Mysterious Files PH

My old friend Jeff was always vocally upset that he didn’t come up with the idea of a string trimmer, commonly known as a Weed Eater or Weed Whacker. On the one hand, the idea is totally simple: spin some nylon line and cut grass and other relatively soft things. But, it turns out, that making the device actually usable requires a little bit of mechanical engineering.

Of course, the noisy part is a motor. The motor — driven by an engine, a battery, or a power cord — spins a flexible nylon line fast enough that the line becomes rigid from centrifugal force. That’s not the important part.

The humble spool at the bottom of the trimmer is where decades of mechanical engineering, questionable patents, consumer frustration, and genuine cleverness all meet. The earliest string trimmers were primitive. [George Ballas], who patented the Weed Eater in the early 1970s, reportedly got the idea from the rotating brushes in a car wash. Attach flexible cords to a spinning head, and they become cutting tools. In fact, the prototype used a tin can for the head. Elegant. But once the line wears down — which it does constantly — you need a way to expose fresh line. That turns out to be harder than it sounds.

The Simplest System

The easiest approach is fixed-length line. Some trimmers still work this way. You cut short pieces of heavy line (or buy it precut) and insert them into holes in the head. No spool. No springs. No moving parts.

These systems are rugged and are popular on commercial units designed to survive abuse. They also work well with thicker lines or even plastic blades. But they are annoying because every time the line wears out, you stop working and manually replace it. Spool-based systems became dominant very quickly.

The basic spool idea is straightforward enough. Wind a long nylon filament onto a reel. Some reels have two sections to feed line out on two sides of the rotating head. As the line wears away, feed out more line from the spool. But how do you do that while the thing is spinning at several thousand RPM?

Bump Feed

If you’ve ever lightly smacked the bottom of a running trimmer against the ground, you’ve used a bump feed mechanism.

Inside the head is a spool loaded with line and pressed upward by a spring. The line exits through eyelets on the side of the head. Under normal operation, friction and centrifugal force keep the spool from turning freely.

When you bump the bottom of the head against the ground, inertia momentarily compresses the spring and disengages locking tabs or detents. The spool can rotate briefly, paying out a short amount of line. When you release pressure, the spring re-engages the lock.

At least, that’s the theory. In practice, bump heads have to balance several competing requirements. The spool must not unwind accidentally. The line can’t bind. Dirt and grass clippings can’t jam the mechanism. The head must survive repeated impacts with concrete, rocks, and fence posts because users inevitably abuse them.

And then there’s the line itself. Nylon trimmer line is more complicated than it looks. Different diameters, shapes, and stiffnesses affect how well the feed works. Star-shaped line cuts aggressively but tangles more easily. Round line feeds smoothly but cuts less efficiently. Humidity even matters because nylon absorbs water. Anyone who has left old trimmer line in a garage for years has probably discovered brittle line snapping constantly. We’ve heard people suggest you soak the line — especially old line — in water overnight before loading it.

The bump feed mechanism has another subtle trick. Many heads rely on centrifugal force not only to stiffen the line but also to help lock the spool during operation. At speed, the line pulls outward hard enough to increase friction on the spool. When rotation slows, the spool loosens slightly. A simple mechanical solution.

Of course, they don’t always work and when that happens, you might find some troubleshooting advice in the video from [Will Shackleton] below.

Automatic Feed

Of course, someone decided bump feed was too much work and, thus, the automatic feed was born. These heads attempt to sense when the line has become too short and feed more automatically. These systems are common on electric consumer trimmers.

There are several ways to do this, but many use a ratchet-like mechanism tied to motor speed. When the load on the motor changes because the line becomes shorter, the system advances the spool slightly. Some units feed line every time the motor starts. Others use centrifugal clutches or vibration-sensitive mechanisms. Great when it works.

