Mysterious Files PH

One of the problems with good science fiction is that it introduces us to all kinds of cool devices that we can’t actually have in real life. [Huy Vector] has tried to fix that a little with this fantastic smartwatch build inspired by everybody’s favorite wrist computer from the Fallout series.

The build is based around a Xiao ESP32-S3 board, which hosts the capable microcontroller and has all that useful wireless connectivity built in. It’s hooked up to a MAX30102 heart rate sensor to collect the wearer’s vital signs, as well as a 1.54″ LCD screen for displaying the fantastic Pip Boy themed interface. Power is courtesy of a small lithium-ion cell tucked in behind the display. A little copper tubing and brass hardware helps tie everything together, with the latter serving as capacitive touch points for controlling the device. A simple leather watch strap completes the build.

It’s a bit of a diversion from the classic Pip Boy design, in that it’s a small smartwatch instead of a chunky device that takes up most of the wearer’s forearm. However, this isn’t so bad in reality—it’s far more practical while still rocking those classic green-on-black graphics that we all love so much.

If you’re craving a more authentic Pip Boy recreation, we’ve featured a few of those, too.

Everyone loves those rather bouncy wooden lounge chairs that got popularized by a certain Swedish seller of furniture, but as tough as they are, the laminated wood can still break at some point. The chair that [John’s Furniture Repair] got in for repair had cracked right around where a bolt hole had been drilled, apparently creating a weak spot that over the years turned into a crack.

The way to fix this issue is to recreate the one piece of curved, laminated wood as demonstrated in the video. This starts with tracing the contours of the original part on a piece of MDF, which then gets doubled up by a second plate of MDF. After cutting out the contours this then creates the two halves of a mold for the laminated part.

Next is preparing the layers of wood that will become the new part, making sure to keep the same final thickness as the original. With everything glued up the layers are put into the mold, clamped down and the glue left to dry.

Finally, the part is freed from the mold, cut to its final size, and sanded down to prepare it for final treatment and installation on the lounge chair. Perhaps the only negative one can say about this kind of fix is that after you’re done, you really get that itch to sand down and re-lacquer all of the other parts as well so that they also look new and shiny.

If you’ve ever had a medical team investigating cardiac issues, you’ve probably had a bunch of electrodes stuck all over your chest and been hooked up to an electrocardiogram. This is the gold standard when it comes to understanding electrical activity in the heart and can diagnose a great many conditions. However, sometimes doctors just need the basic information—your pulse rate, and whether or not there’s actually any oxygen in your blood.

Thankfully, there’s a cheap and simple device that can offer that exact information. It’s the pulse oximeter, and it’s a key piece of equipment that’s just about vital for monitoring vitals. Let’s learn how it works!

Pump It

If you’re unfamiliar with pulse oximeters, they’re that little plastic thing that clips on your finger at the doctor’s office. The device places two LEDs on one side of your finger, and a photodiode on the other. With just these simple components, it’s possible to determine the percentage of your blood’s hemoglobin that is currently carrying oxygen. It’s also possible to discern pulse rate, which also comes in handy when you’re trying to determine a patient’s current status at a glance.

Pulse oximetery was the brainchild of Takuo Aoyagi, an electrical engineer at Nihon Kohden in Tokyo. In 1972 he was working on a non-invasive way to measure cardiac output using the dye dilution method, which involves injecting a tracer dye and watching how its concentration in the blood decays over time. He was reading that decay optically through an ear oximeter. These devices used red and infrared light passed through the ear tissue to determine blood oxygen levels, but required frustrating calibration to work properly and often required fussy steps like first squeezing blood out of the tissues prior to measurement. The problem was that early oximeters worked based on the total absorption of light, and were affected by things like the skin, tissue, and venous blood, when really the goal was to measure the oxygen levels in the arterial blood itself.

As Aoyagi worked with the device, he noted that the patient’s pulse kept showing up as an annoying ripple in the output. He spent some effort trying to cancel that ripple by balancing red and infrared signals against each other. Then he noticed that when a patient’s oxygen saturation dropped, the cancellation fell apart. This led to the realization that the ratio of how much red and infrared light was absorbed could be used to determine the oxygen saturation of the arterial blood.

It all comes down to the nature of blood itself. Hemoglobin comes in two flavours relevant here: oxyhemoglobin, which is carrying an O₂ molecule, and deoxyhemoglobin, which isn’t. They are different colours, which is why arterial blood is bright red and venous blood is darker. They absorb light differently, to the point that it’s actually clinically useful. At a wavelength of 660 nm (red)—deoxyhemoglobin absorbs noticeably more light than its oxygenated cousin. At around 940 nm (near-infrared), oxyhemoglobin absorbs more. Almost every pulse oximeter uses these two wavelengths; both penetrate tissue quite easily, and it’s easy to find LEDs that spit out these wavelengths.

