May 31, 2013
A brief post to catch everyone up while I'm at work:
Project Byzantium has been hard at work building a PTT (push-to-talk) circuit to support the third milestone of the ISC grant. What we're trying to do, in a nutshell, is this:
We have a couple of Baofeng UV-?R radios that we're trying to interface with laptops running Byzantium Linux. This is a known technology - ham radio operators have been doing datacomm over amateur radio frequencies for a couple of decades but this is a first for the three of us. What is posing a problem for us is the slightly unusual jack pinout that Baofeng uses for its radios. What we're trying to do is build a circuit that activates the PTT (push to talk) feature of the radio so that it's not transmitting continually, just when the computer has a data packet to send. There are many of them out there that we've been trying but Haxwithaxe discovered that the circuits he was building were either jamming open (not triggering to transmit) or jamming closed (triggering the transmitter and staying that way). A colleague of ours at HacDC spent the evening with us on Wednesday hunched over an oscilloscope, and discovered that nearby RF interference (probably from the radio itself) was causing the circuit to act up. Some experimentation with filtering capacitors resulted in a functional device that we're going to be testing, replicating, and documenting this weekend.
Photographs of the circuit as constructed by Haxwithaxe on a breadboard and sketches of the diagram have been uploaded here. Click each image after it loads to enlarge.