MP3 Playback in CircuitPython with Lars the Sloth Puppet

by John Park

published May 27, 2020, last edited January 22, 2025

posted in Feather CircuitPython Breakout Boards/ Amplifiers/Sound

Beginner

Project guide

Overview

You can now play MP3 files directly in CircuitPython with no dedicated decoding hardware! You'll be able to use this new capability in all sorts of projects, including this creepy Lars the Sloth voice box project!

The benefit of MP3 over other non-compressed file formats, such as WAV, has always been their small file size, meaning you can pack a lot of songs or samples into limited memory. However, decompressing those small files for playback was too taxing for older microcontrollers, so we had to use dedicated decoder chips at an additional cost, including licensing fees.

Thanks to the raw speed of M4, the days of requiring a dedicated MP3 audio decoder chip are behind us. And, since the patents have expired, we can now distribute MP3 playback libraries free of charge!

Parts

Angled shot of a Adafruit Feather M4 Express.

Adafruit Feather M4 Express - Featuring ATSAMD51

It's what you've been waiting for, the Feather M4 Express featuring ATSAMD51. This Feather is fast like a swift, smart like an owl, strong like a ox-bird (it's half ox,...

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$22.95

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Adafruit Prop-Maker FeatherWing

The Adafruit Feather series gives you lots of options for a small, portable, rechargeable microcontroller board. Perfect for fitting into your next prop build! This FeatherWing will...

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$9.95

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Mini Oval Speaker with pico blade connector

Mini Oval Speaker - 8 Ohm 1 Watt

Hear the good news! This wee speaker is a great addition to any audio project where you need 8 ohm impedance and 1W or less of power. We particularly like...

$1.95

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Slim Lithium Ion Polymer Battery 3.7v 400mAh with JST 2-PH connector and short cable

Lithium Ion Polymer Battery Ideal For Feathers - 3.7V 400mAh

Lithium-ion polymer (also known as 'lipo' or 'lipoly') batteries are thin, light, and powerful. The output ranges from 4.2V when completely charged to 3.7V. This...

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Tactile On/Off Switch with Leads

Squeeze once to turn on, squeeze again to turn off! This clicky switch makes a great power switch or mode toggler. We like this switch because it's easy to embed in a seam for...

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Micro B Round Panel Mount Extension Cable - 30cm

If you need to add a panel-mount connection for USB Micro B, but don't have the time or ability to cut a custom oval or square hole, this Round Panel Mount...

$4.95

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Five pack of 20-pin 0.1 Female Header - Red plastic

20-pin 0.1" Female Header - Red - 5 pack

Female header is like the duct tape of electronics. It's great for connecting things together, soldering to perf-boards, sockets for wires or break-away header, etc. We go through...

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Break-away 0.1 inch 36-pin strip male header - Red plastic

Break-away 0.1" 36-pin strip male header - Red - 10 pack

In this world nothing can be said to be certain, except we need headers, headers, and more headers!Each pack contains ten red 36-pin 0.1" pitch...

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Black Nylon Screw and Stand-off Set with M2.5 Threads, kit box

Black Nylon Machine Screw and Stand-off Set – M2.5 Thread

Totaling 380 pieces, this M2.5 Screw Set is a must-have for your workstation. You'll have enough screws, nuts, and hex standoffs to fuel your maker...

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Plush Doll

The choice is yours, but if you want to embed creepy, semi-threatening voice samples into a murder puppet, you could do worse than a Sickening Sloth Fuggler.

The teeth. Oh the teeth.

Who is Lars?

Lars is the sworn enemy of CVT Joseph, the ice cream truck godfather of Los Angeles, and owner of CVT Soft Serve.

Lars is angry that Joe won't reveal the location of his Wonka-style Golden ice cream gift cards. This video linked below will make everything clear:

https://www.instagram.com/p/B--_P8JJWWy/

Actor Matt Biedel (Umbrella Academy, Narcos) has kindly given us permission to use Lars's voice. At least that's what Joe claims.

This project is not sponsored by CVT Soft Serve. John is is just really obsessed with Lars and great ice cream.

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Build the MP3 Player

You can play .mp3 files with just a Feather M4 plus an amplifier and speaker. However, for the puppet sound playback, we'll make a nice, compact player with some extra features.

The Propmaker FeatherWing gives us a very convenient built-in amplifier and speaker output port, as well as an accelerometer with tap-detection we will use to trigger the sample playback.

With a small battery, USB port extender, and toggle switch for powering the board on and off via the Enable pin, we'll have a great little player that can be embedded inside the puppet, while still allowing it to be triggered from the outside, recharged and even coded without requiring disassembly.

Add Header Pins

Solder a set of female and male header pins to the Feather and FeatherWing respectively as shown here.

This guide shows how to solder in the headers.

Enable Switch

The Enable pin on the Feather can be used to turn the board on and off.