Part of the problem is that the operating environment is terrible. Grass juice, dirt, vibration, heat, and impacts are all happening simultaneously. It is hard enough to make reliable machinery in a clean factory. Designing a precise mechanism that lives inches from flying mud is another matter entirely. That’s why many professionals prefer simple bump heads despite the inconvenience. Simpler systems usually fail less dramatically.

You can see several head styles in the video below.

The Eyelets Matter More Than You Think

One overlooked component is the eyelet where the line exits the head. That little metal or ceramic ring takes an enormous amount of abuse. The line is moving at perhaps 200 miles per hour at the tip, vibrating continuously, and carrying abrasive dirt particles. A plain plastic hole would wear out quickly.

Some trimmers use hardened steel inserts. Others use aluminum oxide ceramics. The better heads often have replaceable eyelets because manufacturers know they are consumable parts.

The angle matters, too. The line should exit smoothly with minimal friction but still maintain enough control to prevent tangling. You probably don’t notice how important the eyelet is, but you’d notice if it were poorly designed.

Why Tangling Happens

Anyone who has reloaded a spool badly knows the pain of internal tangles. The spool effectively stores torsional energy. If the line is wound unevenly or crosses over itself, it can dig into lower layers under centrifugal load. Once that happens, the line jams. Pulling harder only makes it worse.

This is why most spools have directional arrows molded into them. The line must wind in the correct direction, so rotational forces tighten the winding instead of loosening it.

Modern “easy load” heads try to solve this by allowing users to thread the line straight through the head and then twist a knob to wind it automatically. These systems are genuinely better than older designs, although many still become incomprehensible the first time you disassemble one accidentally.

One trick we’ve heard is that if you spray a lubricant like WD-40 into the eyelet before you use the trimmer, it will help the mechanism feed more smoothly. Let us know if you’ve ever tried that and how it works.

Batteries Changed the Game

Cordless electric trimmers have altered feed mechanism design in subtle ways. Gas trimmers typically run at nearly constant speed, which makes centrifugal systems predictable. Battery trimmers vary speed more often due to electronic controls and power-saving logic. That means newer designs increasingly depend on passive mechanical systems rather than RPM-sensitive tricks. Electronic control also allows some high-end trimmers to detect load changes more intelligently.

Ironically, while motors and batteries have become dramatically more sophisticated, the line feed mechanism is still mostly springs, friction surfaces, tabs, and molded plastic. No microcontroller. No electronic sensors. Go figure.

The string trimmer looks like a brute-force tool. But hidden inside that disposable-looking plastic head is a surprisingly nuanced mechanical system balancing centrifugal force, friction, vibration, inertia, wear, and user abuse. Poor [George Ballas]. He took his prototype to toolmakers, who were all uninterested in the invention. He started the Weed Eater company and launched a lucrative product category.

We love finding all the strange tech around us, from shopping carts to gas pumps.

Featured image: “String trimmer” by Hedwig Storch

[Nick Electronics] had an idea to build a stylish lamp that could transform its shape while lit. This goal was achieved beautifully with the aid of many, many filament LEDs.

If you’re unfamiliar with filament LEDs, they’re basically thin plastic filaments stuffed with lots of individual LEDs that are very close together. This effectively creates a continuous, flexible, glowing string that can be used for all sorts of creative purposes.

[Nick] packed the lights into an interlocking stack of PCBs that make up the lamp’s structure. Each PCB layer hosts four filaments mounted around the outer edge, and has a pin that locks into a groove in the next layer to allow them to tug each other around as they turn. The PCBs rotate around a central shaft, with power passed from one to the other via interlinking wires. Drive is via a stepper motor on top of the lamp, controlled by an A4988 driver. There’s also an ATmega48 microcontroller onboard, which is the brains of the operation. A DC-DC converter onboard steps up the 5 V input voltage from USB-C to 10 volts for the stepper motor.

It’s neat to watch the lamp in action, glowing and slowly shifting in patterns as the layers catch and rotate in and out of alignment. We’ve seen interesting builds in this vein before, like this fantastic origami lamp from a few years ago.

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.