Lychee/PCB
This page details the various aspects of the PCB(s) designed for the Lychee HAB project. The aims of the project are to demonstrate basic hardware capabilities and flight-test the altimeter for verification before the launch of Martlet IV... if that day ever comes.
Architecture
The board is based on the Project Strix hardware, and as such the core of the project is the STM32F405RGT6 microcontroller, stolen from some old hardware Henry dug out of the shelves. The inner workings of each are covered in more detail below, but the rough architecture of the subsystems looks like:
- PSU: A simple battery balancer IC and two linear regulators providing 1.8 and 3.3V.
- Altimeter: A 7-DoF IMU from TDK which includes an accelerometer, gyroscope, and barometer. A redundant barometer was also included, which is why the system is called the 'altimeter' and not simply the IMU. Both peripherals share an SPI bus.
- Radio: includes a 2-way RF transceiver and the GPS. Really should be two separate systems but all the RF kit has historically been grouped into a single 'radio' subsystem, and why bother breaking tradition ;) . The radio system uses a single SPI bus, while the radio uses one singular UART.
- USB input, just for fun.
- A 'blackbox' connector. Has a single UART input, which then gets converted to differential RS-422/485 (depends on who you ask) to be transmitted to the blackbox board. More on that later.
Subsystems
PSU
The PSU must take unregulated input from the 2S Li-ion battery pack, and distribute regulated power to the various onboard devices. It has three main components:
- The battery balancer
- 1v8 regulator
- 3v3 regulator
Let's take a look at each in detail!
Battery Balancer
The battery pack for the system consists of two 18650 cells in series, called a 2S configuration. To balance these, we just buy and off-the-shelf IC that does the job. The chip chosen for this purpose was the BQ29200.
Note from the author:
Well, I said that chip does the job, but I kinda lied :P
I noticed pretty late in the game that it looks like it doesn't actually handle over-discharge/under-voltage protection. Oops! I'm slightly concerned about that, so to the engineers of the future: how did I do?