This Mallet Has Backwards Dovetails… That’s Impossible!

February 22, 2020 by Mike Szczys 14 Comments

Dovetails are a wedge-shaped joint found in woodworking. The wedge makes for strong joinery because a force that tries to pull it apart also increases the friction on the joint. This mallet has dovetails on either side that keep the head from flying off, but there’s also a through tenon in the center. This is an impossible joint as there’s no way to slide the mallet head onto the handle. The two pieces of wood must have grown that way!

As with everything, there’s a trick here, let it scratch your brain for a while before reading on… if you can guess how it’s done it’ll be very satisfying when you confirm your theory. Both the trick of the impossible mallet and the superb hand joinery are shown off in this video from the [Third Coast Craftsman].

The trick comes in the form of internal voids hidden from view once the two pieces of the mallet have been assembled. The through tenon is exactly as you’d expect: a straight tenon slides into a straight mortise in the mallet. The dovetails to either side of the handle and the pockets they mate with in the mallet head are not at all what you’d expect. The edges of the dovetail have been chamfered at 45 degrees so you can’t pull them to the outside of the mallet as you slide them into place. The opposite is the actual trick. Each of the dovetails bends inward until a ramp at the very end of the mallet pocket pushes it back into place.

Underside of mallet head shows “ramp” detail

Tenons clamped during assembly

The impossible mallet isn’t a new concept and stands as a formidable challenge for any accomplished woodworker. The images above are of [Jim Guilford’s] impossible mallet. Here the trick is fully exposed, showing the dovetail tenons of the handle clamped together as it is driven into place. Two things are striking here; the joints cannot be tested and must be perfect before assembly, and there is a real chance the tenons will break or the mallet head will split apart from the force of assembly. This project will test your courage as much as it will your patience.

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Wearable Cone Of Silence Protects You From Prying Ears

February 17, 2020 by Dan Maloney 36 Comments

Careful, the walls have ears. Or more specifically, the smart speaker on the table has ears, as does the phone in your pocket, the fitness band on your wrist, possibly the TV, the fridge, the toaster, and maybe even the toilet. Oh, and your car is listening to you too. Probably.

How does one fight this profusion of listening devices? Perhaps this wearable smart device audio jammer will do the trick. The idea is that the MEMS microphones that surround us are all vulnerable to jamming by ultrasonic waves, due to the fact that they have a non-linear response to ultrasonic signals. The upshot of that is when a MEMS hears ultrasound, it creates a broadband signal in the audible part of the spectrum. That creates a staticky noise that effectively drowns out any other sounds the microphone might be picking up.

By why a wearable? Granted, [Yuxin Chin] and colleagues from the University of Chicago have perhaps stretched the definition of that term a tad with their prototype, but it turns out that moving the jammer around does a better job of blocking sounds than a static jammer does. The bracelet jammer is studded with ultrasonic transducers that emit overlapping fields and result in zones of constructive and destructive interference; the wearer’s movements vary the location of the dead spots that result, improving jamming efficacy. Their paper (PDF link) goes into deeper detail, and a GitHub repository has everything you need to roll your own.

We saw something a bit like this before, but that build used white noise for masking, and was affixed to the smart speaker. We’re intrigued by a wearable, especially since they’ve shown it to be effective under clothing. And the effect of ultrasound on MEMS microphones is really interesting.

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Making Your Own Maple Syrup Just Got A Little Easier

February 16, 2020 by Lewin Day 23 Comments

[ctstarkdesigns] had fond memories of collecting maple syrup as a child. At the same time, he also remembered the work involved: from lugging buckets around on an unstable snow mobile to accidentally burning the mixture and making all the effort for naught. So he set out to make things a little easier this time around by building his own evaporator.

The build starts as many do, with a surplus 44-gallon drum. With an off-the-shelf kit, and some cutting and welding, it’s readily repurposed into a stove capable of burning wood in a roaring fire. From there, it’s a simple matter of making a few further incisions to install warming trays, used to hold the takings from the maple trees. There, the mixture can be boiled down into the tasty, delicious substance that goes so perfectly on pancakes.

The build has the dual benefits of both easing the boiling process and keeping the user warm while doing so. Already, the rig has proven itself as an adept heater, and we’re sure it will only prove more popular once it’s producing sweet maple syrup en mass. If that’s not enough, consider building an entirely automated system in your back yard!

BBQ Burners Built From Scratch

February 13, 2020 by Lewin Day 23 Comments

Building a barbecue is a common DIY pursuit, and one that comes with a tasty payoff at completion. While many projects focus on charcoal or wood-fired designs, [Andrew] is more of a gas man. Not one to simply buy off the shelf, he designed his own burners from scratch.

This quest wasn’t just unnecessary yak shaving; burners to suit [Andrew]’s desired size and power simply weren’t available. The burner is designed around the Venturi effect, wherein the propane gas is passed through a small orifice, creating a jet and pulling air along with it as it enters the burner tube. This causes the gases to mix, and they can then be ignited when passing through the outlet holes of the burner. Get the orifice and outlet holes sized just right, and you’ll have a burner that produces a hot, blue flame, perfect for efficient cooking.

