brainwagon "There is much pleasure in useless knowledge." — Bertrand Russell

14Aug/110

ARISSAT-1’s battery appears to be failing quicker than expected…

I didn't get a chance to record any more ARISSAT-1 data this weekend, but I did catch up on some reading. Apparently, it's batteries are giving out quicker than expected: the voltage is dropping low enough to cause a reset when the satellite goes into eclipse each orbit. If you were thinking of grabbing some telemetry/SSTV from the satellite, or even attempting some QSOS, perhaps sooner would be better than later.

You can read up on the Power System of the satellite here, which includes a link to this AMSAT Journal article which gave many details. Apparently the silver-zinc batteries which were used aboard the satellite (largely for safety, a dead short of these batteries does not cause a fire) have many features which were seemingly not that well understood. These batteries are typically used in a deep-discharge situation, and only recharged five times. Aboard ARISSAT-1, they are recharged in a shallower cycle, more times. There also seems to be some issues regarding temperature performance. I skimmed the article, but will reread it and rethink it some more soon.

14Aug/111

Arduino PWM Laser Transmitter

A couple of weeks ago, I programmed an Arduino to take digitized sound stored in its rom, and send it out via PWM of an LED. A couple days after that, I used the same code to send voice using a small 5mw laser module. Ever since then, I thought it would be good to use the Arduino's analog to digital converters to sample the sound from a small microphone, and then use that as the signal to send over PWM. Here's my first test:



There is lots of noise remaining in the signal. Some further experimentation showed that the voltage regulation wasn't very good: I did tap 5v from the Arduino, but the load must have been near its limit, because the overall voltage levels were varying by nearly 100 millivolts, which is only slightly less than the signal amplitude. I also have many leads which are too long, poor layout, and unoptimized values for AC blocking caps. But it does seem to work. I'll be revising this over the next couple of weeks.

Addendum: I did measure the current through the op amp preamp, and the laser diode module. The op amp circuit only needs to drive the Arduino input, which is very high impedance, so it draws less than four milliamps. The laser diode averages 20ma, but has a peak power of twice that.