Brass And Nickel Work Together In This Magnetostrictive Earphone

When you go by a handle like [Simplifier], you’ve made a mission statement about your projects: that you’ll take complex processes and boil them down to their essence. So tackling the rebuilding of the humble speaker, a device he himself admits is “both simplified and optimized already,” would seem a bit off-topic. But as it turns out, the principle of magnetostriction can make the lowly speaker even simpler.

Most of us are familiar with the operation of a speaker. A powerful magnet sits at the center of a coil of wire, which is attached to a thin diaphragm. Current passing through the coil builds a magnetic field that moves the diaphragm, creating sound waves. Magnetostriction, on the other hand, is the phenomenon whereby ferromagnetic materials change shape in a magnetic field. To take advantage of this, [Simplifier] wound a coil of fine copper wire around a paper form, through which a nickel TIG electrode welding filler rod is passed. The nickel rod is anchored on one end and fixed to a thin brass disc on the other. Passing a current through the coil causes the rod to change length, vibrating the disc to make sound. Give it a listen in the video below; it sounds pretty good, and we love the old-time look of the turned oak handpiece and brass accouterments.

You may recall [Simplifier]’s recent attempt at a carbon rod microphone; while that worked well enough, it was unable to drive this earphone directly. If you need to understand a little more about magnetostriction, [Ben Krasnow] explained its use in anti-theft tags a couple of years back.

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Wood And Carbon Rods Used For This Handsome And Effective Microphone

Anyone who was active in the phreaking scene or was even the least bit curious about the phone system back in the Ma Bell days no doubt remembers the carbon capsule microphone in the mouthpiece of many telephone handsets. With carbon granules sandwiched between a diaphragm and a metal plate, they were essentially sound-driven variable resistors, and they worked well enough to be the standard microphone for telephony for decades.

In an attempt to reduce complicated practices to their fundamentals, [Simplifier] has undertaken this surprisingly high-fidelity carbon microphone build that hearkens back to the early days of the telephone. It builds on previous work that was more proof of concept but still impressive. In both builds, the diaphragm of the microphone is a thin piece of wood, at first carved from a single block of softwood, then later improved by attaching a thin piece of pine to a red oak frame. The electrical side of the mic has four carbon rods running from the frame to the center of the diaphragm, where they articulate in a carbon block with small divots dug into it. As the diaphragm vibrates, the block exerts more or less pressure on the rods, varying the current across the mic and reproducing the sound. It works quite well, judging by the video after the break.

Congratulations to [Simplifier] for another great build and top-notch craftsmanship. We’ve seen homebrew vacuum tubes, conductive glass, and solar cells from him before, which sort of makes him the high-tech version of Primitive Technology. We’re looking forward to whatever comes next. Continue reading “Wood And Carbon Rods Used For This Handsome And Effective Microphone”

Capture A Star In A Jar With Sonoluminescence

If nothing else, [Justin Atkin] is persistent. How else do you explain a five-year quest to create sonoluminescence with simple tools?

So what exactly is sonoluminescence? The short answer is as the name suggests: a release of light caused by sound. In [Justin]’s case, he used an ultrasonic transducer to set up a standing wave at the resonant frequency of a flask of water. A drop of water is used to entrain a small air bubble, which is held in a stable position in the flask in much the same way as styrofoam beads are in an acoustic levitator. Turn off the lights and you’ll see that the bubble glows with a ghostly blue light.

What causes the glow? Good question. According to [Justin], we just don’t know for sure what causes it, although the leading theory is that cavitation of the bubble causes the trapped gas to compress and heat violently, turning into a brief bit of plasma. But there are problems with that theory, which is one of the reasons he wanted to show just how easy the process can be – now that he’s shaken out the bugs with five years of effort. It wasn’t easy getting the transducers attached and the driver circuit properly tuned, but with little more than a signal generator, an audio amp, and a spool of magnet wire, you too can make your own “star in a jar.”

We applaud [Justin]’s determination to bring this project to a successful conclusion. It’s not unlike his dogged effort to make a cold plasma torch, or even his desktop radio telescope.

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Simple Ultrasound Machine Shows The Skeleton Lurking Inside Us All

That first glimpse of a child in the womb as a black and white image on a screen is a thrilling moment for any parent-to-be, made possible by several hundred thousand dollars worth of precision medical instrumentation. This ultrasound machine cobbled together from eBay parts and modules is not that machine by a long shot, but it’s still a very cool project that actually gives a peek inside the skin.

