Mysterious Files PH

In a recent video [Saša Karanović] revisits the DIY filament dryer that he gave a shot a couple of years ago. Back then he reused an existing filament dryer, adding a custom controller and such to improve its performance. This technically-not-fully-DIY dryer got some feedback since then, and thus the V2 version is an example of how to better DIY such a dryer, including a custom PCB and a GitHub project for all the details.

Those who just want to dive into the documentation for assembly and the BOM can look at the available documentation. At its core the whole assembly consists of some kind of container like the shown 5L food storage type, along with an SHT30 temperature and humidity sensor and 100K NTC temperature sensor. These connect to the controller board which then switches on or off the 12V polymide resistive heater.

One thing that could be improved here is that the saturated warm air has nowhere to go. This is a common issue with filament dryers and why it’s recommended with even commercial filament dryers like the common Sunlu types to leave them slightly ajar so that the moist air can be replaced with cooler air that can much more readily absorb moisture.

A potentiometer is a simple electrical device that allows resistance to be varied at will. Most everyone in the electronics field is intimately familiar with how they work on a fundamental level. Of course, we all had to be taught once, though, and a great way to do that would be with a teaching tool like the one [DiscoLapy] built.

What you’re looking at here is a very simple potentiometer that bares its function for all to see. It consists of a 3D printed base and knob, which form the mechanical part of the device. A paper track is then laid on top to act as the main resistive element, once properly covered with graphite from a regular old pencil. From there, it’s as simple as adding the necessary contacts and wiper to the device, and you’ve got a potentiometer sitting in front of you.

What’s great about this build is that it’s very intuitive. Just by looking at it or putting it together, you get a straightforward understanding of everything that’s going on. By drawing the resistive trace, and by turning the knob, particularly if hooked up to an LED or something like in the demonstration, it’s easy to see how the potentiometer varies its resistance and affects a circuit.

We’ve featured some other fantastic teaching tools in the past, too. If you’ve got your own educational gems, be sure to let us know.

Normally, when you think of a radio transmitter, you want the strongest signal and range. But if your radio operator is secretly operating as a spy, broadcasting their position isn’t a feature; it is a liability. This fact didn’t escape World War II radio designers.

In late 1942, the British realized they needed a way for Special Operation Executive agents, resistance members, and other friendly forces to communicate with an aircraft without attracting undue attention. Two engineers from the Royal Corps of Signals developed a pair of transceivers — the S-Phone — operating around 380 MHz just for this purpose. Frequencies this high were unusual at the time, which further deterred enemy detection.

The output power was below 200 mW, and the ground equipment consisted of a dipole strapped to the operator. No transistors, so with rechargable batteries, the rig weighed about fifteen pounds and reused some parts of a paratrooper radio, Wireless Set Number 37. The other side of the connection was installed in an airplane.

Close Air Support

The low power and directional antenna meant that it was nearly impossible to pick up any signal on the ground if you were more than a mile away. The airplane that the operator was facing, on the other hand, could pick up the voice signal up to 30 miles away. Unfortunately, they also had to be under 10,000 feet, exposing the plane to enemy fire.

The highly directional gear could give the pilot a clue that he was closing on the target, and when the signal suddenly went out, it indicated that the aircraft was directly overhead the transmitter.

The Special Operation Executive had a lot of cool gear, and you can learn more about their gadgets and methods in the 1943 film “School for Danger” that you can see below. Look for the S-Phone at around the 7-minute mark. Interestingly, the two main characters are actual Special Operation Executive agents who actually did the things that are fictionalized in the movie.

The CryptoMuseum has a scan of the S-Phone manual. There are many interesting tidbits there. For example, the set came with a lamp that could show if the transmitter was working. These radios used early NiCad batteries. The manual goes to great lengths to explain that you should not try adding sulpheric acid to the batteries.

Joan-Eleanor

Where the British had the Special Operation Executive, the United States had the Office of Strategic Services. Working at RCA laboratories, OSS engineers along with [Al Gross W8PAL] who would become a pioneer in the development of walkie-talkies, pagers, and cordless telephones, designed the Joan-Elanor, named after the engineer’s wife and a WAC member.

Joan was the field tranceiver, technically SSTC-502, while Eleanor, SSTR-6, was mounted in the aircraft. Joan weighed less than four pounds, using a super-regenerative dual triode that doubled as the transmit oscillator. Originally, the radio was set for 250 MHz, but when it was found that the Germans had the ability to receive at that frequency, they pushed Joan-Eleanor to 260 MHz.

The radio had a range of about 20 miles and, unlike the S-Phone, the aircraft could fly overhead at 30,000 feet. It also took ordinary batteries, so you didn’t need a charger as the S-Phone did.

The system recorded transmissions on a wire recorder in the aircraft. The intent was that agents behind enemy lines could secretly transmit intelligence reports to aircraft flying what appeared to be routine reconnaissance flights.

The radio gear was usually jammed in the rear of the host aircraft, usually a DeHavilland Mosquito, along with an operator aft of the bomb bay. The operator entered the position through a side hatch and remained there the entire flight. You can see an OSS film about the system, which was classified until 1976, in the video below.

Tech

These radios had a few things in common. Both used frequencies that were uncommon at the time, making it less likely the enemy could overhear or even detect conversations. This made it less risky to speak “in the clear” so agents didn’t need incriminating code books and cumbersome encoding and decoding steps.

Similarly, both systems used voice, meaning that agents didn’t need to learn Morse code. They probably needed a little training to use the equipment, but that was far easier than expecting a resistance fighter to study Morse code for weeks.

While the S-Phone depended on directionality, Joan seemed content to rely on being high in frequency. Both had to be lightweight, easy to conceal, and quick to set up and take down.

The Joan radio was critical for agents going behind enemy lines. They’d be brought to an airbase in a car with blacked-out windows to prevent them from knowing where they were leaving from. They’d be given forged papers, an entrenching tool, local money in a belt, a pistol, a food package, a silk map, and, of course, a Joan radio.

We wonder if any Joan radios were captured during the war? A lot of wartime high-tech was highly protected, and we’re sure the agents were instructed on how to destroy the radios. Spies were also famous for using suitcase or even shoe radios.

Most people take the Moon for granted, not considering its slow cycle where the sun gradually illuminates different parts of it. A recent project from [Karsten Mueller] helps you keep our nearest celestial neighbor in mind by putting a tiny version on your desk. (German)

The device itself is made with a circular display, an ESP32-S3, and a simple 3D printed case. But the interesting part is the software — it’s not just a moon phase display, it actually takes your local time, latitude and longitude into account. The resulting image is an approximation of what the moon looks like if you were to look at it, even if you wouldn’t actually be able to see it, such as when it is obscured by the Earth or barely visible during the daylight sky. Initially the project actually used a photograph of the Moon that [Karsten] personally snapped, but there’s also an option to pull the imagery from NASA.

The original write-up is in German, but there’s also an English page for the project on Hackaday.io, and the source is available on GitHub if you’d like to put one together yourself.

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.