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Showing posts from January, 2011

Calculating, rather than experimenting to find, resistor values for the NTX2 voltage divider

A previous blog post showed the voltage divider circuit I'm using to drive the Radiometrix NTX2 transmitter in GAGA-1. And another blog post gave a spreadsheet for working out the voltages based on resistor values. What you really want, though, is a way to say "given this required frequency shift what should the resistor values be?". Recall the circuit diagram I was using. Arduino Radio 7 ---27K--------------\ ---------- TX 8 ---22K----2K2-------/ Now, replace the specific resistances on the left with R1 and R2 and the input impedance to ground with 100K (the value from the datasheet): Arduino Radio 7 ---R1--------------\ ----------100K---Gnd 8 ---R2--------------/ Pins 7 and 8 are connected to either 5V or Gnd to make the divider so you have two configurations: Arduino Radio 5V ---R1--------------\ ----------100K---Gnd Gnd ---R2

The Genius of Turing

For some time I've been involved (albeit only slightly) in a new documentary about Alan Turing. Happily, I can now talk about it. Here's the official blurb: Alan Turing was the British WW II code breaker and father of computer science who faced persecution for being gay. In his short life, Turing profoundly changed our world. Historians believe that his WW2 code breaking work helped save millions of lives and shortened the war by two years. He also founded three new scientific fields: computer science, artificial intelligence, and morphogenesis. In 1952, he was arrested by British police for having a relationship with another man and eventually was forced to undergo chemical castration to "fix" his sexual orientation. Two years after his arrest, Turing killed himself at age 41. Turing is one of the most important scientists ever, yet his tragic story and lasting legacy remain largely unknown. This film will change that fact. Almost 100 years after his

Voltage divider calculator for Radiometrix NTX2

In the configuration being used to drive the Radiometrix NTX2 in GAGA-1 there are two resistors (called them R1 and R2) that are creating a voltage divider and a third resistance (the input impedance of the module). The module takes 0-3V on the TX pin and converts it to a 0-5Khz frequency shift from the centre frequency. So to make calculations easy I made a little spreadsheet that allows you to punch in the two resistor values, the voltage supplied to the voltage divider and the input impedance to get a predicated pair of voltages sent to the TX pin, plus the corresponding frequency shift. It's an Apple Numbers file and you can get it here (Excel version ).

GAGA-1: Flight Computer radio's first transmission

Work is progressing on the flight computer with the first RTTY transmission. In the picture you can see the computer (the Arduino ) in the foreground, connected with test leads to a small breadboard containing a voltage divider made from three resistors which then connects to the radio itself. The radio is connected to the ground plane antenna in the capsule. The radio module is a Radiometrix NTX2 which transmits using FSK on 434.650Mhz. Its TX pin takes a voltage between 0 and 3V and converts that into a frequency shift of 0 to 5kHz. For the RTTY transmission I'll be using a shift of 425Hz between the mark and space frequencies. That shift corresponds to a difference in voltage on the TX pin of 0.255V. The configuration I'm using is based on picoatlas where a pair of pins from the Arduino are used to power a voltage divider. The set up is as follows: Arduino Radio 7 ---27K--------------\ ---------- TX 8 ---22K----

GAGA-1: Getting close to completion

The last time I updated on GAGA-1 it was when I cut the hole for the camera . Since then I've had a little time to work on it and things are getting close to completion. There's still a bunch of software to write for the flight computer, but that shouldn't take long. The launch will now be in the early spring when the winds are favourable. First, here's a nice shot of where I was. It shows the interior of the capsule with the recovery computer mounted in place, a rough location for its battery pack and the camera. The two SMA connectors are for the GPS and the GSM antennas. The first thing I did was make a couple of battery connectors with leads of the right length. I've bought two battery holders (one holds 4 AAs for the recovery computer, the other holds 6 AAs for the flight computer). Both have snap on connectors in the PP3 style and so I soldered up some short leads. The next job was to shorten the leads on the GSM and GPS antennas. They came with

The code injected to steal passwords in Tunisia

It's been floating around the net for weeks now, but I finally took a look at how someone in Tunisia (assumption is the government) was stealing usernames and passwords from common sites like Google Mail and Facebook. The attack worked like this: 1. When a user visited a site like Facebook JavaScript would be injected into the page where the user types in their username and password. On Facebook these pages are served via HTTP and so the injection is possible if you can intercept at the ISP level. The actual username and password are sent via HTTPS but once the JavaScript is in there it's game over . 2. The login form itself is modified to include an onsubmit handler that calls the JavaScript function hAAAQ3d (which reads as hacked). That function reads the username and password and makes an HTTP call to a bogus page on Facebook. This page (named wo0dh3ad, which I think you can read was woodhead) has the username and password appended as parameters with some code to