What is it?

This is a three channel audio extension board with real time clock and LED PWM driver to create you own DigitalRooster Internet radio/Podcast player/alarm clock. Of course you can use it for other projects than building an alarm clock.

The Mk3b is compatible with both Raspberry Pi Zero W and Banana Pi M2 Zero using some solder jumpers. So you have a wider choice of more powerful hardware!

Specs of the Mk3 board:

• Uses standard 40 Pin Raspberry Pi Headers, with some solder jumpers it can be adapted for Banana Pi models.

• Three Channel audio using a 3W MAX98357A I2S driver. Since this IC is a Class-D amp I equipped the channels with passive LC filter to improve audio quality. The left/right channels have a cut-off frequency of around 20kHz while the mid channel filter is a 16kHz low-pass

• A hardware shutdown line for the audio codecs so ‘off’ is really ‘off’ and there is no popping noise when changing tracks

• A BQ32000 Real Time Clock chip with trickle-charge circuit that charges a 200mF SuperCap. Although the software synchronizes time using NTP I wanted a Real Time Clock in case the network is down. Also having a the correct time during boot makes logging a lot more insightful and avoids TLS error for certificates that are not yet valid on Jan 1st of 1970…

• A TLC59208F PWM LED driver for backlight control and a possible RGBA light to wake up with a sunrise simulation. The PWM pins on both Raspberry Pi and BananaPi are scarce an used for other functions, e.g. I2S. Software PWM is not really a nice option so I was willing to spend some 2.5$ for a nice I2C chip that integrates nicely with the Linux LED driver framework

• Integrated circuitry for the Grayhill rotary encoder that requires voltage dividers and a debounce for the push button

• The possiblity to connect an external I2C device. I have a APDS9660 gesture and light sensor in mind for dynamic back-light control or gesture snooze.

Included parts in the kit

DigitalRooster Audio Extension MK3b Kit includes following parts that need to be soldered:

• 3x 5mm terminal blocks for loud speakers (e.g. TE Connectivity 282836-2 or similar)
• 1x 6-pin-header 90°, 2.54mm (e.g. Adam Tech PH1RA-06-UA or similar) for the Greyhill rotary encoder with push-button.

For more information see digitalrooster.dev/mk3b and digitalrooster.dev/pages/hardware

Why did you make it?

Readily available Internet radio devices all have their nuisances. I wanted a fully hackable extensible alarm clock. Other extension boards did not include all features and circuitry I needed for instance Adafruit's SpeakerBonnet does not have a audio filters or a shut-down line for the codecs. I also was quite annoyed that I couldn't control the backlight of the display while using the audio board extension - Raspberry Pi unfortunately uses the same pins for I2S as for PWM.

Most important: It is fun to learn new stuff.

What makes it special?

It can be configured to run with BananaPi (Allwinner H2) SoCs or Raspberry Pi which have different I/Os for I2S audio. Also, it uses a Super Cap for the RTC, controls the back light of the display.

Notes:

You have to solder at least two jumpers and the connectors for loud speakers and rotary encoder.

I run it on a custom buildroot generated Linux root file system, for information how to build it see the README.md in the github project.

I haven't tried the Raspbian distribution in quite a while. At the time it ran a heavily patched 4.19.x kernel that unfortunately does not include drivers for the MAX98357A I2S audio driver. However it should be possible to use the Hifiberry-DAC device tree overlay.

For private consumers in the EU sales I have to charge 19% tax (VAT, USt, IVA...) this is included in the price.

Regulatory advice

The Mk3b-extension board is a module without independent function and is exclusively intended for development purposes under laboratory conditions. It is not a product to be resold individually or integrated in another product without getting the required certification applicable in the respective country (think FCC, UL etc.) For use or operation specialist knowledge is required. Please also read the safety and environmental notes