Author: Aaron Beckendorf

138 Articles

A web interface is shown providing information about a cellular network base station.

Running Your Own 3G Network

May 9, 2026 by 9 Comments

CDMA2000 was one of the protocols defined for 3G networks and is now years out of date and being phased out worldwide. Nevertheless, there are still vast numbers of phones that will happily connect to it, creating an opportunity for hackers seeking to run their own cellular networks. [Chrismoos] recently made this endeavour significantly easier by releasing 1xBTS, a Rust implementation of the lower three layers of a CDMA2000 network.

The lowest layer of the stack is an SDR for the actual radio communications. It’s been tested with the USRP B200 and B210, the LimeSDR Mini 2, and the BladeRF Micro 2.0. The code might work with certain other SDRs using the SoapySDR abstraction layer. The SDR is controlled by the base station (BTS) software, which, in turn, is controlled by the base station controller (BSC) over an Abis link. The BSC manages channels and mobile device associations, and exchanges frames with the mobile switching center (MSC), which handles message switching.

Continue reading “Running Your Own 3G Network” →

A black and yellow robot dog stands in the middle of the floor, with a GoPro camera mounted on its back. A picture-in-picture view in the bottom left corner shows the view from the camera.

An Improved Robot Dog For Senior Design

May 8, 2026 by 6 Comments

[Aaed Musa] has been building robot dogs for a long time now, so it was only natural that he would make one for the senior design project of his mechanical engineering degree. Since this meant working with potential customers, the requirements were somewhat more stringent than for previous dogs, but [Aaed] and his team were able to deliver CARA 2.0, their most agile, versatile robot yet.

Based on conversations with potential customers, [Aaed] and his team aimed for a price around $1,000 USD, a weight under 20 pounds, and a durable design. Like the original CARA, this used capstan drives to actuate the joints, which reduced costs. The drives were printed in resin and powered by brushless drone motors. These motors were designed for speed, not torque, so the team had to rewind them with more wire, an ordeal which paid off by roughly tripling the torque. As far as durability, one joint motor was tested by running it continuously back and forth, and it lasted for over 1,000 hours without obvious damage.

Since the joints don’t contain any absolute encoders, each motor has to home on startup by extending to its limit, as detected by a rise in motor current. As a happy side effect, this creates a lifelike stretching motion on startup. Compared to the earlier iteration, CARA 2.0 takes shorter, quicker steps, and thanks to angled step movements can turn much more quickly. In testing, it originally skewed to the left, which turned out to be due to an asymmetric leg design. Once corrected, CARA 2.0 could walk in straight lines, walk sideways, turn in place, crouch, jump, and keep its balance on an inclined surface. It didn’t quite make the price goal, but $1,450 is still cheap for such a capable robot dog, and it reached every other customer requirement. Most importantly, all the team graduated.

For another take on a capstan-powered robot dog, check out Stanley. We’ve also taken a look at TOPS, one of [Aaed]’s earlier designs.

Continue reading “An Improved Robot Dog For Senior Design” →

A fine steel gear is shown held between a man's fingertips.

Cutting Steel Gears With Homemade EDM

May 5, 2026 by 16 Comments

Electric discharge machining (EDM) may be slower than alternatives like laser cutting, water jets, or a milling machine, but for some applications there’s no alternative: it can cut through any conductive material, no matter how hard, and it leaves no mechanical or thermal stress in the workpiece. Best of all, they’re relatively accessible for a resourceful hacker, such as [Inofid], who recently built the second iteration of his desktop wire EDM.

The EDM’s motion system comes from a cheap desktop CNC router, which had a water tank mounted in its workspace and had the spindle replaced with a wire-management mechanism. The wire-management mechanism needs to continuously wind a tensioned brass wire from one spool through the cutting zone onto another spool. The tensioning system uses two motors: one to pull the wire through, and one to maintain tension by slightly counteracting it, with a tension sensor and Ardunio to maintain the proper tension. If it detects that the wire has broken, it can stop the CNC controller. To keep the wire from breaking or short-circuiting with the workpiece, a current monitor counts sparks between the wire and workpiece and uses this to predict whether the wire is getting too close to the metal, in which case it slows down the movement.

As a first test, [Inofid] cut through a five by three centimeters-thick block of aluminium, taking two hours but producing a clean cut. To speed up the next cut, [Inofid] added a pump and filter to remove sludge from the cutting area. The next cut was an aluminium gear, and then a meshing steel gear, which took about ten hours but turned out well.

EDMs of various kinds appear here from time to time, particularly since the popularization of 3D printers. We’ve even seen one built into a lathe.

Thanks to [Keith Olson] for the tip!

A red box with a yellow front panel is shown. The front panel contains a power switch, an indicator light, and a small OLED display.

A Shortwave Sensor To Monitor The Ionosphere

May 4, 2026 by 9 Comments

The ionosphere is of great importance to shortwave radio transmissions, since it allows radio waves to be refracted and reflected over the horizon, and it’s therefore unfortunate that the height and thickness of the ionosphere depends on the time of day or night, weather, season, and the solar cycle. To get a better idea of current transmission conditions, [mircemk] built this shortwave propagation monitor.

