person who needed one for their new home in the short term as a stopgap until they get further with renovation work. Even got some good chocolate for that! The only point of concern with this change is that I don’t really trust the new refrigerator to last as long as the old one. The previous one was good for 30 years if you look past the whole ice buildup, heat and noise, but with the new one I suspect that it’s not going to last as long. At least my new refrigerator doesn’t have a Wi-Fi-connected screen on it! honestly, I miss that a lot, the ice cream was colder for longer, I ate it in smaller bites and savored it more.  ↩︎ old refrigerator: 2.6 kWh new refrigerator: 0.7 kWh honestly, I miss that a lot, the ice cream was colder for longer, I ate it in smaller bites and savored it more.  ↩︎

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Daniel Mangum 1 months ago

Using a Laptop as an HDMI Monitor for an SBC

Though I spend the majority of my time working with microcontroller class devices, I also have an embarassingly robust collection of single board computers (SBC), including a few different Raspberry Pi models, the BeagleV Starlight Beta (RIP), and more. Typically when setting up these devices for whatever automation task I have planned for them, I’ll use “headless mode” and configure initial user and network credentials when writing the operating system to the storage device using a tool like Raspberry Pi’s Imager.

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Stavros' Stuff 1 months ago

I made a really small LED panel

I bought a really small 8x8 LED panel a while ago because I have a problem. I just can’t resist a nice WS2812 LED panel, much like I can’t resist an e-ink display. These days I manage to stay sober, but once in a while I’ll see a nice cheap LED panel and fall off the wagon. It has now been thirteen minutes that I have gone without buying LED panels, and this is my story. This isn’t really going to be super interesting, but there are some good lessons, so I thought I’d write it up anyway. On the right you can see the LED panel I used, it’s a bare PCB with a bunch of WS2812 (Neopixel) addressable LEDs soldered onto it. It was the perfect excuse for trying out WLED , which I’ve wanted to take a look at for ages, and which turned out to be absolutely fantastic. As with every light-based project, one of the big issues is proper diffusion. You don’t want your LEDs to show up as the points of light they are, we really like nice, big, diffuse lights, so you need a way to do that. My idea was to print a two-layer white square out of PLA (which would be translucent enough to show the light, but not so translucent that you could see the LEDs behind it. I also printed a box for the square to go in front of: I printed the diffuser (the white square) first, held it over the LED panel and increased or decreased the distance of the square from the LEDs until the LEDs didn’t look like points, but the colors also didn’t blend into the neighboring squares’ colors. This turned out to be around 10mm, so that’s how thick I made the box. The eagle-eyed among you may want to seek medical assistance, but if you have normal human eyes, you may have noticed that there’s nowhere in the box for the microcontroller to go, and you would be correct. For this build, I decided to use an ESP8266 (specifically, a WeMos dev board), but I didn’t want to make the whole box chunky just to fit a small microcontroller in there, so I did the next best thing: I designed a hole in the back of the box for the cables that connect to the LED panel, and I glued the ESP8266 to the back of the box. YOLO. Look, it works great, ok? The cables are nice and shortish, even though they go to the entirely wrong side of the thing, the USB connector is at a very weird place, and the ESP8266 is exposed to the elements and the evil eye. It’s perfect. Here’s the top side, with the diffuser: And here’s the whole mini tiny cute little panel showing some patterns from WLED (did I mention it’s excellent? It is). That’s it! I learned a few things and made a cute box of lights. I encourage you to make your own, it’s extremely fun and mesmerizing and I love it and gave it to a friend because I never used it and it just took up space and then made a massive 32x32 version that I also never use and hung it on my wall. Please feel free to Tweet or toot at me, or email me directly.

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Jeff Geerling 1 months ago

Qualcomm's buying Arduino – what it means for makers

Qualcomm just announced they're acquiring Arduino , the company that introduced a whole generation of tinkerers to microcontrollers and embedded electronics. The Uno R3 was the first microcontroller board I owned. Over a decade ago, I blinked my first LED with an Uno; the code for that is actually still up on my GitHub .

