Hello everyone. It has been a while since I posted and in that time I have done a lot of research for the Pi Portable Recorder and I should be ordering components this week! Firstly, I bought a new screen - a SPI ST7735 128 x 128 LCD to try out. I favoured this screen over other screens I have tried due to it's cost (£2.54 on Ebay), colour (not just two colours like a SSD1306 OLED), speed (uses SPI not I2C) as well as ease of use (already supported by luma.lcd) and easy to use pinout (no pins we do not need or SD card sockets on the underside of the screen etc.).
Secondly, I have decided on a audio board. After searching for a good I2S audio board for the Pi (with high quality input of course), I decided I was going to design my own PCB using the WM8731 chip which supports I2C commands, stereo audio output and stereo audio input, electret mic bias and much more . However, whilst researching how to design a good WM8731 breakout for the Pi, I came across this amazing board created by Github user skiselev. It uses the WM8731 as I wanted and breaks out all the inputs and outputs I wanted - big thanks to skiselev for making this board available! The board is so perfect, I don't need to make any PCB changes but I will try out an EM172 instead of the average electret mic included in the BOM - I say 'try out' because despite the EM172 being popular within the bird recording community, information of how well it works at a lower mic bias voltage (around 2.5V for the WM8731 if I recall correctly) is thin on the ground. I plan on assembling this board this side of the New Year and will post when I have finished.
Next up is the RTC, I like a recorder that keeps time so your recordings are easily timestamped. I noticed on the Zoom H1 taking out the battery resets the time which I find very annoying. For this reason I decided on the precision DS3231 RTC chip using a supercapacitor as backup power. I will try out a simple cheap breakout to start but plan to add it to the main board in the future.
Another thing I thought about was battery type used. I decided on using 1 18650 due to ease of use (booster and chargers easily available), capacity (around 2500 mah) and size (not too big). I decided not to use lipo as they are easily punctured and bent and more difficult to source (18650 is are readily available and can easily be bought from vape shops). This leads me onto the power management integrated circuit (PMIC) I plan to use. I currently plan on using the AXP209 (you may not of heard of it). As I have recently become involved in the Zerophone project a little bit (good experience for making Linux portable projects like mine), I was alerted to the presence of the cool AXP209 PMIC by the creator of Zerophone. It is 'China only' chip but very cheap! The AXP209 supports the charging and boosting of an 18650 whilst also supporting soft and head resets/shutdowns, under/over voltage protection and more. To see all the features of this chip go here. A breakout for this chip is being designed and made at the moment but it has not currently been made to work but when/if it does the breakout should be available to buy on Tindie in the near future.
Of course, as well as output (the LCD) we need input, I have decided to use some through hole 4 pin tactile push buttons for prototyping but in the future I plan to use these buttons and maybe a bigger button to start recording. These buttons were chosen for their tall caps and low cost.
As well as the essentials the Pi Portable Recorder allows for use of add ons. There should be a template in the future for users to create their own add on boards. Useful pins that are can be shared or are not used (any spare SPI pins, power pins, I2C etc.) will be broken out from the Pi on the main board to allow the easy addition of add on boards.
As well as the possibility of adding your own sensors etc., I am also designing a Premium...
Read more »