Explanation of Electronics Design

Introduction
This post will only discuss the electronics on the balloon and not the tracking electronics on the ground.  The electronics payload consists of a printed circuit board(PCB), camera module, GPS module, radio module, and battery pack.  The above schematic is of the printed circuit board only.  As such, anything not on the PCB is not shown in it.  The PCB is responsible both for controlling the camera, GPS, and radio, and taking power from the battery pack and redistributing it to the other modules.

Battery Pack (not on PCB)
The battery pack we are using (http://www.sparkfun.com/products/550) holds 4 lithium ion AA batteries.  We chose to use lithium ion AAs because they are much less massive than alkaline batteries, they store more energy, are less affected by low temperatures, and don't cost that much more than alkaline batteries.  Each AA has a voltage of ~1.5V, so the output of the battery pack is ~6V.
Power Regulation
The power regulation electronics take the input voltage from the battery pack and step the voltage down to 5V and 3.3V for use by the other electronics.  I decided to run the output of the 5V regulator into the input of the 3.3V regulator because the 5V parts draw very little current relative to the 3.3V part.  The power dissipated by a linear voltage regulator is equal to the voltage drop across it times the current, so this daisy chaining reduces the power dissipated in the 3.3V regulator by about one third.  The increase in power dissipation in the 5V regulator is not a problem because the 5V parts draw such a small amount of current.
Controll Unit
What I have decided to call the "control unit" consists of  the Teensy 2.0 (a micro controller module), a microSD card adapter, a level shifter, and a mux/demux.  UART stands for universal asynchronous receiver transmitter, and is used by the Teensy to communicate with the camera, radio, and GPS.  The Teensy is a 5V part, but all of the other modules are 3.3V parts, so the output of the voltage of the Teensies UART port must be "shifted" down to the 3.3V range.  This is done by the SF LLC shown on the schematic.  Because the Teensy only has one UART port, a 74HC4052 is used to allow the single port communicate with more than one device.  A microSD card is used to store images from the camera as well as store a flight log consisting of all GPS coordinates received, all messages sent to the radio, all pictures taken by the camera, and any errors that occur.
Interface
The interface portion of the board has connectors on it to allow communication with the radio module, camera, and GPS.
Radio Module (not on PCB)
I chose to use a 1W COTS radio module, the  AC4790-1000, for communications.  I chose this module over building my own because although it would be cheaper to build my own, it would significantly increase debug and design times.