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Christian Jauvin 1 months ago

Ignore Your Check Engine Light at Your Own Peril

Currently in my car I have the “check engine” light being on, but it’s ok, because I know what is the problem, my mechanic told me that it’s , and that even though it’s not ideal, it can wait while he finds the part to repair it (apparently it’s not so easy to find). There is something I don’t like about this though: if there is a NEW problem appearing, I won’t know about it, because this check engine light has only one state, and now it’s being used.

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Stone Tools 1 months ago

Electric Pencil on the TRS-80

As the story goes, 40-year old filmmaker Michael Shrayer was producing a Pepsi-Cola commercial in 1974 when he had a crisis of conscience on set. He looked around at the shoot, the production, the people, the apparatus and found a kind of veil lifted from his eyes. Prior to that moment, it was work. Suddenly it was insanity in service to a "dancing bottle cap." Almost on-the-spot, he swore off Hollywood for good, tore up his union card (forfeiting union pensions in the process!), and moved to California to semi-retire as a beach bum . After moving to California, with time on his hands, he found a distraction in a new-ish gizmo on the market, the MITS Altair 8800 . Once a creator, always a creator, Shrayer put his efforts into learning how the machine worked, and more importantly how to make things with it. That was not an easy task back then on a machine whose base model used physical toggle switches to input byte code. Undeterred, he put together Software Package 1 , a collection of public domain development tools, then started work on ESP-1 ("Extended" Software Package 1). Tired of using a typewriter to write documentation, he wondered if he could use the Altair itself to help him compose the document. That software didn't exist, so he wrote it himself . Dubbed the Electric Pencil by he and his wife , it was used it to write its own manual, and began its spread through the home computing scene. Landing on the TRS-80 series in 1978 opened it to a mass market, and the word processor genre was well and truly born. For the first couple of years Electric Pencil was the only word processing option for home microcomputers. This gave Michael Shrayer what some may call arrogance, but which I'll call "work/life balance" to utterly ignore the support phone line come 5pm sharp. Customers literally had no choice but to wait until morning to call again. Shrayer was living the dream, I guess is what I'm saying. (He also said he sold 10,000 copies. At ~$200/per. In late 70s money. Again, "living the dream.") In 1982, James Fallows called Electric Pencil , "satisfying to the soul." My expectations are considerably lower than having my soul soothed, but maybe I can at least find a place for it in my heart? I can tell already this is going to be a little painful. No undo. A bit of a clunky UX with the underlying system. No spell checking. Limited number of visible lines of text. Basic cut-copy-paste is nowhere to be seen. But maybe I don't rely on those things as much as I assume I do in this context? The primary work I'm committing to is to write this very section, the bulk of the review, within Electric Pencil itself. I'll also use it to revise the document as much as possible, but this blog platform has unique tools I need to finalize the piece. Once I feel comfortable using Pencil , I think I'll take a stab at writing some fan-fiction in it. Any Mork & Mindy fans out there? I am off to a rough start. The simple fact of the matter is that the interaction between a modern keyboard, an emulator, and Electric Pencil is not strictly harmonious. One of the primary interactions with the software worked best with a key that wasn't even part of the original TRS-80 hardware. I monkey-paw the keyboard, trying to deduce which modern key maps to which retro key and finally figure it out. I write it down on my memo pad and refer to it constantly during my evaluation. Once I'm into Electric Pencil though, I'm rather struck by its stark simplicity. I am here to write, just me and my words; alone together with no external pressure to do anything but the task at hand. It's nice! I'm definitey reminded of any number of "distraction free word processors" in recent years and dozens of attempts to return to a clean, uncluttered interface for writing. I suppose, like Pepsi executives reinventing souless product shoots over and over, we are similarly doomed to reinvent "a blank screen with a blinking cursor" from time to time. Once I start typing though, I realize the blank screen is less a great, untapped plain of fruitful potential and more of a Mad Max style desert. War Boys are at the ready, poised to steal my blood and guzzoline. Initially I was concerned about how I would translate my writing into something I can use on the blog. It struck me that I kind of don't need to worry about it. Electric Pencil saves in plain ASCII format so Markdown is a realistic option. Rather than bring esoteric file formats of the past to the present, we'll instead bring text formatting of the present into the past. Well, that was the working theory until I learned that there is simply no way to type things like or or and even requires a weird combination. I hadn't mentally prepared myself for the idea that the physical keyboards of the past might present their own