Last night's hacking adventure was inspired by a couple of recent posts from Roger, G3XBM having to do with light based communication. The first was his simple beacon design: it's just a keyer chip, a MOSFET, a voltage regulator, and some (beefy) current limiting resistors. Roger used this beacon to do some non-line-of-sight communications via "cloudbounce".
It's inspiring, so I've been thinking about trying some similar experiments. As it would happen, I have some reasonably high power LEDs floating around, and rigging up the transmitter really wouldn't be too hard. I don't have any of the K1EL chips that he used, but I do have a few of the Atmel ATtiny13s floating around, so I figured I'd go ahead and write up a simple beacon program to send my call via QRSS3 and (if a switch is tossed) 12wpm Morse. After my ATtiny powered Halloween Pumpkin and my ATtiny13 powered Christmas hat, I figured it would be a snap. I fetched an Arduino, programmed it with the AVRISP sketch, and then went to wire up the Arduino to program the code (did I mention I wrote some code?) to an ATtiny13 which I stuck on a breadboard.
And that's when I realized that I didn't take very good notes about how I did it last time, and refinding the necessary connections and commands was both difficult and some sources didn't even have the information correct.
So I'll remember, here's the list of connections you need:
- ATtiny pin 4 goes to ground.
- ATtiny pin 8 goes to 5V.
- ATtiny pin 1 goes to Arduino pin 10.
- ATtiny pin 5 goes to Arduino pin 11.
- ATtiny pin 6 goes to Arduino pin 12.
- ATtiny pin 7 goes to Arduino pin 13.
- Many sources say that for Unos, you must put a 10uF cap between the RESET and GROUND on the Arduino. I had one, so I did it. I've used older Arduinos (such as the NG) which have different reset circuits (you must manually reset them to program them) and for those it isn't necessary. Your mileage may vary.
Lastly, you need the right avrdude command. It took me an embarrassingly long time to come up with the right command, mostly because it appears that I needed to manually set the baud rate to 19200 (what the AVRisp sketch expects). Archived for posterity:
avrdude -v -c avrisp -p t13 -P /dev/tty.usbserial-A800eKtX -b 19200 -U flash:w:beaker.hex
I discovered a couple of things: while the previous versions of the ATtiny I used seemed to have fuses set so that the internal oscillator (nominally 9.6Mhz) would be divided by 8, this one did not. I sorted this out by defining F_CPU appropriately, but it was late and I didn't get my PWM code (which generates the 800hz sidetone) working properly. Tonight, I'll dust it off and hook it to the scope and see what's going on.
When this is done, it should be fairly simple to get the rest of the prototype transmitter together. Roger used a 4" or so lens as the collimating optic for his transmitter. As it happens, I have a dusty 6" Newtonian telescope that I purchased for a pittance, whose mirror really isn't up to the kind of standards I appreciate. I think I can mount a high power LED into a 35mm film can and insert that into the telescope eyepiece, and it should make an excellent collimator (I may need an auxillary lens if the cone from my LED is too broad, I think the telescope is around f/5 or so). Perhaps by this weekend, I'll have the transmitter setup, and I can start working on the (much harder) receiver.
Hope this article saves someone some time. I know I'll be referring back to it in the future.