BT audio player: Circuit and PCB

This is the second part of a six-part post:

Part 1: System design and planning
Part 2: Circuit and PCB design
Part 3: Mechanical design
Part 4: Firmware
Part 5: Installation and external power
Part 6: Conclusion, problems and screwups

Circuit design and prototyping

The prior post already detailed the key electronic components choices. Putting all of these together:


The circuit design is not complicated. The main sections are:

  • MCU with 8MHz crystal and supporting caps.
  • The RN52 with serial connection to the MCU
  • The OLED display connected to the MCU over SPI.
  • OLED 8V boost power supply.
  • Environmental, ambient light, proximity sensors and RTC connected to the MCU over I2C.
  • Several logic-controled "switches" to isolate power to specific devices. Primarily MOSTFETs, but one high-side switching IC.
Breadboarded Prototype


Firmware development was primarily done on a prototype design

PCB design

The PCB was one of the more mechanically constrained designs I have done. The next post in this series describes why that was. Namely, alignment requirements between the screen's FPC connection and the proximity/light sensor daughterboard, Bluetooth antenna location, analog audio routing, and capacitive button connections.

There is plenty of real estate on the board, but the functional layout was challenging.

Below are the design images and the actual assembled board, which was reflowed in my toaster reflow oven (project will be linked here shortly).