Progressive Or Thrash? How Metal Detectors Discriminate

Metal detecting is a fun pastime, even when all you can find is a little bit of peace and a whole lot of pop tabs. [Huygens Optics] has a VLF-based metal detector that offers much more feedback than just a beep or no beep. This thing is fancy enough to discriminate between types of metal and report back a numerical ID value from a corresponding range of conductivity.

Most pop tabs rated an ID of 76 or 77, so [Huygens Optics] started ignoring these until the day he found a platinum wedding band without looking at the ID readout. Turns out, the ring registered in the throwaway range. Now thoroughly intrigued by the detector’s ID system, [Huygens Optics] set up a test rig with an oscilloscope to see for himself how the thing was telling different metals apart. His valuable and sweeping video walk-through is hiding after the break.

A Very Low-Frequency (VLF) detector uses two coils, one to emit and one to receive. They are overlapped just enough so that the reception coil can’t see the emission coil’s magnetic field. This frees up the reception coil’s magnetic field to be interrupted only by third-party metal, i.e. hidden treasures in the ground.

Once [Huygens Optics] determined which coil was which, he started passing metal objects near the reception coil to see what happened on the ‘scope. Depending on the material type and the size and shape of the object, the waveform it produced showed a shift in phase from the emission coil’s waveform. This is pretty much directly translated to the ID readout — the higher the phase shift value, the higher the ID value.

We’ve picked up DIY metal detectors of all sizes over the years, but this one is the ATtiny-ist.

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Solenoid Engine Adds Three “Pistons”

The earliest piston engines typically had only one cylinder, and at best, produced horsepower measured in single digits. But once you have a working engine, it’s a relatively short step to adding cylinders and increasing the power output. [Emiel] made a similar upgrade to one of his engines recently, upgrading it from one cylinder to four. But this isn’t an internal combustion engine, it gets its power from electric solenoids.

We featured his single-cylinder build about a month ago, and since then he’s been busy with this impressive upgrade. The new engine features four cylinders arranged in a V4 pattern. Of course, this greatly increases the mechanical complexity. To start, he had to machine a crankshaft to connect all four “pistons” to a shared output shaft. He also had to build a set of cams in order to time the firing of the cylinders properly, so they don’t work against one another.

The build is just as polished and impressive as the last, which is saying a lot. [Emiel] has a quality machine shop and built the entire motor from scratch, including winding the solenoids, machining the connecting rods and shafts, and building a very picturesque wooden base for the entire contraption to sit on. It’s definitely worth checking out.

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3D-Printed Transformer Disappoints, But Enlightens

Transformers are deceptively simple devices. Just coils of wire sharing a common core, they tempt you into thinking you can make your own, and in many cases you can. But DIY transformers have their limits, as [Great Scott!] learned when he tried to 3D-print his own power transformer.

To be fair, the bulk of the video below has nothing to do with 3D-printing of transformer coils. The first part concentrates on building transformer cores up from scratch with commercially available punched steel laminations, in much the same way that manufacturers do it. Going through that exercise and the calculations it requires is a great intro to transformer design, and worth the price of admission alone. With the proper number of turns wound onto a bobbin, the laminated E and I pieces were woven together into a core, and the resulting transformer worked pretty much as expected.

The 3D-printed core was another story, though. [Great Scott!] printed E and I pieces from the same iron-infused PLA filament that he used when he 3D-printed a brushless DC motor. The laminations had nowhere near the magnetic flux density of the commercial stampings, though, completely changing the characteristics of the transformer. His conclusion is that a printed transformer isn’t possible, at least not at 50-Hz mains frequency. Printed cores might have a place at RF frequencies, though.

In the end, it wasn’t too surprising a result, but the video is a great intro to transformer design. And we always appreciate the “DIY or Buy” style videos that [Great Scott!] does, like his home-brew DC inverter or build vs. buy lithium-ion battery packs.

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Build Your Own Solenoid Engine

A solenoid engine is a curiosity of the electrical world. By all measures, using electricity to rotate something can be done almost any other way with greater efficiency and less hassle. But there’s just something riveting about watching a solenoid engine work. If you want to build one of your own and see for yourself, [Emiel] aka [The Practical Engineer] has a great how-to.

