Broadcast Engineer at BellMedia, Computer history buff, compulsive deprecated, disparate hardware hoarder, R/C, robots, arduino, RF, and everything in between.
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Hello Kitty museum exhibit headed to Seattle in November

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The massive museum exhibition "Hello! Exploring the Supercute World of Hello Kitty" will land at Seattle's EMP Museum in November. Read the rest

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tekvax
58 minutes ago
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Burlington, Ontario
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What is the lethal dose of various substances?

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Drinking six liters of water in a short period could kill you. One gram of vaporized polonium is enough to kill 50 million people. What about alcohol, caffeine, and cannabis? This video will tell you.

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tekvax
59 minutes ago
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Burlington, Ontario
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Quitting sugar "changed my brain"

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5.51-sugar-adMichael Grothaus quit sugar, soon regretted it, but came through after a week or so. Read the rest

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tekvax
1 hour ago
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Burlington, Ontario
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RIP Dean Jones, 1931-2015

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Dean Jones, the iconic superstar of so many early Disney films, has passed away. The Chicago Tribune shares a wonderful remembrance.

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tekvax
1 hour ago
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Burlington, Ontario
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Decoding Satellite-based Text Messages with RTL-SDR and Hacked GPS Antenna

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[Carl] just found a yet another use for the RTL-SDR. He’s been decoding Inmarsat STD-C EGC messages with it. Inmarsat is a British satellite telecommunications company. They provide communications all over the world to places that do not have a reliable terrestrial communications network. STD-C is a text message communications channel used mostly by maritime operators. This channel contains Enhanced Group Call (EGC) messages which include information such as search and rescue, coast guard, weather, and more.

Not much equipment is required for this, just the RTL-SDR dongle, an antenna, a computer, and the cables to hook them all up together. Once all of the gear was collected, [Carl] used an Android app called Satellite AR to locate his nearest Inmarsat satellite. Since these satellites are geostationary, he won’t have to move his antenna once it’s pointed in the right direction.

Hacked GPS antenna
Hacked GPS antenna

As far as antennas go, [Carl] recommends a dish or helix antenna. If you don’t want to fork over the money for something that fancy, he also explains how you can modify a $10 GPS antenna to work for this purpose. He admits that it’s not the best antenna for this, but it will get the job done. A typical GPS antenna will be tuned for 1575 MHz and will contain a band pass filter that prevents the antenna from picking up signals 1-2MHz away from that frequency.

To remove the filter, the plastic case must first be removed. Then a metal reflector needs to be removed from the bottom of the antenna using a soldering iron. The actual antenna circuit is hiding under the reflector. The filter is typically the largest component on the board. After desoldering, the IN and OUT pads are bridged together. The whole thing can then be put back together for use with this project.

Once everything was hooked up and the antenna was pointed in the right place, the audio output from the dongle was piped into the SDR# tuner software. After tuning to the correct frequency and setting all of the audio parameters, the audio was then decoded with another program called tdma-demo.exe. If everything is tuned just right, the software will be able to decode the audio signal and it will start to display messages. [Carl] posted some interesting examples including a couple of pirate warnings.

If you can’t get enough RTL-SDR hacks, be sure to check out some of the others we’vefeatured in thepast. And don’t forget to send in links to your own hacking!


Filed under: radio hacks
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tekvax
2 days ago
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Burlington, Ontario
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Core Memory for the Hard Core

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[Brek] needed to store 64 bits of data from his GPS to serve as a last-known-position function. This memory must be non-volatile, sticking around when the GPS and power are off. Solutions like using a backup battery or employing a $0.25 EEPROM chip were obviously too pedestrian. [Brek] wanted to store his 64 bits in style and that means hand-wired core memory.

OK, we’re pretty sure that the solution came first, and then [Brek] found a fitting problem that could be solved, but you gotta give him props for a project well executed and well documented.

Core memory is basically just a bunch of magnetizable rings on wires. When you pass enough current through a ring it becomes magnetically charged (North or South) depending on the direction of the current. Once magnetized, if you try to re-magnetize the core in the same direction, nothing changes. But if you flip the polarity of the ring, it emits a short electric pulse in the process. Sensing this pulse (and re-writing the bit back to its original state if necessary) buys you one-bit-per-ring of memory that remembers even when the power goes off.

2818981437205560722You could string the cores up independently, but that’s a lot of wiring. The trick to making core memory (halfway) reasonable is the fact that a current that’s not quite strong enough to flip the polarity of a ring doesn’t do anything.

Look at the way the cores are wired up in a matrix. If you want to select a single core, you can apply half the current to one of the y-axis wires, for instance, and then another half current to a single x-axis wire. Now the one ring to get enough current to flip state is the core in the cross-hairs; all the other rings in the x or y direction only get half.

What’s amazing to us young(er) whipper-snappers is that this was the dominant form of computer memory from the 1950s to the beginning of the transistor age in the mid-1970s. (Come to think of it, my father’s PDP-8 had core memory cards that I vaguely remember seeing as a kid. The sheer wiring required for 4KB was ridiculous.)

7564961439448793132Now back to [Brek]’s project. He’s added some shift registers and H-bridge drivers to handle the logic and current requirements respectively. The sense amplifier lives in a tidy copper cage. The whole build is a sweet testament to over-the-top, bespoke retro engineering. And he gets extra points for the hysteresis logo on the top cover. Go check out his project.

Thanks, [Brek] for all the work and documentation!


Filed under: classic hacks, hardware
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tekvax
2 days ago
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Burlington, Ontario
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