The human body does plenty of cool tricks, but one of the easiest to take advantage of is persistence of vision (POV). Our eyes continue to see light for a fraction of a second after the light goes off, and we can leverage this into fun blinkenlight toys like POV staffs. Sure, you can buy POV staffs and other devices, but they’re pretty expensive and you won’t learn anything that way. Building something yourself is often the more expensive route, but that’s not the case with [shurik179]’s excellent open-source POV staff.

There’s a lot to like about this project, starting with the detailed instructions. It’s based on the ItsyBitsyM4 Express and Adafruit’s Dotstar LED strips. You could use the Bluetooth version, but it’s already quite easy to load images to the staff because it shows up as a USB mass storage device. We like that [shurik179] added an IMU and coded the staff so that the images look consistent no matter how fast the staff is spinning. In the future, [shurik179] might make a Bluetooth version that’s collapsible. That sounds like quite the feat, and we can’t wait to see it in action.

As cool as it is to wave a POV staff around, there’s no real practical application. What’s more practical than a clock?

Plants are great to have around, but they all have different watering needs. If only they could cry out when they’re thirsty, right? Well, now they can. All you need to hear them suffer is your very own Klausner Machine. [RoniBandini] based the Klausner machine on one of Roald Dahl’s short stories, which features an inventor who builds a machine that can make audible the sound of plants shrieking whenever they’re cut.

In [RoniBandini]’s version of the Klausner Machine, the point is to judge the plant’s feelings based on its soil moisture content. An Arduino Nano reads in from the soil moisture sensor, and if the soil is dry, the plant screams. If the soil is moist, the plant emits happy sounds from DF Player Mini and SD card.  We think the analog meters are a great touch, and the jumping needles really anthropomorphize the plant.

Go forth and gain a better appreciation for your plants’ feelings, because this project is wide open. Maybe it will help you water them more often. Some plants need to be cut back, so we think it would be cool if you could make it scream when you take a cutting. Check out the demo after the break.

This is isn’t the first time we’ve seen an analog meter used in conjunction with soil moisture. What is a VU meter, anyway? Our own [Dan Maloney] really moved the needle on the subject a while back.

While you might think of radar pointing toward the skies, applications for radar have found their way underground as well. Ground-penetrating radar (GPR) is a tool that sends signals into the earth and measures their return to make determinations about what’s buried underground in much the same way that distant aircraft can be located or identified by looking for radar reflections. This technology can also be built with a few common items now for a relatively small cost.

This is a project from [Mirel] who built the system around a Arduino Mega 2560 and antipodal Vivaldi antennas, a type of directional antenna. Everything is mounted into a small cart that can be rolled along the ground. A switch attached to the wheels triggers the radar at regular intervals as it rolls, and the radar emits a signal and listens to reflections at each point. It operates at a frequency range from 323 MHz to 910 MHz, and a small graph of what it “sees” is displayed on an LCD screen that is paired to the Arduino.

Using this tool allows you to see different densities of materials located underground, as well as their depths. This can be very handy when starting a large excavation project, detecting rock layers or underground utilities before digging. [Mirel] made all of the hardware and software open-source for this project, and if you’d like to see another take on GPR then head over to this project which involves a lot of technical discussion on how it works.

Our old math teacher famously said, “You have to take what you know and use it find what you don’t know.” The same holds true for a lot of microcontroller designs including [rgco’s] clever metal detector that uses very little other than an Arduino. The principle of operation is simple. An Arduino can measure time, a coil and a resistor will create a delay proportional to the circuit values, and metal around the coil will change the coil’s inductance. As the inductance changes, so does the delay and, thus, the Arduino can sense metal, as you can see in the video below.

The simple principle is also simple in practice. Besides the Arduino and the coil, there’s a single resistor. You want a small coil since larger coils won’t detect smaller objects. If you don’t want to wind your own coil, [rgco] suggests using a roll of hookup wire as long as the resistance is under 10 ohms.

