BBC:microbit projects

SORTING OUT THE DESIGN AESTHETICS AND COMPLETING THE PROJECT

Part Two of making a music box

We skipped an issue of the magazine with this project but here is the second and final instalment on how to make a music box. Part one was in Issue No. 86 of The Shed.

The design of the Arduino MP3 player in part one was the first step to confirm the feasibility of this project.

To complete the build, I temporarily stopped working on the electronic part to concentrate on the aesthetics of the box. Following the style of this series of Micro Bit projects, I decided to use recycled cardboard to create the container. The box was metal painted with a brownish finish to simulate rugged metal.

It required a considerable amount of design and 3D printing to make the visible parts of the music box and the spring-charger simulator. The 3D-printed parts are used to control the music box and provide several effects simulating the behaviour of an old spring-charged music box.

Charge-level simulator

A metal-painted arrow simulating the

charge level of the spring moves at precise angle positions between zero and 90 degrees (from horizontal to vertical position). I have programmed nine positions, corresponding to the Arduino MP3 player’s nine sound tracks. When a track ends, the charger moves down one position until it reaches zero. You have to recharge the ratchet simulator to restart playing.

Charger simulator

The charge simulator is built in two parts. The internal part simulates a spring charger — the old music boxes are fully mechanical, of course — with a 3D-printed ratchet and a notch.

Rotating the ratchet, a small brushed DC motor driven by a couple of pulleys and an elastic belt produces a sufficient voltage pulse to be detected by the Micro Bit.

The external part is a big yellow charging key that is rotated to activate the music box.

Music-changer button

The Arduino stops when the track ends, and when the red button on top of the box is pressed the music box starts playing the next track. The Arduino will also skip to the next track when the button is pressed.

The output of the Arduino MP3 player is sent to a smartphone-sized speaker and the sound is amplified by a 3D-printed blue cone, which is visible on one side of the music box.

Lamp

I installed a 220V lamp simulating a yellow burning flame on one side of the box. The lamp is powered when the musical box is fully charged and the charger indicator is in the top position (90 degrees). The lamp remains powered until the charge level reaches zero. The lamp power is digitally controlled by the Micro Bit through a relay because of the power difference.

I assembled all the parts inside the box with hot glue, including the 9V power supply. Then I powered up the music box and enjoyed playing it.

It required a considerable amount of design and 3D printing

The music box assembled and ready for the first charge. On the right side is the charging key of the ratchet and the charge level indicator is on the left

Detail of the DC motor used as a pulse generator with the 3D-printed pulley

Assembling the ratchet. The 3D-printed ratchet mechanism simulates a real spring-charger ratchet. The rotating components — the notch and the retain spring — are assembled between two plates

The 3D-printed ratchet with the pulley connected to the DC motor. A half rotation to the ratchet wheel is required to make the DC motor (with the small pulley) rotate fast and generate the current pulse

This page, clockwise from main image: The completed ratchet simulator with the motor connected to the ratchet pulley by an elastic band The core of the music box is the Micro Bit plugged on a robotic board by Kitronik (kitronik.co.uk). The board controls and manages all the components of the music box through the robotic board (the servo, the signals to the Arduino MP3 player, and the lamp-powering relay) and exposes all the available pins of the Micro Bit for any use The ratchet connected to the 3D-printed charging key and the other 3D-printed components used to build the music box

Before completing the assembly of the music box, I had to calibrate the level of the DC motor generating the pulse to simulate the spring-load charger

Detail of the 220V glowing lamp simulating a flame. To digitally control the light, the on-off signal sent by the Micro Bit through the robotic board changes the state of a relay

Below: The music box before closing the container with all the components: 1. Arduino MP3 player 2. Smartphone-sized speaker with the 3D-printed blue cone to amplify the sound 3. Servo connected externally to the charger arrow of the box 4. Micro Bit plugged in the robotic Kitronik box 5. Printed circuit board (PCB) wiring the digital control signals 6. Relays and 220V lamp group 7. Ratchet module with the DC generator connected to the spring-charger simulator

The charge-level indicator is controlled by the Micro Bit through a servo, positioned at precise angles. When the nine levels of the charge are complete, the music box starts playing the first track. When the red button is pressed a new track is played. After every track is played the servo is moved down by one step until the music-box spring simulation needs a new charge