writing challenges unique from the software. Still, and exist, making HTML input an option for the missing characters. I move on and discover organically that deletes a character. The brief adrenaline rush of feeling attuned to the vibe of the program is nice, until I saved the document. Until I tried to save the document. No lie, this represents is my sixth attempt at getting things working. Look, I do not claim to be a smart person. I overlooked an obvious warning message or two, simply because I didn't expect them. I was looking to the left for cross-traffic when I should have looked to the right for the coming train. A fun fact about Electric Pencil is that when you save your work, you are saving from the current location of the cursor to the end of the document. Think for a moment about the implications of that statement. There will be no advanced warning when you do this, only an after-the-fact message about what was done. It will obediently wipe out large chunks, or even all, of your document, depending on the current cursor position. If you make a mistake and lose your work, the manual explicitly puts the blame on you , "The truth of the matter is that...you screwed up." It then says there are recovery techniques you can learn if you buy a copy of the book TRS-80 Disk & Other Mysteries by Harvard Pennington, the same guy who bought the rights to Electric Pencil from Shrayer. The same guy who published this very version of the program. Hate the game, not the player? But wait, the emulator itself has its own quirks. With the various things that could go wrong having actually gone wrong, dear reader I doubt you'll be surprised to learn that I lost my work multiple times. I cannot blame Electric Pencil for everything, as the emulator kind of worked against me as well, but the total experience of using this in 2025 was quite frustrating. I persevered for your entertainment . The tip jar is to your right. Back in the day, Electric Pencil was notorious for losing the user's typing. In Whole Earth Software Review , Spring 1984, Tony Bove and Cheryl Rhodes wrote, "(It) dropped characters if you typed too fast. During “wraparound” it nibbles at your keystrokes while it does what it wants." I did not personally encounter this in my time with the program. It may have been addressed as a bug fix in v2.0z, though I can't find evidence it did. It may be that the emulator itself provides more consistent keyboard polling than the original hardware did and keeps up with my typing. Or maybe I didn't flex my superior Typing of the Dead skills hard enough? No, for me the most immediate and continually frustrating aspect of using Electric Pencil is its "overtype" mode. This is a feature you still see in text editors and word processors, maybe hidden under the "Advanced" preferences, requiring a conscious choice to enable it. The modern default is to "insert" when typing. Place a cursor, start typing, and your words are injected at the point of the cursor. The text to the right moves to accommodate the new text. Overtype, as the name suggests, types over existing words replacing them. The amount of time I've spent reversing out inadvertent overtyping when I just wanted to add a space, or especially a line break, must surely have subtracted a full hour of my life by now. I have to remember to jump into "insert mode" when I want to retroactively add more space between paragraphs. I'm not one to suggest it is without its merits, though if my life were dependent on finding a reason for its existence my life would be forfeit. But what I can say absolutely is that losing your words to overtype really sucks in a program with no "undo" option. I mentioned Harvard Pennington earlier, and I want to spend a little time talking about the transfer of Electric Pencil from Shrayer to Pennington. I'm using v2.0z of Electric Pencil and it would be unfair to fail to note Michael Shrayer's absence in its development. According to the v2.0z manual, "Shrayer continued to sell ( Electric Pencil ) until January of 1981. By this time (it) was losing ground...Michael was not ready to devote his life to the daily chore of...doing business around the world." Harvard Pennington had already published a book written in Electric Pencil and wanted to keep his beloved software alive. Shrayer sold Pencil to Pennington's company International Jewelry Group. Yes, you read that right, "jewelry." But it's OK, they just called themselves IJG and their hard pivot was papered over nicely. "Pennington got together with...fans and programmers" to create a patched, improved version. Now I say "improved" because I read a review or two that suggested that. I can say absolutely that the command menu in v2.0z is a marked improvement over v1. It helps push some nuclear options (like to clear all text) behind a much-needed protective wall. But it also has, in hindsight perhaps, a couple of baffling decisions. First, if IJG was trying to improve the product, I don't really understand why the save file mechanism remains so inextricably linked to cursor position. Rather than fix it, a slightly snarky lambaste in the manual was added. Good job team, problem solved? The biggest change is the utter removal of WordStar -esque cursor controls. v1 had it, v2 doesn't. The cursor control keys are "unused" according to the v2.0z manual. The functionality was removed and replaced with nothing . Why concede a feature to your competitor that is so useful they literally named their