Reading the blood oxygen level is relatively straightforward. The device will typically alternate the two LEDs on and off, many times a second, also including a third phase with both off so the photodiode can subtract out ambient room light as well. The photodiode sees light that has passed through an entire finger, including the skin, bone, fat, as well as the venous and arterial blood. Most of that doesn’t change from second to second, but the arterial blood does, with every pump of the heart. Thus, when sampling light from the infrared and red LED pulses, the photodiode puts out a signal that’s mostly a continuous level from light passing through the finger, with a little wiggly bit on top that throbs at a human pulse rate. That’s due to the pulsing of the arterial blood, and the frequency can be used to measure pulse rate. Meanwhile, the continuous component is removed by subtracting the trough of both the infrared and red signals from the peak, which solely leaves the component of light absorption due to the fresh arterial blood itself.

The level of oxygenation in the arterial blood itself can then be measured by comparing the ratio of red to infrared light picked up in this part of the signal. The light ratio is converted into an human-parseable number via a lookup table, based on the Beer-Lambert law of concentration of substances in a solution. The displayed number is flagged as “SpO₂.” The “p” stands for “peripheral,” to indicate it’s an optical measurement rather than determined directly with blood-gas measurement techniques. This distinction is important, as there are a range of conditions under which pulse oximetry readings can be inaccurate. At a very base level, pulse oximeters can get confused if a patient is moving while wearing the device, which makes the pulsatile signal itself less clear.  The device also cannot tell carboxyhemoglobin from oxyhemoglobin, because they absorb light very similarly at 660 nm.  Carboxyhemoglobin is the result of carbon monoxide entering the blood, so a smoke inhalation victim can display a high apparent SpO₂ figure while their blood is carrying very little oxygen. Nail polish and skin tone can impact the amount of light transmitted through the finger, impacting readings, while limited bloodflow to the fingers can also frustrate things.

It may not be perfect, but pulse oximetry is nevertheless very useful a lot of the time. It enables medical teams to get a near-instant look at a patient’s most vital signs in a completely non-invasive manner. The use of this technology has revolutionized both emergency care and surgery, where it has played a huge role in patient monitoring under anaesthesia. Plus, the simplicity of the device has made this critical medical insight accessible to anyone that can afford a $20 device with a few LEDs and a photodiode in it. It’s now even possible to track your oxygen saturation during sleep with an off-the-shelf smartwatch due to developments from this technology, helping aid in the diagnosis of complex conditions like sleep apnea. All because blood tends to pass light a little differently depending on how oxygenated it is. Sometimes you have to thank nature for those little conveniences.

Biofeedback is the idea of making one conscious of a biological process or feature, and then using this to try and exert control over the very same. [Mariia Hruntes] demonstrates this ably with a fluttering build of her own design.

In this case, the biological process being made clear is that of the user’s heartbeat. This is tracked with a MAX30102 pulse oximetry sensor, which can be used to measure both heart rate and blood oxygen levels if so desired. It’s hooked up to an Arduino Uno, which polls for pulse rate data, and then actuates an SG90 micro servo in turn. This operates the wings of a 3D printed butterfly, such that they flap in pace with the wearer’s pulse. The goal is to observe this, and then try and calm one’s self to relax and slow the flapping through the power of the mind.

It’s a simple build, but one that clearly demonstrates the concepts of biofeedback in action. We’ve seen similar principles applied to everything from aiding sleep to improving the practice of mediation. If you’re working on your own neat biofeedback project, be sure to let us know on the tipsline.

You could make a clock with three hands spinning about nested central shafts. If you did that, we probably wouldn’t publish it on Hackaday unless you really found a way to make it interesting. Make a clock out of voltmeters, however, and that usually catches our eye. [lcamtuf] has done just that.

The heart of the build is an AVR128DB28 microcontroller, an 8-bit microcontroller that is still currently in production. It runs at 8MHz, and drives a series of three Baomain 65C5 voltmeters to display hours, minutes, and seconds. Each has a custom printed face with the correct number of 13 or 61 divisions as needed. The voltmeters are driven by a continuous stream of 1-bit pulses with a software-controlled duty cycle determining exactly how far the needle moves. Yes, it’s using simple pulse width modulation, coded by hand by [lcamtuf] to do the job. All the components are wrapped up in a beautiful wooden case, with delicately kerf-bent panels to create the attractive curved lines.

We’ve featured similar builds before, too. As it turns out, hackers just really love clocks and old-school dials. Video after the break, which is worth watching for the rollover behaviour alone.

Most consumer-grade night vision devices are basically a standard camera without the usual filter to block near infrared (NIR) light, which are then paired with a NIR light source that’s not visible to the human eye. Unlike the passive night vision provided by a photomultiplier tube, these can’t resolve objects beyond the beam of their illumination source. On the other hand, if, as [Project 326] did, you use an infrared laser to illuminate the scene, you can still get a very long range out of these devices.

[Project 326]’s device consists of a previously-built reflecting telescope focusing a distant scene in to a webcam with the infrared filter removed, with the infrared laser illuminating the scene. Finding a suitable laser took some effort: the first option, a secondhand fiber-coupled industrial laser, was accidentally over-volted and destroyed during testing. The second had a fiber output which proved extremely hard to terminate, and a third laser couldn’t be collimated correctly. The final laser was a Vertical-Cavity Surface-Emitting Laser (VSEL) diode array element driven at about two Watts and collimated by a small lens.