The ultrasound transducer used by [stoppi71] in this build has an unusual source: a commercial paint-thickness meter. Cue the jokes about watching paint dry, but coatings measurement is serious stuff. Even so, the meter in question only ran about $40 on eBay, and provided the perfect transducer for the build. The sender needs a 100V pulse at about 5 MHz, so [stoppi71] had some fun with a boost converter and a 74121 Schmitt-trigger one-shot driving a MOSFET to switch the high voltage. On the receive side, the faint echo is sent through a three-stage amp using AD811 op amps before going through an LM7171 op amp acting as a rectifier and peak detector. Echos are sent to an Arduino Due for display on a 320×480 LCD. The resolution isn’t great, but the video below shows that it’s enough to see reflections from the skin of [stoppi71]’s forearm and from the bones within.

[stoppi71] says that he was inspired to tackle this build by Murgen, an open-source ultrasound project. That project got further refined and entered into the “Best Product” category in the 2018 Hackaday Prize. We like that because focusing on turning projects into products is what this year’s Hackaday Prize is all about.

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Flex PCBs Make Force-Mapping Pressure Sensor For Amputee

What prosthetic limbs can do these days is nothing short of miraculous, and can change the life of an amputee in so many ways. But no matter what advanced sensors and actuators are added to the prosthetic, it has to interface with the wearer’s body, and that can lead to problems.

Measuring and mapping the pressure on the residual limb is the business of this flexible force-sensing matrix. The idea for a two-dimensional force map came from one of [chris.coulson]’s classmates, an amputee who developed a single-channel pressure sensor to help him solve a painful fitting problem. [chris.coulson] was reminded of a piezoresistive yoga mat build from [Marco Reps], which we featured a while back, and figured a scaled-down version might be just the thing to map pressure points across the prosthetic interface. Rather than the expensive and tediously-applied web of copper tape [Marco] used, [chris] chose flexible PCBs to sandwich the Velostat piezoresistive material. An interface board multiplexes the 16 elements of the sensor array to a PIC which gathers and records testing data. [chris] even built a test stand with a solenoid to apply pressure to the sensor and test its frequency response to determine what sorts of measurements are possible.

We think the project is a great application for flex PCBs, and a perfect entry into our Flexible PCB Contest. You should enter too. Even though [chris] has a prototype, you don’t need one to enter: just an idea would do. Do something up on Fritzing, make a full EAGLE schematic, or just jot a block diagram down on a napkin. We want to see your ideas, and if it’s good enough you can win a flex PCB to get you started. What are you waiting for?

DIY Ribbon Element Upgrades A Studio Microphone

For those with some experience with pro audio, the term “ribbon microphone” tends to conjure up an image of one of those big, chunky mics from the Golden Age of radio, the kind adorned with the station’s callsign and crooned into by the latest heartthrob dreamboat singer. This DIY ribbon mic is none of those things, but it’s still really cool.

Of course the ribbon mic isn’t always huge, and the technology behind it is far from obsolete. [Frank Olsen]’s ribbon mic starts out with gutting a run-of-the-mill studio mic of its element, leaving only the body and connector behind. The element that he constructs, mostly from small scraps of aluminum and blocks of acrylic, looks very much like the ribbon element in commercial mics: a pair of magnets with a thin, corrugated strip of foil suspended between them. The foil was corrugated by passing it through a jig that [Frank] built, which is a neat tool, but he says that a paper crimper used for crafting would work too. There’s some pretty fussy work behind the cartridge build, but everything went together and fit nicely in the old mic body. The video below was narrated using the mic, so we know it works.

Fun fact: the ribbon microphone was invented by Walter Schottky. That Walter Schottky. Need more on how these mics work? Our colleague [Al Williams] has you covered with this article on the basics.

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72 Tranducers For Acoustic Levitation

Levitation has a way of arousing curiousity and wonder wherever it appears. There’s a multitude of ways to do it, each with their own strengths and weaknesses and ideal use cases. [Julius Kramer] tried his hand at acoustic levitation, and decided to share his build.

The build relies on an astounding number of ultrasonic transducers – 72, in fact. The device operates at 40 kHz to be well above the human range of hearing. 36 each are placed in the top and bottom shells of the device’s 3D printed chassis. Through careful construction, the transducers are placed an integer multiple of half the wavelength apart. This allows the device to create a standing wave, with several low-energy nodes in which small objects can be levitated. In this case, [Julius] uses small scraps of styrofoam, but notes that water droplets can also be used if one is careful to avoid spilling any on the electronics.

The transducers are energised with a square wave generated by an Arduino Nano. This allows the possibility of the frequency and phase of the wave to be altered, which can help tune the device and allow some movement in the vertical axis. Unfortunately, movement in the other axes isn’t possible as the transducers appear to be connected in parallel. However, this could be a good upgrade in a later revision.

This project shows that a device relying on incredibly precise measurement and control can now be constructed at home with a 3D printer and some off the shelf electronics.

Now that you’ve whet your whistle, perhaps you’d like to tackle laser levitation?

[Thanks to Baldpower for the tip!]