Variable capacitors may be useful, but the air gap that provides their capacitance is their greatest weakness. Rather than deal with the poor dielectric properties of air, some high-end variable capacitors replace it with a vacuum, which presents some obvious mechanical difficulties, but does give the resulting capacitor a remarkable quality factor, high-voltage performance, and higher capacitance for plate area than their air-gapped brethren. [Shahriar] of [The Signal Path] managed to acquire a pair of these and took a detailed look at their construction and performance in a recent video.
If you are sitting on a horde of negatives, waiting for the digital photography fad to die off, it may be time to think about digitizing your old film. [Kinpro1024] can help with the PiDigitzier, an open-source film scanning solution. The build centers around a Pi Zero 2, a Pi HQ camera, and a diffusing LED lighting fixture. Of course, there’s also some miscellaneous hardware and a camera lens; the example used a Pentax 50 mm f1.8 lens.
Half of the project is mechanical. An MDF tower provides a stable 250 mm workspace and decks that can slide up and down using threaded rods and curtain rods. Apparently, leveling the platforms is important not only for the optics but also to allow the MDF to move along the rods without binding.
Modern fertilizer manufacturing uses the Haber-Bosch and Ostwald processes to fix aerial nitrogen as ammonia, then oxidize the ammonia to nitric acid. Having already created a Haber-Bosch reactor for ammonia production, [Markus Bindhammer] took the obvious next step and created an Ostwald reactor to make nitric acid.
[Markus]’s first step was to build a sturdy frame for his apparatus, since most inexpensive lab stands are light and tip over easily – not a good trait in the best of times, but particularly undesirable when working with nitrogen dioxide and nitric acid. Instead, [Markus] built a frame out of aluminium extrusion, T-nuts, threaded rods, pipe clamps, and a few cut pieces of aluminium.
Once the frame was built, [Markus] mounted a section of quartz glass tubing above a gas burner intended for camping, and connected the output of the quartz tube to a gas washing bottle. The high-temperature resistant quartz tube held a mixture of alumina and platinum wool (as we’ve seen him use before), which acted as a catalyst for the oxidation of ammonia. The input to the tube was connected to a container of ammonia solution, and the output of the gas washing bottle fed into a solution of universal pH indicator. A vacuum ejector pulled a mixture of air and ammonia vapors through the whole system, and a copper wool flashback arrestor kept that mixture from having explosive side reactions.
After [Markus] started up the ejector and lit the burner, it still took a few hours of experimentation to get the conditions right. The issue seems to be that even with catalysis, ammonia won’t oxidize to nitrogen oxides at too low a temperature, and nitrogen oxides break down to nitrogen and oxygen at too high a temperature. Eventually, though, he managed to get the flow rate right and was rewarded with the tell-tale brown fumes of nitrogen dioxide in the gas washing bottle. The universal indicator also turned red, further confirming that he had made nitric acid.
Thanks to the platinum catalyst, this reactor does have the advantage of not relying on high voltages to make nitric acid. Of course, you’ll still need get ammonia somehow.
Whatever your day job, many of us would love to jump behind the controls of a dump truck for a lark. In the real world, that takes training and expertise and the opportunity is denied to many of us. However, you can live out those dreams on your desk with this 3D-printed build from [ProfessorBoots.]
The build exists as two separate parts—the tractor, and the trailer. The tractor is effectively a fairly straightforward custom RC build, albeit with a few additional features to make it fit for purpose. It’s got six wheels as befitting a proper semi, and it has a nifty retractable magnetic hitch mechanism. This lets it hook up to various trailers and unhitch from them as desired, all from a press on the remote. The hitch also has provision for power and control lines that control whatever trailer happens to be attached.
As for the trailer, it’s a side-dumper that can drop its load to the left or right as desired. The dumping is controlled via a linear actuator using a small DC motor and a threaded rod. A servo controls a sliding locking mechanism which determines whether the truck dumps to the left or right as the linear actuator rises up.
The design video covers the 3D printed design as well as some great action shots of the dump truck doing its thing. We’ve featured some builds from [ProfessorBoots] before, too, like this neat 3D-printed forklift . Video after the break.
An ESP32 runs the show in this build. Via a set of A4988 stepper motor drivers, it controls two NEMA 17 stepper motors which control the motion of the cutting and stripping blades via threaded rods. A third stepper controls a 3D printer extruder to move wires through the device. There’s a rotary encoder with a button for controlling the device, with cutting and stripping settings shown on a small OLED display. It graphically represents the wire for stripping, so you can select the length of the wire and how much insulation you want stripped off each end. You merely need select the measurements on the display, press a button, and the machine strips and cuts the wire for you. The wires end up in a tidy little 3D-printed bin for collection.
The build should be a big time saver for [Red], who will no longer have to manually cut and strip wires for future builds. We’ve featured some other neat wire stripper builds before, too. Video after the break.
At the end of the day, a skateboard boils down to a plank of wood with some wheels. They are wonderfully simple and fun and cheap modes of transportation. But this is Hackaday, so we are not here to talk about any normal skateboard, but one you can download and print. [megalog_’s] Skateboard MK2 is made almost entirely of 3D printed plastic, save some nuts and bolts.
The board’s four piece deck comes in at a modest 55cm length and features a rather stylish hexagonal pattern for grip. While you could presumably bring your own trucks, 3D printable ones are provided as well. The pieces bolt together to create a fairly strong deck with the option to make a rather stylish two tone print if you have the printer for it. Where the pieces meet is also the location of the truck mounting, further increasing the board’s strength. The weakest point is where the tail meets the main deck, which if pressed down to wheelie or ollie, the print breaks apart at the layer lines.
While you might be able to bring your own trucks, all be it with some modification to the deck, [megalog] also provided models for those as well. Not only were the bushings made of flexible TPE filament, but the outer wheel tire is too. It’s a little strange to see a wheel tire combo on a skateboard, when they are traditionally over moulded plastic with enough tire that you would be forgiven for thinking there is no wheel. While some reported using the more traditional threaded rod, the trucks used a metal rod with shaft collars to attach the wheels.
This is a neatly executed skateboard build with a well thought out design. Let us know in the comments if you will (or have) made one yourself! While you’re at it, maybe cast your own resin wheels for it!
There are no shortage of CNC machines in the DIY space these days, but sometimes you just need to do things your own way. That’s what [Chris Borges] decided when he put together this rock-solid, concrete-filled CNC milling machine.
The concrete body of this machine is housed inside a 3D printed shell, which makes for an attractive skin as well as a handy mold. Within the concrete is a steel skeleton, with the ‘rebar’ being made of threaded rods and a length of square tubing to hold the main column. You can see the concrete being poured in around the rebar in the image, or watch it happen in the build video embedded below.
In goes the concrete, up goes the rigidity.All three axes slide on linear rails, and are attached to lead screws driven by the omnipresent NEMA 17 steppers. The air-cooled spindle, apparently the weak-point of the design, is attached to a pivoting counterweight, but make no mistake: it is on rails. All-in-all, it looks like a very rigid, and very capable design — [Chris] shows it cutting through aluminum quite nicely.
Given that [Chris] has apparently never used a true mill before, this design came out remarkably well. Between the Bill of Materials and 45 page step-by-step assembly instructions, he’s also done a fantastic job documenting the build for anyone who wants to put one together for themselves.
![An image of the main column of [Chris]'s CNC mill as the concrete is added. The steel reinforcement is clearly visible.](https://hackaday.com/wp-content/uploads/2025/04/pouring-concrete-mill.png?w=400)
