Practical Transistors: JFETs

Transistors come in different flavors. Tubes used an electric field to regulate current flow, and researchers wanted to find something that worked the same way without the drawbacks like vacuum and filament voltages. However, what they first found — the bipolar transistor — doesn’t work the same way. It uses a small current to modulate a larger current, acting as a switch. What they were looking for was actually the FET — the field effect transistor. These come in two flavors. One uses a gate separated from the channel by a thin layer of oxide (MOSFETs), and the other — a junction or JFET — uses the property of semiconductors to deplete or enhance carriers in the channel. [JohnAudioTech] takes a decidedly practical approach to JFETs in a recent video that you can watch below.

The idea for the FET is rather old, with patents appearing in 1925 and 1934, but there were no practical devices at either time. William Shockley tried and failed to make a working FET in 1947, the same year the first point-contact transistor appeared, which was invented while trying to create practical FETs. In 1948, the bipolar junction transistor hit the scene and changed everything. While there were a couple of working FETs created between 1945 and 1950, the first practical devices didn’t appear until 1953. They had problems, so interest waned in the technology while the industry focused on bipolar transistors. However, FETs eventually got better, boasting both very high input impedance and simplified biasing compared to bipolar technology.

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3D Printed Protection Against “Under-Door” Attacks

“Under-door” style attacks are when an attacker slides a tool through the gap underneath a door, hooks the interior handle from below, and opens the door by pulling the handle downward. This kind of attack works on the sort of doors and locks commonly found in hotels, where turning the handle from the inside always results in an open door. [Michal Jirků] found himself in a hotel room with a particularly large gap underneath the door, and decided to quickly design and print a door guard to protect against just such an attack.

It’s a simple object, and twenty minutes of printing and a little double-sided tape is all it takes to deploy. Because an attacker performs an under-door attack with a sizable mechanical disadvantage, it doesn’t take much to frustrate the attempt, and that’s exactly what the object does. Physical security in hotels is especially important, after all, and crooks have been known to exploit known flaws like the face-palmingly bad Onity key card lock exploit.

If you’re having trouble picturing how it all works, this video demonstrates an under-door attack in action, so you can see how blocking the space by the handle would easily prevent the tool from getting where it needs to go.

3D Printing With Sound, Directly

Canadian researchers at Concordia University want to change how you do 3D printing. Instead of using light or thermal mechanisms, they propose using ultrasound-activated sonochemical reactions. Sounds wild? You can see a video about it below, or read the paper in Nature.

The idea is that sound causes bubbles of cavitation. This requires a focused ultrasonic beam which means you can actually print through items that are transparent to ultrasonic energy. Wherever the cavitation bubbles form, liquid polymer turns solid.

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An electric eraser built into a mechanical pencil

Power Up Your Pencil With The 30,000 RPM Erase-O-Matic

There are some inventions that look completely pointless to the untrained eye: who would ever need a motorized garbage can, an electric pencil sharpener or a battery-powered eraser? Quite often, it turns out that there is some niche use case where such tools make complete sense, as is the case for motorized erasers. Having a tiny piece of rubber spinning at high speed gives artists and drafters a way to very precisely delete or lighten bits of their drawing, something that’s nearly impossible to do with a regular eraser.

[Franklinstein] however decided to design a high-speed eraser integrated into a mechanical pencil that brings the whole concept straight back to the pointless category, although not without showing off his advanced engineering skills. The Erase-O-Matic contains a miniature electric motor sourced from a quadcopter, together with an ultra-small lithium-ion battery and a power switch. The spinning bit is held by a tiny bearing, with the whole setup covered by a machined aluminium housing.

Tests with a laser tachometer show a rotational speed of about 30,000 RPM, which is almost three times as fast as a commercial electric eraser. And although it has very good erasing performance, it also wears out its tip in a few seconds, so a bit less speed could actually make this device more useful. If you’re planning to build one of these yourself, you might want to first read our primer on small DC motors.