You could omit it, but the original design has a buzzer and an electrolytic capacitor connected to generate a buzz in addition to the built-in LED indicator when metal is near. The LED also shows a blink pattern if the coil is open, too short, too long, or has too much resistance.

The biggest problem is that the poor Arduino needs to measure delays down in the nanosecond range. It can’t actually do this directly, so the code takes a ranging measurement to get in the ballpark and then produces appropriately-sized pulses and adds them up to get a better idea of the total delay. There are several videos in the post of a prototype and the final device built in a Tic Tac container, which you can see below.

If you want something a bit fancier, here’s another simple design that has a few more parts. Or you can go for one that is ultrasensitive.

One of the worst things about sewing is finding out that your bobbin — that’s the smaller spool that works together with the needle and the larger spool to make a complete stitch — ran out of thread several stitches ago. If you’re lucky, the machine has a viewing window on the bobbin so you can easily tell when it’s getting dangerously close to running out, but many machines (ours included) must be taken halfway apart and the bobbin removed before it can be checked.

Having spare bobbins ready to go is definitely the answer. We would venture to guess that most (if not all) machines have a built-in bobbin winder, but using them involves de-threading the machine and setting it up to wind bobbins instead of sew. If you have a whole lot of sewing to do and can afford it, an automatic bobbin winder is a godsend. If you’re [Mr. Innovative], you build one yourself out of acrylic, aluminium, and Arduinos.

Here’s how it works: load up the clever little acrylic slide with up to twelve empty bobbins, then dial in the speed percentage and press the start button. The bobbins load one at a time onto a drill chuck that’s on the output shaft of a beefy 775 DC motor. The motor spins ridiculously fast, loading up the bobbin in a few seconds. Then the bobbin falls down a ramp and into a rack, and the thread is severed by a piece of nichrome wire.

An important part of winding bobbins is making sure the thread stays in place at the start of the wind. We love the way [Mr. Innovative] handled this part of the problem — a little foam doughnut around a bearing holds the thread in place just long enough to get the winding started. The schematic, BOM, and CAD files are available if you’d like to make one of these amazing machines for yourself. In the meantime, check out the demo/build video after the break.

Still not convinced that sewing is cool enough to learn? Our own [Jenny List] may be able to convert you. If that doesn’t get you, you might like to know that some sewing machines are hackable — this old girl has a second life as a computerized embroidery machine. If those don’t do it, consider that sewing machines can give you a second life, too.

Thanks for the tip, [Baldpower]!

Full-size metal detectors are great for narrowing down a region to start digging through. But what if you had a smaller metal detector that could pinpoint the location? Then you could spend far less time digging and way more time sweeping for metal.

Metal detectors work because of the way metal behaves around electromagnetic fields. [mircemk] reused the ferrite core from an old MW radio to build the antenna coils. When metal objects are close enough, the induced electromagnetism changes the frequency, and the Arduino blinks an LED and beeps a buzzer in time with the new frequency.

[mircemk]’s handheld metal detector is quite sensitive, especially to smaller objects. As you can see in the demo video after the break, it can sense coins from about 4cm away, larger objects like lids from about 7 cm, and tiny things like needles from a few millimeters away. There’s also an LED for treasure hunting in low light.

Don’t want to pinpoint a bunch of useless junk? Build in some phase detection to help you discriminate.

[Christofer Hiitti] found himself with the latest Microsoft Flight Simulator on his PC, but the joystick he ordered was still a few weeks out. So he grabbed an Arduino, potentiometers and a button and hacked together what a joke-yoke.

The genius part of this hack is the way [Christopher] used his desk drawer for pitch control. One side of a plastic hinge is attached to a potentiometer inside a drawer, while the other side is taped to the top of the desk. The second pot is taped to the front of the drawer for pitch control and the third pot is the throttle. It works remarkably well, as shown in the demo video below.