own product for it? Just above I even called it WordStar -esque despite Pencil being first-to-market! In Creative Computing Magazine , November 1984, Pennington, newly elected chairman of the board at IJG, wrote an article about the future of computing. A jewelry salesman turned author and Pencil fanboy, now in charge of stewarding Pencil 's future, saw the coming wave of Macintosh-inspired GUIs. What did he think of this sea-change? "So what is in our future? Windows? A mouse? Icons? This is the hot stuff for 1984. How do we know it is the hot stuff? Because the computer press tells us. And how do they know? Because the marketing people tell them (and they know) because the finance people have determined the next "hot" item. How does the financial community know? They don't. However, no one is going to tell them to take their money elsewhere ... If you can come up with an idea that is bizarre enough, you can probably raise (millions) to bring it to market ... It is hype." Within two years of this bold, anti-GUI stance, Pennington would sell Electric Pencil to Electric Software Corporation. Later, PC Magazine would review ESC's v3.0 in the February 24, 1987 edition. They praised how much you got for $50, but also called it "not easy to learn" and "not for beginners." By then, with so much competition in the genre it had birthed, Electric Pencil was effectively dead. As the v1 manual states, "THE BEST WAY TO LEARN TO OPERATE THIS SYSTEM IS TO USE IT !!!" That's proving true! Why did the v2.0z manual remove that statement?! The learning curve is relatively shallow, benefitting from the software being sparse; there's just not that much to learn. From the perspective of a "daily driver," it's not growing on me. Getting into a flow is proving difficult. (future Chris here; as I come back to edit this document later, the editing flows more freely though I can't claim it is "natural." More like I'm just better at anticipating quirks, and there are plenty.) Part of editing is rearranging, but we need to forget what we know about "cut, copy, and paste." Those words had not yet been commonly adopted to describe those actions. Instead, we have the "block" tool. adds a visual marker to indicate the start of a block. Move the cursor and do that again to add a second marker at the end of a block. The text between markers is your block. Place the cursor elsewhere in the document and will clone the delimited block into the current cursor position; the original block stays as-is. You can clone as much as you like. "Cut" as we understand it today doesn't exist. deletes the block. It is GONE, no longer in memory at all. Remember also, there is no "undo" in this program, so gone really does mean GONE. Good enough for government work, I guess, just watch your step. The only feature left worth discussing is find/replace. brings up a modal screen for searching for a word. In James Fallow's discussion on it, he noted that he would use it for complicated words. He would insert a or some other unusual character to stand-in for a complex name, for example the Russian surname . Then, when he was done editing he would do a find-and-replace on the character for the surname. It only looks for the literal, case-sensitive text you type, although wild cards may be used. This is also your only method for jumping to known parts of the document. Search for a word and replace it with nothing to jump from search result to search result. Aside from some era-specific, esoteric commands, that's basically all Electric Pencil has to offer. It would have been fun if I could have tried the tape-dictation controls to transcribe a conversation. Spell-check isn't part of the base system, though it was available as an add-on. It is bare-bones, utilitarian, and sometimes frustrating for a modern user. It's forcing me to evaluate my personal threshold between "just enough" and "not enough" in such software. For me, this one is "not enough." With a few quality of life additions I suppose it could be sharpened up for the "distraction free" crowd. Maybe v3.0 of Electric Pencil PC is just right, if overtype is your jam? As-is, it is hard to recommend it on the TRS-80 for much more than writing a review of it. But don't worry, I teased some fan-fiction and I am a man of my word. I'd like to return to the Creative Computing Magazine issue where Pennington kind of poo-pooed windows, mice, and icons, "What is their future? They are here to stay. That does not mean that they will be used." In that same issue, Shrayer also gave his thoughts about the future of computing. Michael Shrayer died October 19, 2006 in Arlington, Texas. He was 72. Ways to improve the experience, notable deficiencies, workarounds, and notes about incorporating the software into modern workflows (if possible). Speaking honestly, too much is missing to recommend Electric Pencil on the TRS-80 for a modern workflow. trs80gp v2.5.5 on Windows 11 Emulating the default TRS-80 Model III 48K RAM 4 floppy drives TRSDOS v1.3 Electric Pencil 2.0z Move the cursor to the start of the document with (move to the "beginning") to open the command tool screen. type as Can't remember the filename? type Don't see your file listed? That's because only lists /pcl files. will list everything on disk, including /txt files like I'm using. In the menu, go to an empty drive number Select to get a blank, formatted disk for your work See how the disk name has around it? That means it is referencing an internal copy of