The monitor provides a basic measure of ionosphere conditions by measuring the strength of received shortwave signals: if the conditions for transmission are good, it should receive a relatively high level of existing signals, and a weak signal if conditions are bad. It has an external antenna connected to a signal strength indicator circuit based on the CA3089, which amplifies signals in the 1-40 MHz range and outputs a smoothed voltage indicating the RF energy in this range. The output signal can be read by any voltmeter, in this case an Arduino Nano with an OLED display. Assuming the same antenna is always used, the signal should noticeably fluctuate between night and day as the solar wind affects the ionosphere.

Of course, the distance at which you’ll be receiving a signal means nothing unless you have a receiver, which can range from the antique to the modern.

A man's hand is shown holding a color photograph of a vase of flowers against a black background.

True-Spectrum Photography With Structural Color

May 3, 2026 by 34 Comments

Although modern cameras can, with skill and good conditions, produce photographs nearly indistinguishable from the original scene, this fidelity relies on the limitations of human vision. According to the trichromatic theory, humans perceive light as a mixture of three colors, which can be recorded and represented by cameras, displays, and color printing; a spectrometer, however, can detect a clear distance between the three colors present in a photograph and the wide range of spectra in the original scene. By contrast, one of the earliest color photography methods, Lippmann plates, captured not just true color, but true spectra.

A Lippmann plate, as [Jon Hilty] details, starts with a layer of photographic gel containing extremely fine silver halide crystals over the back of a glass plate. This layer is placed on top of a mirror, traditionally a mercury bath, and put in the camera. When light passes through the emulsion and reflects off the mirror, it interferes with incoming light to create a standing wave. The portions of the emulsion at the wave’s antinodes absorb the most energy, converting local silver halide crystals into reflective silver. The spacing of the silver particles depends on the incoming light’s wavelength, and is fixed in place during the development process.

This creates a matrix of vertically-stacked diffraction gratings, each diffracting back the original wavelength when illuminated with white light. Unlike normal diffraction gratings, the wavelength of diffracted light doesn’t depend strongly on the viewing angle; since the interference structure here is vertically-arranged, it refracts a narrow range of wavelengths across all possible viewing angles. The viewing angles, however, are limited; unlike with dye-based photographs, you can only view the colors nearly straight-on. This, along with the necessity for long exposures, the chance of producing washed-out colors, and the impossibility of creating reprints, kept Lippmann plates from ever really catching on. The basic concept lives on in holograms, which encode spatial information in a similar kind of photographically-formed diffraction pattern.

For a more conventional method of color photography, we’ve also seen a recreation of the autochrome method. Alternatively, check out this homemade silver halide photography emulsion.

Thanks to [Stephen Walters] for the tip!

An ultrasonic transducer with two wires attached to it by alligator clips floats very slightly suspended over a glass surface.

A Different Kind Of Ultrasonic Levitation

April 27, 2026 by 3 Comments

Ultrasonic levitation is by now a familiar trick: one or more ultrasonic transducers create a standing wave, and small objects can be held in the nodes of this standing wave. With a sufficiently large array of transducers, it’s even possible to control the movement of the object. This isn’t the only form of ultrasonic levitation, however, as [Steve Mould] demonstrated with his ultrasonic air hockey table.

This less familiar form of levitation was discovered by [Bob Collins] while working on torpedo guidance systems: when he tried to place a glass lens on an ultrasonic transducer it immediately slid off. He found during further experimentation that an ultrasonic transducer would levitate over any sufficiently flat and smooth surface. It works by trapping a very thin layer of air between the transducer and the smooth surface. When the transducer moves sharply toward the surface, it compresses a layer of air in between, and forces some air out, and the reverse happens while pulling back. However, during the downstroke, the gap through which air can escape is narrower than during the upstroke, and there is more surface-induced drag, meaning that the inflow and outflow of air through a narrow gap isn’t completely equal. At a certain distance, inflow and outflow balance, and the transducer floats on a thin layer of air. Continue reading “A Different Kind Of Ultrasonic Levitation” →

A drill bit, not apparently rotating, descends toward a block of aluminium and throws up aluminium shavings as it passes through the block.

Freeze Moving Tools With A Stroboscopic Camera

April 26, 2026 by 20 Comments

If you take a video of a spinning wheel, you’ll probably notice that the spokes appear to turn more slowly than the wheel is actually rotating, and sometimes in the wrong direction. This is caused by a near match in the frame rate of the camera and the rate of rotation of the wheel – each time the camera captures a frame, the wheel has rotated a spoke into nearly the same position as in the last frame. If you time the exposures carefully, as [Excessive Overkill] did in his latest video, this effect can seemingly freeze moving objects, such as a fan or saw blade.

Most cameras only allow relatively coarse, fixed adjustments to frame rate, making it difficult to synchronize the shutter to an object’s motion. To get around this, [Excessive Overkill] used an industrial camera (previously used in this aimbot), which has fine frame rate control and external triggering. He connected the external trigger to a laser sensor, which detects a piece of retroreflective tape every time it passes by (for example, on one blade of a fan). When the laser sensor sends a signal, it also triggers a powerful LED flash. The flash is so powerful that dark materials create a hum when exposed to it, as pulses quickly heat the material, but each pulse is also so brief that the flash board doesn’t require any cooling.

Continue reading “Freeze Moving Tools With A Stroboscopic Camera” →