drempelbox

kids friendly audio system

Building

Using cross

This project uses cross for compiling to different target architectures. Cross requires a regular rust install via rustup, docker or podman, and a Linux kernel mit binmft_misc support, see also cross dependencies.

Then install cross using cargo:

cargo install cross --git https://github.com/cross-rs/cross

To build for 64bit RaspberryPi OS (aarch64-unknown-linux-gnu), use

make build-aarch64-unknown-linux-gnu

To build for build-x86_64-unknown-linux-gnu, use

make build-aarch64-unknown-linux-gnu

Cross uses a containerized build environment to make cross compilation easy, install any external dependencies for the build step in Dockerfile.cross. Docker buildx is required for monitoring the build progress.

Building directly on the host

You can also build this project directly on your host machine (for its architecture) using just cargo. You will need to install the ALSA dev libs for your system, though (libasound2-dev on many platforms).

Building on the RaspberryPi

Don't do this unless you really need to! Compiling on the Zero 2W is very slow!

To install the rust toolchain on the Zero 2W, you need to increase swap size. From https://gist.github.com/tstellanova/0a6d8a70acc58a0d5be13ebaa7c935d4

sudo dphys-swapfile swapoff
sudo nano /etc/dphys-swapfile

Change CONF_SWAPSIZE=100 to CONF_SWAPSIZE=512 (or 1024)

sudo dphys-swapfile setup
sudo dphys-swapfile swapon
sudo reboot

Change to any swap size you feel appropriate for your needs.

Running on RPi Zero 2W

Assuming a 64 bit RaspberryPI OS lite, install these packages to enable audio support:

• pipewire
• pipewire-alsa

Enable SPI for NFC ready support (using the raspi-config tool, for example).

Modify /boot/firmware/config.txt, adding this line at the end:

dtoverlay=gpio-poweroff,gpiopin=4,active_delay_ms=5000,inactive_delay_ms=1000,active_low=1

This enables the Pi to keep itself powered during normal operation and switch itself off after poweroff. See the schematic for details.

Create the service user and its group: sudo adduser --system --no-create-home --group drempelbox --uid 1337

Add the user to these system groups to give it the necessary permissions:

sudo usermod -a -G gpio drempelbox
sudo usermod -a -G audio drempelbox
sudo usermod -a -G spi drempelbox

Create a polkit rule to allow the user to shutdown the system. Create the file /etc/polkit-1/rules.d/40-allow-shutdown.rules and add these contents:

/* Allow members of the drempelbox group to shutdown without authentication */
polkit.addRule(function(action, subject) {
   if ( ( action.id == "org.freedesktop.login1.power-off" ||
          action.id == "org.freedesktop.login1.power-off-multiple-sessions"
        ) && subject.isInGroup("drempelbox") ) {
     polkit.log("Powering Off permitted for subject" + subject)
     return polkit.Result.YES;
   }
});

Restart the system or the polkit service for the changes to take effect:

systemctl restart polkit.service

Hardware

Rough block diagram of system components:

classDiagram
    direction BT
    USB_PD_Decoy o-- Powerbank : USB A to USB C
    StepDownReg o-- USB_PD_Decoy : 12V
    5VFilter <|-- StepDownReg
    RPi <|-- 5VFilter : 5V micro USB
    RPi <|--|> NFC_Module : 5V, SPI
    USB_Soundcard *-- RPi : USB
    Amplifier <|-- USB_Soundcard : 3.5mm TRS
    Amplifier  o-- USB_PD_Decoy : 12V
    Speakers <|-- Amplifier

    class NFC_Module {
    }
    class RPi["RPi Zero 2W"] {
    }
    class USB_Soundcard["USB Soundcard"] {
    }
    class Powerbank{
        USB PD
        SoC Display
        USB C in/out
        USB Micro in
        USB A out
        charge()
        discharge()
    }
    class USB_PD_Decoy["USB PD Decoy"]{
        12V
    }
    class StepDownReg["5V DC/DC Buck"]{
        12V -> 5V
    }
    class 5VFilter["5V Filter"]
    class Amplifier{
    }
    class Speakers{
    }

Schematic

Software

flowchart
    subgraph NTAG
        present{token presence changed?} -- no --> delay((delay)) --> present
        present -- added --> read[read token]
        present -- removed --> sendreq_ntag_stop[send player stop  request]
        read --> parse --> parse_ok
        parse_ok{is url?} -- yes --> sendreq_ntag_play[send player play request]
    end

    subgraph Player
        sendreq_ntag_stop --> stop{{stop all playback}}
        sendreq_ntag_play --> type{file or https}
        type -- file --> playfile{{stop spotify, play file}}
        type -- https --> isspotify{starts with 'open.spotify.com'?}
        isspotify -- no --> error
        isspotify -- yes --> playspotify{{stop file, play spotify}}
    end

    subgraph HTTP
        post_play[POST to play endpoint] --> query{has url query param?}
        query -- yes --> type
    end