The May issue of QST arrived in the mail today and an article about building a “fox” for hidden-transmitter hunting was included. That brought back memories of a teenage project of mine that I had once thought of writing up for QST, but now just makes a good story for the blog.
The first (and only) handheld radio I’ve owned is an Alinco DJ-580T. Like most HTs of a certain age, it has provision for an external (“speaker”) microphone. The microphone input is a sub-miniature (3/32-inch, “2.5 mm”) stereo phone plug. As a high school student, one of my passions was tinkering with a graphing calculator—the venerable Texas Instruments TI-85—do kids these days even use this stuff or have they gone the way of slide rules and nomograms? The TI-85 offered the provision to link to a computer or another calculator through a similar sub-miniature stereo phone plug.
Well, one afternoon in probably 1997, I was sitting with the DJ-580T in one hand and the TI-85 in the other…and it hit me…I wonder if I can use the the TI-85 to drive the DJ-580T microphone input?
A few preliminaries are now in order. Thanks to a helpful (and still operational, albeit now with a CMS and the attendant spam) web site called ticalc.org, a few friends and I had learned to load our TI-85s with third-party binary machine code programs with considerably faster execution times than the built-in scripting language. This allowed us to play relatively powerful video games surreptitiously on a school-sanctioned platform…a tactic that worked well until the English teacher wisened up to the fact that the five students with their calculators out were not typing essays on them. Not satisfied to just play games—although I did set a very high score in Tetris during Spanish class—I sought to harness the power of the Z80 microprocessor in the TI-85 for myself. Recall that this was before widely-available and inexpensive microcontroller development systems like the PIC, Arduino, and AVR.
I gathered the tools and eventually managed to write some fairly sophisticated (given my utter lack of formal training in computing) software in Z80 assembly language, including a crude clone of Space Invaders and a crude adventure game I called “Kashmir.” Maybe some screenshots or stories about them will come later.
But, for the story at hand, I learned how to manipulate the link port. Fortunately, the sleeve was ground on both the TI-85 and the DJ-580T. So, it was just a matter of tip and ring—one was audio and the other was PTT on the radio, and both were settable on the TI-85 for some kind of two-wire communication link. So, I reasoned that I could write up a bit of assembly code that would key the PTT by pulling it low, then toggle the audio line back and forth at 500 Hz or so to generate a rough audio tone. It worked!
This was an expensive, although trivially so since I had the hardware, way to build a hidden transmitter. So, I modified the software to send my callsign in Morse code (using a look-up table) and stuffed the whole thing in a cigar box. It was good fun for a few of us teenage boys.
And, for the interested, I found the original source code, which is sadly not well commented or dated. But, it does have my old callsign (AA8UP) listed by the lookup table.