This illumination setup is safe at a long range, but only at a long range. The laser was strong enough to burn cardboard at close range, but out at about 500 meters, the beam had spread until it was less than a hundredth of the standard safety limit. To make sure that nothing else would get in the way of the beam, it was shone down from the top of a tall building. Testing with a power meter also showed that at a long range, the beam was weaker than expected. It turned out that the wavelength used (940 nm) is attenuated by water vapor, to the point that up to 70% of the beam’s strength was lost before reaching the target. Despite this, and despite a rather linear beam profile, a somewhat dark image was still visible at 650 meters.

If you’re looking for a somewhat more versatile long-range night vision device, check out one based on a photomultiplier tube. Another approach is to use a very high-sensitivity camera.

Thanks to [Keith Olson] for the tip!

There are plenty of porous materials out there that we’re all readily familiar with. Fabrics and wood are great examples, allowing liquids or gases to pass through to a certain degree—a property which is useful or problematic depending on the application.

Metals, however, are not something we would readily consider to be porous. They are solid, unyielding, and impermeable. However, with the right techniques, it is possible to produce so-called “breathable” steel, which has particularly interesting applications in the molding industry.

Breathe Into Me And Make Me Real

Imagine you’re making tooling for an injection molding operation. You’re using steel, of course, because you need a hard, resilient material that can deal with the high temperatures and pressures involved. It’s tough, and readily able to be machined into the desired geometry for your application. Of course, it doesn’t let liquid or gas pass, since it’s a solid impermeable material. This means that when you inject your mold full of hot plastic, you need to find somewhere for the air inside to go. Otherwise, the gas in the mold will end up dissolved in the molten plastic, causing voids, surface imperfections, and other irregularities. Chasing away gas porosity defects in finished parts is one of the major jobs of casting engineers the world over, an endless battle against the forces of heat transfer and fluid mechanics.

Traditionally, this is deal by designing a mold with exhaust ports or vacuum hookups to allow the air to vent out as needed. This takes a great deal of work to get right, particularly when it comes to getting your defect rates as low as possible in mass production. If your gas can’t vent fast enough, or if there are areas where it gets trapped, you end up with defects, and you have to go back to the drawing board.

Breathable mold steel attempts to solve this problem by venting gas through the tooling itself. It allows the creation of a steel mold that is full of tiny little pores that allow air to pass through, while still acting largely impermeable to the molten plastic being molded.

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Breathable mold steel is quite something to behold, behaving quite unlike a normal steel part in this regard.

As you might imagine, it’s quite difficult to make a steel mold with complex geometry that also has lots of tiny contiguous holes that allow gases to pass through. It is possible, however, by using some tricky additive manufacturing techniques.

As outlined in one research paper, it’s possible to produce breathable steel via selective laser melting (SLM) 3D printing techniques. This involves using a high-powered laser to fuse metal powder together, layer by layer, to produce a final part. Combining a foaming agent with the metal powder enables the creation of 3D-printed metal parts with incredibly fine interconnected pores.

The pores need to be particularly small, on the order of 80 micrometers or less, such that they allow gas in the mold to pass freely while blocking the flow of the larger polymer molecules of the injected plastic.

Chromium nitride is one foaming agent typically used, for the fact that the Cr and N released during its decomposition both lend beneficial properties to the steel of the finished product. The foaming agent is mixed in with the steel powder, and melts along with it as the part is being produced. The breakdown of the foaming agent releases gas bubbles which creates pores in the steel part as it is produced in a relatively predictable manner.

The level of porosity can be controlled by the amount of foaming agent mixed in to the steel powder, as well as the laser settings. Lower melt pool temperatures caused by faster scanning speeds or lower laser powers tend to favor more porous structures, due to the fluid mechanics involved and how the cooler liquid steel flows into existing pores.

There have been earlier attempts to vent molds with special breathable steel inserts in the past. These consist of premade rectangular inserts or round bars which have been made with so-called “ventilated steel” like PM-35. This material is made by sintering steel powders together in such a way to create a porosity of 20-30%. However, this process isn’t always great for advanced geometry that one might find in a injection mold. Thus, the creation of breathable rods and bars that can be used as an insert in a larger mold, acting as a localized vent. It’s a useful technique, but comes with more constraints on mold and part geometry than being able to simply create the whole mold itself out of breathable steel.

There are other powder metal techniques that allow the production of more complex vented parts, but they can be expensive and difficult to execute well down to smaller pore sizes, especially compared to the simplicity of SLM printing with an additional foaming agent. The 3D-printing based process has also proven to have more admirable mechanical properties compared to products like PM-35 steel in some cases, with impressive compressive strength as well as hardness and corrosion resistance.

Breathable steel is probably not something you’ll come across in your everyday life unless you happen to work in particular manufacturing fields. Still, if you have the expensive 3D printing hardware on hand to work with metal powders, and you really want to make a complex metal part that’s also porous, this is a great way to go. You could probably use it to make some very weird magic tricks at the very least. Ultimately, it just goes to show that modern material processing techniques can upend everything we think we know about a common material like steel. It’s amazing what can be done!