For this build though he used a few tools that some of us may not have on hand, such as a lathe and a drill press. The lathe was used to make the plastic spool to hold the wire, and also to help wind the wire onto the spool itself rather than doing it by hand. He also milled the wood mounts and metal bearings as well, and the quality of the work really shows through in the final product. The final touch is the transistor which controls power flow to the engine.

If you don’t have all of the machine tools [Emiel] used it’s not impossible to find substitute parts if you want to build your own. It’s an impressive display piece, or possibly even functional if you want your build to have a certain steampunk aesthetic (without the steam). You can even add more pistons to your build if you need extra power.

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Turn A Ceiling Fan Into A Wind Turbine… Almost

It’s not uncommon to drive around the neighborhood on trash day and see one or two ceiling fans haphazardly strewn onto a pile of garbage bags, ready to be carted off to the town dump. It’s a shame to see something like this go to waste, and [Giesbert Nijhuis] decided he would see what he could do with one. After some painstaking work, he was able to turn a ceiling fan into a wind turbine (of sorts).

While it’s true that some generators and motors can be used interchangeably by reversing the flow of electricity (motors can be used as generators and vice-versa) this isn’t true of ceiling fans. These motors are a type called induction motors which, as a cost saving measure, have no permanent magnets and therefore can’t simply be used as a generator. If you make some modifications to them, though, like rewiring some of the windings and adding permanent magnets around them, you can get around this downside of induction motors.

[Giesbert] does note that this project isn’t a great way to build a generator. Even after making all of the changes needed to get it working, the motor just isn’t as efficient as one that was built with its own set of magnets. For all the work that went into it, it’s not that great of a time investment for a low-quality generator. However, it’s interesting to see the theory behind something like this work at all, even if the end result wasn’t a complete wind turbine. Perhaps if you have an old ceiling fan lying around, you can put it to better use.

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Flexible PCB Robot Flops Around To Get Around

In his continuing quest to reduce the parts count of a robot as far as possible, [Carl Bugeja] has hit upon an unusual design: robots built of almost nothing but PCBs.

Admittedly, calling these floppy four-legged critters robots is still a bit of a stretch at this point. The video below shows that while they certainly move under their own power, there’s not a lot of control to the movement – yet. [Carl]’s design uses an incredibly fragile looking upper arm assembly made from FR4. Each arm holds a small neodymium magnet suspended over the center of a flexible PCB coil, quite like those we’ve seen him use before as actuators and speakers. The coils are controlled by a microcontroller living where the four legs intersect. After a few uninspiring tethered tests revealed some problems with the overly compliant FR4 magnet supports, [Carl] made a few changes and upped the frequency of the leg movements. This led to actual motion and eventually to untethered operation, with the bot buzzing around merrily.

There are still issues with the lack of stiffness of the magnet arms, but we’re optimistic that [Carl] can overcome them. We like this idea a lot, and can see all sort of neat applications for flapping and flopping locomotion.

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Turn Old Pinball Parts Into A Unique Digital Clock

It’s getting ever harder to build a truly unique digital clock. From electronic displays to the flip-dots and flip-cards, everything seems to have been done to death. But this pinball scoring reel clock manages to keep the unique clock ball in play, as it were.

It’s not entirely clear whom to credit with this build, but the article was written by [Lucky]. Nor do they mention which pinball machine gave up its electromechanical scoring display for the build. Our guess would be a machine from the ’60s, before the era of score inflation that required more than the four digits used. And indeed, the driver for the display is designed so that a scoring unit from any pinball machine from the electromechanical era can be used. An ESP8266 keeps the time with the help of an RTC and drives the coils of the scoring unit through a bunch of MOSFETs. The video below shows that it wouldn’t make a great clock for the nightstand; thankfully, it has a user-configured quiet time to limit the not inconsiderable noise to waking hours. It also flashes the date every half hour, rings solenoid operated chimes, and as a bonus, it can be used to keep score in a pinball game built right into the software.

We like the idea of honoring the old pinball machines with clock builds like this. We’ve seen a word clock built from the back-glass of an old machine, and one that uses a four-player back to display the date and alarm time too.

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