a master blank disk. Your changes to it will not be saved to this disk In the drive menu for this disk, select and give your disk a name. In the drive menu for this disk, select for the named disk Now and select the disk you exported above. This disk is your personal copy, saved to the host operating system, ready to accept your writes. When you finish working and want to shut down the emulator, check the menu to see if any disks are prefixed with . If so, that disk has changes in memory that have not yet been written to the filesystem. Those changes will be lost if you shut down. Use on that diskette to save its changes out. trs80gp offers relatively minimal options for speeding up performance. It automatically kicks in "Auto Turbo" mode when accessing the disk drive, so I didn't find it annoyingly slow to read/write even though I'm using virtual floppies. A virtual hard disk is an option, but configuration looks... complex . I'll dig into that later. mode makes a noticeable difference in input and scroll speed. I didn't notice any troubles using that mode with this software. It was definitely a time-saver to set up the Windows shortcut with launch parameters (Properties > Shortcut > Target) to auto-insert program and save disks on launch, enable "Turbo", and set the emulator's typing mode to "Typing Layout" (see Troubleshooting, below) Options for those not running on Windows The TRS-80 emulator scene itself seems fairly anemic, especially outside of Windows. There is a Javascript emulator , but it feels a little heavy for my lightweight needs, and the hosted online versions seem to disallow arbitrary disk insertion. I'm completely unclear how to get at my personal data even if I did manage to save my work. That said, it is open source on Github and may be a better option than my initial tests indicated. So I suppose you could run a Node server? Or run it in a web browser? to interface with an emulator which runs the software. How many abstraction layers are you willing to put up with? For a native macOS app, the only option I can recommend is kind of non-native: run trs80gp and trstools in WINE. No other app is maintained to work on modern macOS, or if it runs its "experimental" or broken in some fundamental way to render it unusable. On Linux, I'm still investigating. Remember: in trs80gp's Diskette menu if you see beside a drive, that means it has been written to virtually but has not been written to the host operating system yet. This can happen if you have a diskette with brackets, indicating a virtual copy of an internally defined master diskette. Export that diskette, stat! Keyboard input has three options. One is notably called "Typing Layout" and it addresses the issues I encountered with certain character inputs doing a kind of double-input. For example typing always resulted in printing to screen. "Typing Layout" felt much more stable and behaved as expected, though it had its own quirks (see What's Lacking, below) If you get an error saying , you probably have the cursor at the end of the file and are trying to save. to move to the start of the file before saving. Cannot stress this enough. In a sense it is easy, and in a sense it is annoying. The easiest way I found to get my data out of TRSDOS world is through the utility trstools . Use it to browse your TRSDOS disk, then simply open a file and copy out the contents. It's just plain ASCII; there are no special word processing code embeds. Caveat! There are no embeds unless you use the printer formatting functions! Then there are absolutely embedded codes and they're unfriendly to a modern workflow. They only apply to doing real printing to real vintage printers, so I recommend ignoring those features. No undo whatsoever. This bit me more than once thanks to (delete TO end of line) and (delete ENTIRE line) being right next to each other. You are essentially restricted to no formatting or a subset of markdown formatting. Getting the emulator keyboard and Electric Pencil to be happy together has simply not panned out for me. If I use "Logical Layout" I get double-input on many keys. If I use "Physical Layout" my muscle memory of where lives (for example) betrays me every time. If I use "Typing Layout" keys like stop working and keyboard commands for marking blocks of text don't work any more. There is no perfect keyboard choice for this program that I can find. No spell-checking without a secondary package like Blue Pencil or Electric Webster's . Search is strictly, only case-sensitive. For writing a basic skeleton of a document for this very blog, it worked well-enough. But to restrict all editing to Electric Pencil means not touching a thing within the Ghost blog platform. That is hard to resist. Limited keyboard support means writing without certain characters that come up in a modern context quite a lot, like The default "overtype" mode definitely has an adjustment period to get through, and will surprise you with how often it deletes the first character of a line of text when all you wanted to do was insert a new line. Getting the data out isn't a horrible process, but adds enough friction to the process to make it frustrating in a rapid write-edit-publish cycle. The small amount of text on screen at one time makes it difficult to read and scan through long text to find specific passages for editing. If you're a visual editor, it's going to be a rough ride. This predates Unicode and software-based fonts, so no international writing!