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LED Heart Keeps Tabs On Your RuneScape Character

The MMORPG RuneScape holds a special place in the hearts of those who played it in the early 2000s. Sure it might seem exceptionally quaint by modern standards, but at the time it was groundbreaking stuff. Plus you could play it for free, which certainly helped get people onboard. While there’s a more modern build available, many who played the game from the early days prefer to stick with what they know, and continue to run a version of the game that has now become known as Old School RuneScape.

[Austin Blake] is one of those early adopters, and the work he put into this LED health indicator should tell you all you need to know about how dedicated he is to the classic game. The 3D printed heart holds an incredible 312 NeoPixel LEDs, which are controlled by a 5 volt compatible Arduino Nano Every located on the back side.  Both the color and “fill level” of the heart will change in real-time to correspond to the health of the player character.

Building the light itself was pretty straightforward, but getting the health value from the game was another story. As [Austin] explains in the video, his first attempt involved using Python and some image recognition routines to literally read the indicator off of the screen. The idea worked, and is frankly a fascinating hack worth keeping in mind on its own, but unfortunately it was too slow to provide the real-time feedback he was looking for.

Eventually he turned his attention to RuneLite, which is an open source client for Old School RuneScape. Thanks to its open source nature he could have hacked a routine to read the current health value and send it off to the Arduino, but thanks to a mature plug-in system, he didn’t have to.

The game’s API let him create a simple and reliable way of getting the data out of the game, similar to what we see in the flight simulator community for driving physical gauges and displays. RuneLite features a repository of community-developed plugins, and [Austin] says that he’d be happy to submit his for inclusion if others are interested in building similar indicators — a perfect match for this motion-sensing RuneScape axe.

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Linux Fu: Easy Widgets

Here’s a scenario. You have a microcontroller that reads a number of items — temperatures, pressures, whatever — and you want to have a display for your Linux desktop that sits on the panel and shows you the status. If you click on it, you get expanded status and can even issue some commands. Most desktops support the notion of widgets, but developing them is a real pain, right? And even if you develop one for KDE, what about the people using Gnome?

Turns out there is an easy answer and it was apparently inspired by, of all things, a tool from the Mac world. That tool was called BitBar (now XBar). That program places a widget on your menu bar that can display anything you want. You can write any kind of program you like — shell script, C, whatever. The output printed from the program controls what appears on the widget using a simple markup-like language.

That’s fine for the Mac, but what about Linux? If you use Gnome, there is a very similar project called Argos. It is largely compatible with XBar, although there are a few things that it adds that are specific to it. If you use KDE (like I do) then you’ll want Kargos, which is more or less a port of Argos and adds a few things of its own.

Good News, Bad News

The good news is that, in theory, you could write a script that would run under all three systems. The bad news is that each has its own differences and quirks. Obviously, too, if you use a complied program that could pose a problem on the Mac unless you recompile.

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Surprisingly Stomp-able Soft Switches

Competition sure brings out the brute in people, doesn’t it? So what do you do when you need a bunch of switches you can let people fist-pound or stomp on repeatedly without them taking damage? You could look to the guitar pedal industry and their tough latching switches, or you could simply build your own smash-resistant buttons as [wannabemadsci] has done.

The main thing about these switches is that they aren’t easily destroyed by shoes or angry fists. That’s because the shiny red push-me part of the button is made by cutting a foam ball in half.

Not easily crush-able Styrofoam, mind you — squishy, coated foam like an indoor football. This is mounted to the top of a sandwich made of hardboard and a couple pieces of easily-compressible foam from craft paintbrushes.

A brass washer is mounted to the middle of both pieces of hardboard, and these have wires soldered to them to read button presses. Then it’s just a matter of hooking it to a microcontroller like any other momentary.

There are all kinds of things you could cut in half for the top, like maybe tennis balls. Or, do what [Sprite_TM] did and use inverted plastic bowls.