Linux Fu: Counter Rotate Keys!

If you’ve done anything with a modern Linux system — including most variants for the Raspberry Pi — you probably know about sudo. This typically allows an authorized user to elevate themselves to superuser status to do things.

However, there is a problem. If you have sudo access, you can do anything — at least, anything the sudoers file allows you to do. But what about extremely critical operations? We’ve all seen the movies where launching the nuclear missile requires two keys counter-rotated at the same time and third firing key. Is there an equivalent for Linux systems?

It isn’t exactly a counter-rotating key, but the sudo_pair project — a prelease open-source project from Square — gives you something similar. The project is a plugin for sudo that allows you to have another user authorize a sudo request. Not only do they authorize it, but they get to see what is happening, and even abort it if something bad is happening.

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Dead Simple Ultrasonic Data Communication

Some of the best hacks are the ones which seem perfectly obvious in hindsight; a solution to the problem that’s so elegant, you wonder how it never occurred to you before. Of course we also love the hacks that are so complex your eyes start to water, but it’s nice to have a balance. This one, sent in by [Eduardo Zola] is definitely in the former group.

In the video after the break, [Eduardo] demonstrates his extremely simple setup for using ultrasonic transducers for one-way data communication. Powered by a pair of Arduinos and using transducers salvaged from the extremely popular HC-SR04 module, there’s a good chance a lot of readers can recreate this one on their own bench with what they’ve got lying around. In this example he’s sending strings of text from one computer to another, but with a little imagination this can be used for all sorts of projects.

For the transmitter, the ultrasonic transducer is simply tied to one of the digital pins on the Arduino. The receiver is a bit more complex, requiring a LM386 amplifier and LM393 comparator to create a clean signal for the second Arduino to read.

But how does it work? Looking through the source code for the transmitter and receiver, we can see it’s about as basic as it gets. The transmitter Arduino breaks down a given string into individual characters, and then further converts the ASCII to eight binary bits. These bits are sent out as tones, and are picked up on the receiving end. Once the receiver has collected a decent chunk of tones, it works through them and turns the binary values back into ASCII characters which get dumped over serial. It’s slow, but it’s simple.

If you’re looking for something a bit more robust, check out this guide on using GNU Radio with ultrasonics.

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Tracking CubeSats For $25

CubeSats are tiny satellites which tag along as secondary payloads during launches. They have to weigh in at under 1.33 kg, and are often built at low cost. There’s even open source designs for these little spacecrafts. Over 800 CubeSats have been launched over the last few years, with many more launches scheduled in the near future.

[Thomas Cholakov] coupled a homemade cloverleaf antenna to a software-defined radio to track some of these satellites. The antenna is built out of copper-clad wire cut to the correct length to receive 437 MHz signals. Four loops are connected together and terminated to an RF connector.

This homebrew antenna is connected into a RTL-SDR dongle. The dongle picks up the beacon signals sent by the satellites and provides the data to a PC. Due to the motion of the satellites, their beacons can be easily identified by the Doppler shift of the frequency.

[Thomas] uses SDR Console to receive data from the satellites. While the demo only shows basic receiving, much more information on decoding these satellites can be found on the SDR Satellites website.

This looks like a fun weekend project, and probably the cheapest aerospace related project possible. After the break, watch the full video explaining how to build and set up the antenna and dongle.

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