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Maurycy 1 months ago

Modifying a radiation meter for (radioactive) rock collecting:

The Ludlum Model 3 is quite a nice radiation meter, as long as you like analog displays and don’t mind it weighing one and a half kilograms: These can be found used for a reasonable price, are easy to fix, nearly indestructible, and have an adjustable HV power supply that will drive just about any Geiger tube or scintillation detector you can throw at it. Geiger tubes are mostly sensitive to alpha and beta particles: great for detecting surface contamination and measuring weakly active specimens but nearly useless for finding them: All it takes is a bit of dirt on top of the rock it and the counter will detect nothing. Scintillation counters are super super sensitive to gamma radiation, which punches right though dirt and rock. However, because of this, they pick up background of several hundred counts per second, making the audio clicks completely useless. Without audio, you have to constantly look at the meter face, guaranteeing that you trip on something. Using the meter for rock hounding is certainly possible, but it’s not fun. The most obvious solution is to divide down the clicks with a counter. While this is a huge improvement over the raw output, it’s not perfect: All the background radiation picked up by a scintillator makes weaker signals imperceptible. Would you notice if one click came a few milliseconds earlier then the last? I certainly can’t, but a computer has no problem: Source code: tone.c Prebuilt binary: tone.elf The microcontroller measures the current count rate, subtracts a background reading and converts the difference into an audible tone. On my meter, I got switch controlled power from the reset button and the event signal from pin 3 of the CD4093BE IC. Because the CD4093BE runs at a high voltage then the battery, I added a 22k resistor to protect the MCU. The Ludlum Model 3 has had multiple redesigns, your meter will probably be different: find an appropriate manual or poke around with an oscilloscope. Here’s what the circuit looks like wired into my meter: The microcontroller is wrapped in tape to avoid shorts, and fits in the gap between the board and case when the meter is closed. I made sure to keep wires away from the high voltage section: a stray arc could easily destroy the microcontroller. After taking the photo, I tapped the microcontroller to the board so it doesn’t get trapped under one of the transformers or other large parts. Here’s the circuit detecting some weakly active granite, that measures ~10% above background: (10 CPS on my detector, and around 0.5 CPS a Radiacode 102) This is a good demonstration of the squelch and background subtraction: A ten percent increase over background would normally be imperceptible. In the field, an increase like this can indicate a deeply buried specimen that would otherwise be missed. The same circuit should also work in other meters, but you’ll be on your own have to find needed signals: The circuit needs power, ground and a digital event signal. You might need to adjust some parameters in the code depending on your scintillator’s sensitivity. Because it relies on making fast and accurate measurements, I wouldn’t recommend it for small detectors like the Radiacode, Raysid, or Better Geiger: Anything with a background rate below 30 counts/second is unlikely to work well. If you want something cheap, look around for used scintillators: The 1.25*1.25*3 inch CsI(Tl) detector in the video cost me $60 delivered, and it’s got a crystal 76 times bigger then the Radiacode 102’s. When powered on, the meter will beep once, take a three second calibration measurement, and then beep a second time to indicate that it is ready to use. During operation, it takes a one second sliding-window average and outputs a tone depending on the measured count rate. The audio is turned off if the count rate is not significantly above the background measurement, which preserves power and makes hot spots easier to notice. If the sound activates, simply go towards the direction in which the pitch increases until you find the source. If … … No audio is produced: Try disconnecting the “Audio switch” line. If this fixes it, you wired it to the wrong side of the switch. Otherwise, check that the microcontroller has power, has been programmed, and that the speaker amplifier is wired correctly. … The meter beeps 10 times after calibration: No counts were detected during calibration. Make sure the probe is connected and working, and check the wiring. … The meter makes noise even with no radiation source: This happens if the meter is taken to an area with more background radiation then during calibration. Recallibrate the background by turning it off, and then on again. … The meter is beeping: This happens if the reading is below the callibrated background, usually because the meter was taken to a different area. Reset the background by turning it off, and then on again. … The meter randomly screams and pegs the needle: This is usually caused by a bad connection somewhere, most commonly in the cable.

