While rarely seen by users, the technology behind telephone exchanges is actually quite interesting. In the first hundred or so years of their existence they evolved from manually-operated switchboards to computer-controlled systems, but in between those two stages was a time when dialling and switching was performed electromechanically. This was made possible by the invention of the stepping switch, a type of pulse-operated relay that can connect a single incoming wire to one of many outgoing wires.
Public telephone exchanges contained hundreds of these switches, but as [dearuserhron] shows, it’s possible to make a smaller system with way fewer components: the Cadr-o-station is built around one single stepper switch. Although it looks rather complicated, the only other components are a bunch of ordinary 24 V relays and a few power supplies. Together they make up a minimal telephone exchange that connects up to ten handsets.
It doesn’t have all the functionality of a larger system however, as only a single voice circuit is made to which all phones are automatically connected. Still, it does allow users to dial a number and let the other phone ring, which might be good enough for a home or indeed the hackerspace where it’s currently sitting. It’s also a fine demonstration of how relatively simple technology can be applied to make a surprisingly complex system.
[dearuserhron] wrote an in-depth article on the workings of electromechanical telephone exchanges, which might come in helpful to anyone who’d like to design such a system for their own home. For a more general introduction into analog phone technology, check out our analysis of a 1970s rotary telephone.
Continue reading “The Simplest Electro-Mechanical Telephone Exchange That Actually Works”
It all started when [Big Clive] ordered a chintzy Chinese musical meditation flower and found a black blob. But tantalizingly, the shiny plastic mess also included a 2 MB flash EEPROM. The questions then is: can one replace the contents with your own music? Spoiler: yes, you can! [Sprite_tm] and a team of Buddha Chip Hackers distributed across the globe got to work. (
Bad pattern number one is XOR. Used correctly, XORing can be a force for good, but if you XOR your key with zeros, naturally, you get the key back as your ciphertext. And this data had a lot of zeros in it. That means that there were many long strings that started out the same, but they seemed to go on forever, as if they were pseudo-random. Bad crypto pattern number two is using a linear-feedback shift register for your pseudo-random numbers, because the parameter space is small enough that [Sprite_tm] could just brute-force it. At the end, he points out their third mistake — making the encryption so fun to hack on that it kept him motivated!
