Goal: To build a sensor to measure some things!

This project was more about learning things that interest me more than developing a complex product. I wanted to learn more about:

• Creating a Bluetooth Low Energy (BLE) application from scratch 
• LiPo battery implementation incl. charging circuitry
• Antenna design, layout tips and tuning
• Designing for ultra low power 
• PCB assembly with stencils and the reflow oven

Still high on motivation I made a sketch of what I was thinking at the time

This was actually when I was considering using LoRa for communication. Of course this ended up being totally different from what I actually made in the end.

Let's break down the different subsystems:

Subsystems

Power management

• LiPo Battery
• Battery level detection - maybe voltage based?
• Charging connector - USB micro B or USB C ?
• Battery Protection - over current, short circuit, over voltage, over temperature
• Charging IC
• Load switch - to turn the sensors off during sleep
• Voltage regulator

Interface

• ON / OFF - Slide switch
• User Input - momentary tactile switch
• Charging LED
• Status LED - RGB?
• External headers for other sensors - 4 x male header. Will not mount unless needed. Probably I2C

MCU 

• Crystal
• General MCU implementation - based on nRF52 reference
• Programming and debugging connector
• Antenna & Matching Circuit

Sensors

• Temperature 
• Pressure
• Humidity

Details

Here are some more details about specific parts of the system that I thought were interesting.