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Fakeman Show 1 months ago

Reverse Engineering a 1979 Camera's Spec

I bought a 1979 Chinon CM-4 film camera in Tijuana. Film is expensive, so before wasting a roll I decided to learn exactly how this machine works — by taking apart its specs, one line at a time. So here is my three step plan Understand what I really have in my hands. Michael Butkus created a comprehensive document that covers the details and specifications of the Chinon CM 4.

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./techtipsy 2 months ago

Those cheap AliExpress 18650 Li-ion cell power bank enclosures suck, actually

I had a few old ThinkPad batteries lying around. They were big, bulky and not holding much of a charge. Inside those were standard 18650 Li-ion battery cells. I have two TOMO M4 power banks around, and they are fantastic for reusing these old 18650 battery cells inside them. You can even mix and match cells without a worry because they are individually addressed, meaning that any issues with battery charge levels and voltages differing between cells are not a concern. Unfortunately the TOMO M4 lacks modern features, such as USB-C ports and USB-C PD outputs at higher voltages and currents, which makes it less useful and convenient in 2025. I haven’t found any newer designs from them as well that are just as cool. I still wanted to reuse those 18650 cells, so I went to AliExpress and bought some 18650 battery enclosures for testing. One holds 8 cells, another one 10 cells, and the largest one could fit 20 cells inside it. Unfortunately, they all suck and are likely a huge fire hazard in the wrong hands. For the 8-cell variant, I used newly bought 18650 Li-ion cells that were charged up to the same level. This battery enclosure worked quite well, until it didn’t. For whatever reason, the enclosure could not charge itself and other devices at the same time. With the 10-cell variant, I used two different batches of used 18650 Li-ion cells from old ThinkPad batteries, charging them up first. That one worked fine, until it also failed in weird ways. It got quite hot during charging/discharging cycles, and eventually the segment display that’s responsible for displaying the charge level stopped showing certain segments. At that point I lost trust in that enclosure, too. I had the most fun with the 20-cell battery enclosure. My first fuck-up involved using two old battery cells with different charge levels, which resulted in some magic smoke coming out of the PCB of the enclosure itself. Somehow that didn’t break the battery bank enclosure, so I crammed 20 charged up used and mixed 18650 Li-ion cells in it and started charging and discharging it. The batteries got quite hot, likely around 50-70°C based on the temperature readings of my hand. 1 At that point I realized I was playing with fire and stopped. The USB-C PD behaviour was different on all power banks. Some were fine with powering a ThinkPad laptop with the appropriate cable, some were flaky with setting the power levels, and some were just useless with certain cable or device combinations. The battery banks rely on a very simple arrangement: the 18650 Li-ion cells are connected in parallel, and the resulting 3.7-4.2V is then boosted up for the appropriate voltage on the control board. This carries risks: if you insert two or more Li-ion cells with different voltages, then one will start charging the others to bring the cells to the same voltage, and that can become uncontrolled and result in a cell overheating and/or exploding. It’s also a horrible idea to mix and match used cells of different capacities and wear levels as they will charge and discharge at different rates. In my experience, a cheap DIY power bank enclosure also carries the risk of attracting attention at an airport security check. After learning how bad these can be, that is an entirely justified suspicion. I ended up throwing all the battery bank enclosures out, the hardware failures and issues made me too concerned about one of these starting a fire. I like controlled fires, but the uncontrolled ones are really not my cup of tea. If you know of a 18650 Li-ion cell battery bank enclosure that works like the TOMO M4 but has modern features (USB-C port, USB-PD, can charge laptops etc.) then please do reach out to me as I’d love to test one out. You can find the contact details below the post. 50-55°C feels very hot to the touch, so it’s a good rule of thumb (no pun intended) for determining the minimum temperature of a hot surface by hand. Disclaimer: not physics advice.  ↩︎ 50-55°C feels very hot to the touch, so it’s a good rule of thumb (no pun intended) for determining the minimum temperature of a hot surface by hand. Disclaimer: not physics advice.  ↩︎

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