The orifice was produced with brass plumbing components, and hooked up to a valve rated for use with gas lines. The burner tube itself was created from stainless steel tube, with slots cut to act as outlet holes and with the end crimped and welded shut. A black iron pipe reducer was then used as the air inlet and orifice mount.

The final result is a powerful barbecue burner that is perfectly sized to [Andrew]’s needs. If you’re keen to build your own custom rig, you may find this a useful and cheap way to go versus sourcing parts off the shelf. We’ve seen [Andrew]’s work before, too. Video after the break.

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Flywheel Stores Energy To Power An Airplane – Eventually

February 13, 2020 by Dan Maloney 7 Comments

Question: Can a flywheel store enough energy to power an airplane? Answer: Yes it can, for certain values of “flywheel” and “airplane.”

About the only person we can think of who would even attempt to build a flywheel-powered airplane is [Tom Stanton]. He’s a great one for off-the-wall ideas that often pay off, like his Coandă effect hovercraft, as well as for ideas that never got far off the ground, or suddenly met it again. For most of the video below, it seems like his flywheel-powered plane is destined to stay firmly in the last category, and indeed, the idea of a massive flywheel taking flight seems counterintuitive. But [Tom] reminds us that since the kinetic energy stored by a flywheel increases as the square of angular velocity, how fast it’s turning is more important than how massive it is. The composite carbon fiber and aluminum flywheel is geared to the propeller of a minimal airplane through 3D-printed bevel gears, and is spun up with an external BLDC motor.

Sadly, the plane never made it very far, no matter how much weight was trimmed. But [Tom] was able to snatch victory from the jaws of defeat by making the propeller the flywheel – he printed a ring connecting the blades of the prop and devised a freewheel clutch to couple it to the motor. The flywheel prop stored enough energy to complete a few respectable flights, as well as suffer a few satisfyingly spectacular disintegrations.

As always, hats off to [Tom] for not being bashful about sharing his failures so we can all learn, and for the persistence to make his ideas take flight.

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Name That Unknown RF Signal With A Little FFT Magic

February 11, 2020 by Dan Maloney 27 Comments

Time was once that the amateur radio bands were an aurally predictable place. Spinning the dial up and down the bands, one heard familiar sounds – the staccato of Morse, the [Donald Duck] of sideband voice transmissions, and the occasional flute-like warble of radioteletype signals. Now, the ham bands are full of exotic signals encoding all manner of digital signals, each one with a unique sound and unique demodulation needs. What’s a ham to do?

Help is on the way. [José Carlos Rueda] has made progress toward automatically classifying unknown signals by modifying a Shazam-like app. Shazam is a popular smartphone app that listens to a few seconds of a song, creates an audio fingerprint of it, and searches a massive database of songs for a match. [Rueda] used a homebrew version of the app to search a SQL-lite database of audio fingerprints populated not with a playlist of popular music, but with samples from every known signal type in the Signal Identification Wiki. The database contains hashes for an FFT of each sample, which can be easily searched. With a five to ten second sample of a signal, captured either live over a microphone or from a recording, he is able to identify the signal automatically.

Whether it be the weird, dissonant wail of PSK-31 or the angry buzzing of PACTOR, the goings-on across the bands no longer have to remain a mystery. We really like the idea here, and wonder if it can be expanded upon to visually decode signals based on their waterfall signatures using TensorFlow. There are some waterfall examples in [Danie Conradie]’s excellent article on RF modulation that could get you started.

[via RTL-SDR.com]

Airport Split-Flap Letters Carry On As Spotify Display

February 11, 2020 by Kristina Panos 25 Comments

Today’s tale of being in the right place at the right time comes from [fabe1999], who was doing an intern gig at the airport when the controller on their split-flap display bought a one-way ticket going south. They were just going to throw away thousands of these letters and replace them with monitors, but the intern intervened.

[fabe1999] grabbed an armload, took them home, and set about making them flap again, one letter at a time. An ATtiny worked okay, but it wasn’t really fast enough to flip them at their full clacking potential, so [fabe1999] switched to an ESP8266. So now there is one ESP for each of the 20 characters, and another that runs a web server where text can be directly entered for immediate display.

Each letter uses two sensors to flap to the right letter. The first one acts as a start sensor, detecting the blackness of a blank character. Another sensor counts the letters and makes the ESP stop the motor on the right one. So far, [fabe1999] hasn’t figured out how to recognize when a blank character can stay blank, so they flap all the way around back to blank for now. It certainly adds to the rich, flappy sound, but that can’t be good for the long-term life of the letters. Your flight is now departing for Post Break Island, where the letters are spending part of their retirement showing song titles from Spotify.

No chance of split flaps falling into your lap? Here’s a tip: you can fab your own flip.

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