Push the wires through the Propmaker FeatherWing's hold as shown (for strain relief) and then solder it to the G and En pads (either wire in either hole is fine).

Battery Power

Connect the battery to the JST battery jack and add a little piece of double-stick foam tape to keep it secure between the two boards.

Fasten with Hardware

Use four M2.5 standoffs and eight short M2.5 screws to fasten the boards together.

Speaker

Next, plug in the speaker and you'll be ready for programming and testing your .mp3 player!

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CircuitPython on Feather M4 Express

CircuitPython is a derivative of MicroPython designed to simplify experimentation and education on low-cost microcontrollers. It makes it easier than ever to get prototyping by requiring no upfront desktop software downloads. Simply copy and edit files on the CIRCUITPY drive to iterate.

The following instructions will show you how to install CircuitPython. If you've already installed CircuitPython but are looking to update it or reinstall it, the same steps work for that as well!

Set up CircuitPython Quick Start!

Follow this quick step-by-step for super-fast Python power :)

Download the latest version of CircuitPython for this board via CircuitPython.org

Click the link above and download the latest UF2 file.

Download and save it to your desktop (or wherever is handy).

Plug your Feather M4 into your computer using a known-good USB cable.

A lot of people end up using charge-only USB cables and it is very frustrating! So make sure you have a USB cable you know is good for data sync.

Double-click the Reset button next to the USB connector on your board, and you will see the NeoPixel RGB LED turn green. If it turns red, check the USB cable, try another USB port, etc. Note: The little red LED next to the USB connector will pulse red. That's ok!

If double-clicking doesn't work the first time, try again. Sometimes it can take a few tries to get the rhythm right!

You will see a new disk drive appear called FEATHERBOOT.

Drag the adafruit_circuitpython_etc.uf2 file to FEATHERBOOT.

The LED will flash. Then, the FEATHERBOOT drive will disappear and a new disk drive called CIRCUITPY will appear.

That's it, you're done! :)

Further Information

For more detailed info on installing CircuitPython, check out Installing CircuitPython.

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Code the MP3 Player

To use with CircuitPython, you need to first install a few libraries, into the lib folder on your CIRCUITPY drive. Then you need to update code.py with the example script.

Thankfully, we can do this in one go. In the example below, click the Download Project Bundle button below to download the necessary libraries and the code.py file in a zip file. Extract the contents of the zip file, open the directory MP3_Tap_Player/ and then click on the directory that matches the version of CircuitPython you're using and copy the contents of that directory to your CIRCUITPY drive.

Your CIRCUITPY drive should now look similar to the following image:

Text Editor

Adafruit recommends using the Mu editor for using your CircuitPython code with the Feather boards. You can get more info in this guide.

Alternatively, you can use any text editor that saves files.

Code.py

Copy the code below and paste it into Mu. Then, save it to your Feather as code.py.

Download Project Bundle

Copy Code

# SPDX-FileCopyrightText: 2020 John Park for Adafruit Industries
#
# SPDX-License-Identifier: MIT

# MP3 playback with tap trigger
# Works on Feather M4 (or other M4 based boards) with Propmaker
import time
import board
import busio
import digitalio
import audioio
import audiomp3
import adafruit_lis3dh

startup_play = False  # set to True to play all samples once on startup

# Set up accelerometer on I2C bus
i2c = busio.I2C(board.SCL, board.SDA)
int1 = digitalio.DigitalInOut(board.D6)
accel = adafruit_lis3dh.LIS3DH_I2C(i2c, int1=int1)
accel.set_tap(1, 100)  # single or double-tap, threshold

# Set up speaker enable pin
enable = digitalio.DigitalInOut(board.D10)
enable.direction = digitalio.Direction.OUTPUT
enable.value = True

speaker = audioio.AudioOut(board.A0)

sample_number = 0

print("Lars says, 'Hello, CVT Joseph. Tap to play.'")

if startup_play:  # Play all on startup
    for i in range(10):
        sample = "/lars/lars_0{}.mp3".format(i)
        print("Now playing: '{}'".format(sample))
        mp3stream = audiomp3.MP3Decoder(open(sample, "rb"))
        speaker.play(mp3stream)

        while speaker.playing:
            time.sleep(0.1)
    enable.value = speaker.playing


while True:
    if accel.tapped and speaker.playing is False:
        sample = "/lars/lars_0{}.mp3".format(sample_number)
        print("Now playing: '{}'".format(sample))
        mp3stream = audiomp3.MP3Decoder(open(sample, "rb"))
        speaker.play(mp3stream)
        sample_number = (sample_number + 1) % 10
    enable.value = speaker.playing

