I had originally planned another ambitious project for my second HAB launch. My enthusiasm is tempered a bit by the idea of hurling another couple of hundred dollars worth of equipment into the sky without having sorted out the logistics of the launch and recovery procedures. I began considering a more modest design, using ordinary party balloons and an expendable payload. The two critical elements of such a payload would be an effective transmitter a simple microcontroller, both of the super-cheap variety. In October 2007, I began looking.
The two microcontrollers that I already had lying around were a Parallax Basic Stamp 2 and my prized Elba ZX-24. I had paid $60 or so for each of these modules. I knew that either would be more than capable as a morse code telemetry beacon, but I wanted to reduce the electronics cost even further.
Checking my usual online sources, I found that many PIC controllers ran $5 or less per chip. Having looked into this option previously, I also knew that most of these controllers (especially the less expensive ones) could only be programmed in assembler. The more advanced models could be programmed in C, but my experience in C is so shallow that this option would be a bigger challenge than I was looking for.
The Spark Fun Electronics website offers what should be an excellent solution. The PICAXE series of microcontrollers are regular PIC chips that have special bootloaders installed by a company in the UK called Revolution Education Ltd. These bootloaders allow a developer to write code in a variant of Basic, compile it with the free compiler provided by Revolution, and load the compiled binary onto the chip using a simple serial connection. The 8-pin version of the PICAXE, which should work well for this project, is $4 from Spark Fun. At that price, I won't feel bad losing a whole handful.
The other challenge is to find a suitable transmitter. My preference would be to use a 2-meter transmitter, so that the chase team can use mobile rigs to track and (hopefully) recover the payload. Homebrewing VHF gear turns out to challenging in many respects, but thankfully VHF radio modules are available that can be easy to integrate.
For now, I have settled on a 20-meter design called the VU Transmitter. This is a transmit-only variation of the classic two-transistor Pixie design. Following some hints from Rick KE3IJ's website, I am using the same value inductor for both the LPF and the collector choke on T2. I found I was able to keep the oscillator off the antenna during key-up by doubling the value of the T2 emitter-to-ground filter cap. This design should be able to produce as much as 400 to 500 mW into a 15-foot 30-gauge trailing wire antenna from a single nine-volt lithium battery.
I am driving the circuit from a 14.318 MHz crystal from an old ISA sound card (a lucky find). I am considering using a 10 MHz crystal grounded through a small cap instead, to use the transmitter on 30 meters. The 50 KHz bandwidth of the 30-meter band is a small target, though, especially for a circuit that may see -50*C at altitude (and thus drift out-of-band).
As much as I would love to be able to have this payload announce its own location over the air, GPS receivers are still too expensive to be disposable. Inertial measuring units (IMUs) may be able to provide an approximate fix, but those modules cost more than a GPS. Most payloads spin like crazy during ascent and descent, so I can't imagine that compass module would provide any useful data. An altitude reading might also be good to have - MEMS barometric sensors are fairly cheap and easily sourced. Air temperature varies with altitude with some consistency, so could maybe be used to estimate altitude.
I have always imagined a balloon mission in terms of achieving a peak altitude of over 100,000 feet using a latex envelope. But there is also the world of floaters - ultra-lightweight balloons of inelastic material such as mylar, which can achieve and sustain an altitude of a few 10s of thousands of feet, and cruise for days at a time. Some folks have even circumnavigated the globe with solar powered computer, GPS, and radio systems weighing as little as ten =grams=.
Options abound!