Lithium Battery pack with on/off switch
How simple can you get for a power supply? Very, if you're us. We used two 6V primary (non-rechargable) Lithium batteries (from Radio Shack) in series to get 12V of very reliable power. At 1.5 amp-hours, we had more than we could possibly use. Given a 5W transmitter and at most 500mW of power for the PIC/Accelerometer, that's 1 amp for over an hour and a half - and given our flight time about 5 minutes, I'd say we over did it a bit. In fact, the batteries still work great 2 months later!
We used a SPDT with "center off" metal toggle switch in the main power supply so we could switch the payload power between "OFF" (no power), "BATT" (onboard batteries) and "EXT" (external power or "shore" power). Although potentially a reliability problem, the switch was very much worth it: to conserve batteries and transmission time, we could wait until right before launch to turn the rocket (and transmitter) on. To get at the switch, we drilled a small 1/4 inch hole in the casing so that we could turn it on with a pair of tweezers. Note that we also sawed off the end of the toggle switch to lessen the moment arm of the switch so that we didn't get spurious switching during the high g forces of launch or recovery ejection.
For our other boards, we simply used distributed LM7805s to produce regulated 5V power from the unregulated 12V batteries. Although in some senses wasteful (the 7805 is a linear regulator which are pretty inefficient), the 7805s are a "one chip", reliable solution to regulated power. We may move to a small one-chip switchi power supply in the future. Again, we'll use one for each board. That way we're more likely to isolate power supply problems to their respectful boards rather than take the entire system down with a single failure.
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