The linearity of the drawer mechanism probably isn’t great, but it was good enough for a temporary solution. The Arduino Leonardo he used is based on the ATmega32u4 which has a built-in USB, and with libraries like ArduinoJoystickLibrary the computer interface very simple. When [Christopher]’s real joystick finally arrived he augmented it with a button box built using the joke-yoke components.

We’ve seen There is no doubt that Microsoft Flight Simulator 2020 will spawn a lot of great controller and cockpit builds over the next few years. We’ve already covered a new joystick build, and a 3D printed frame to turn an Xbox controller into a joystick.

A few years ago, YouTubing madman [Colin Furze] took an old bumper car and made a 600-horsepower beast of a go-kart that managed to clock 100MPH with a headwind. This isn’t that. It’s a miniaturized, remote-control homage to [Colin]’s go-kart that is equally awesome.

[Forsyth Creations] started by CAD-modeling the chassis right on top of a still from the video. The entire body is 3D-printed in four large pieces, which took several days because each piece took around 24 hours. Inside the car there’s an Arduino brain driving a motor in the back and a servo in the front. This bad boy runs on a couple of rechargeable battery packs and can be controlled with either a Wii balance board or a PS2 controller. This thing really moves, although it doesn’t quite reach 100MPH. Watch it zoom around in the video after the break.

Got a Segway lying around that just doesn’t do it for you anymore? You could always turn it into a go-kart. Never had a Segway to begin with? Just roll your own.

We are no stranger to peculiar and wonderful musical instruments here at Hackaday. [James Bruton] has long been fascinated with barcode scanners as an input source for music and now has a procedural barcode-powered synth to add to his growing collection of handmade instruments. We’ve previously covered his barcode guitar, which converts a string of numbers from the PS/2 output to pitches. This meant having a large number of barcodes printed as each pitch required a separate barcode. As you can imagine, this makes for a rather unwieldy and large instrument.

Rather than looking at the textual output of the reader, [James] cracked it open and put it to the oscilloscope. Once inside, he found a good source that outputs a square wave corresponding to the black and white lines that the barcode sees. Since the barcodes [James] is using don’t have the proper start and stop codes, the barcode reader continuously scans.  Normally it would stop the laser to send the text over the USB or PS/2 connection. A simple 5v to 3.3v level shifter feeds that square wave into a Teensy board, which outputs the audio.

A video showcasing a similar technique inspired [James] with this project. The creators of that video have a huge wall of different patterns of black and white lines. [James’s] next stroke of brilliance was to have a small HDMI display to generate the barcodes on the fly. A Raspberry Pi 4 reads in various buttons via GPIO and displays the resulting barcode on the screen. A quick 3d printed shell rounds out the build nicely, keeping things small and compact. All the code and CAD files are up on GitHub.

Thanks [James Bruton] for the awesome project!

What keeps people from playing music? For one thing, it’s hard. But why is it hard? In theory, it’s because theory is confusing. In practice, it’s largely because of accidentals, or notes that sound sour compared to the others because they aren’t from the same key or a complementary key.

What if there were no accidentals? Instruments like this exist, like the harmonica and the autoharp. But none of them look as fun to play as [Bardable]’s Starshine, the instrument intended to be playable by everyone. The note buttons on the outside are laid out and programmed such that [Bardable] will never play off-key.

We love the game controller form factor, which was also a functional choice. On the side that faces the player, there’s a PSP joystick and two potentiometers for adding expression with your thumbs. The twelve buttons on this side serve several functions like choosing the key and the scale type depending on the rocker switch position. A second rocker lets [Bardable] go up or down an octave on the fly. There’s also an OLED to show everything from the note being played to the positions of the potentiometers. If you want to know more, [Bardable] made a subreddit for this and other future instruments, and has a full tour video after the break.

If this beginner-friendly MIDI controller isn’t big enough for you, check out Harmonicade’s field of arcade buttons.