# SPDX-FileCopyrightText: 2020 John Park for Adafruit Industries
#
# SPDX-License-Identifier: MIT

# MP3 playback with tap trigger
# Works on Feather M4 (or other M4 based boards) with Propmaker
import time
import board
import busio
import digitalio
import audioio
import audiomp3
import adafruit_lis3dh

startup_play = False  # set to True to play all samples once on startup

# Set up accelerometer on I2C bus
i2c = busio.I2C(board.SCL, board.SDA)
int1 = digitalio.DigitalInOut(board.D6)
accel = adafruit_lis3dh.LIS3DH_I2C(i2c, int1=int1)
accel.set_tap(1, 100)  # single or double-tap, threshold

# Set up speaker enable pin
enable = digitalio.DigitalInOut(board.D10)
enable.direction = digitalio.Direction.OUTPUT
enable.value = True

speaker = audioio.AudioOut(board.A0)

sample_number = 0

print("Lars says, 'Hello, CVT Joseph. Tap to play.'")

if startup_play:  # Play all on startup
    for i in range(10):
        sample = "/lars/lars_0{}.mp3".format(i)
        print("Now playing: '{}'".format(sample))
        mp3stream = audiomp3.MP3Decoder(open(sample, "rb"))
        speaker.play(mp3stream)

        while speaker.playing:
            time.sleep(0.1)
    enable.value = speaker.playing


while True:
    if accel.tapped and speaker.playing is False:
        sample = "/lars/lars_0{}.mp3".format(sample_number)
        print("Now playing: '{}'".format(sample))
        mp3stream = audiomp3.MP3Decoder(open(sample, "rb"))
        speaker.play(mp3stream)
        sample_number = (sample_number + 1) % 10
    enable.value = speaker.playing

Audio Samples

You can get started by using these .mp3 files. (Of course, you're free to use any files that you like, see below for more info on that.)

Download the .zip file linked below and then uncompress the file. Drag the /lars directory onto the root level of your Feather's CIRCUITPY drive.

lars.zip

Make Your Own

To make your own .mp3, use audio software such as Audacity to save them with these settings:

bit rate: anywhere from 16 to 320 kb/s (lower will be smaller, higher is better quality)
sample rate: 22050Hz or 44100Hz are recommended, although 16kHz, 24kHz, and 48kHz should also work
channels: mono or stereo
must be DRM free

How It Works

Here's what the code does:

Imports the necessary libraries for audiomp3 playback and accelerometer tap detection with the adafruit_lis3dh
Sets up the accelerometer
Specifies the speaker enable pin (this allows the speaker to be disabled when not in use)
If startup_play is set to True, each sample in the /lars directory with the name format of /lars_0*.mp3 will be played using the mp3stream object
In the main loop of the program, the accelerometer is checked for tap detection. When the tap occurs, the next sample is played. When the tenth one is played, the list loops around back to the first one again.

Test It

With the enable switch clicked on, try tapping the Feather/Propmaker to trigger a sample. When a sample has finished you can tap again to trigger the next one.

Note, there is a small volume adjustment screw on the Propmaker FeatherWing that can be turned with a small screwdriver to adjust the maximum volume.

Now, let's add the sound player to our puppet friend Lars!

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Implant the MP3 Player in Puppet

Before making any incisions, we'll lay out the parts on the puppet to get a feel for where things will go. This is a good time to add the USB panel-mount port extender to the micro USB jack on the Feather.

We want to get the speaker up near Lars's inexplicably human-toothed maw, with the Feather board near the center of his back so it is easy to tap it to trigger the sound sample playback.

I decided to place the on/off switch for the enable pin in Lars's right paw-claw-hand thing, and the USB port jack in his left foot.

Seam Ripper

Use a seam ripper or small scissors to open up the seam on the bottom of Lars's left foot.

Feed in the Speaker

Feed the speaker up through the foot hole and work it up toward the head area.

Bag It

To prevent the board from snagging on the doll's batting on the way in, you can wrap the feather in a small bag with a zip tie or rubber band to close off the neck as shown here.

USB Jack

The last thing to push into the plush toy will be the USB panel-mount jack.

Use a medium sized zip tie to secure it to the end of the foot as shown, then trim off the excess zip tie end.

You can now plug a USB cable into Lars's foot for battery charging, swapping out .mp3 files, or even adjusting the CircuitPython code!

Test out your finished puppet now by making sure the Enable switch is clicked on, then tap his back to play!

You may need to wiggle the speaker around to get it to an optimal placement. Also ensure the battery is charged or the USB cable plugged in to get sufficient power.

Note, in the video below I was using a double-tap to trigger.

Page last edited March 08, 2024

MP3 Playback in CircuitPython with Lars the Sloth Puppet

Overview

Parts

Plush Doll

Who is Lars?

Build the MP3 Player

Add Header Pins

Enable Switch

Battery Power

Fasten with Hardware

Speaker

CircuitPython on Feather M4 Express

Set up CircuitPython Quick Start!

Further Information

Code the MP3 Player

Text Editor

Code.py

Audio Samples

Make Your Own

How It Works

Test It

Implant the MP3 Player in Puppet

Seam Ripper

Feed in the Speaker

Bag It

USB Jack

Search

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