The Infinity Mirror Music Player

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The propeller clock module, the primary display screen of the system, is consisted of the following main hardware components: a 12V motor, which is attached under the base board and fixed by a motor holder to the base board, is utilized to rotate the proto board. Here the proto board is a PCB board that is used to hold the other major components, including the Adafruit Dotstar LED strip, the Arduino Pro mini, the Hall sensor, and the Bluetooth to UART module. As for these components on the protoboard, a Dotstar LED strip with 8 LEDs will be used to display the content while rotating. Arduino pro mini will be used to control the LED strip display and communicate with the Raspberry Pi3 to control the whole system. A wireless Bluetooth to UART converter will be used to transmit the control signal from Raspberry Pi 3 to Arduino. A hall sensor, which is fixed on the board, together with a magnet, which is fixed on the regular mirror surface, is used to detect the starting point and refresh the display screen. A DC motor control module is planned to be connected with the board and control the rotating speed of the board( which was removed in the end as it introduced unstability to the motor rotating speed). In this module, balancing of the propeller clock will be our major challenge.

12V motor
LED strip
Arduino Mini Pro
Raspberry Pi
Power supply
Inductive charge module
3D printing motor holder
acrylic board
two way mirror
hall sensor
Bluetooth module HC-05
Skills Overview

Arduino programming; Raspberry Pi programming; Circuit design; 3D printing;

The Challenge

Propeller clock displayer balancing

The propeller displayer was the main display screen for the whole system. It utilizes persistence of vision theory to display the illusion of stable patterns floating in the air, thus it requires demanding rotation balance skills, which does not fall within our familiar expertise.

The protoboard system was even not balanced when we set up the initial system and began the rotation test. We first changed the Li-ion battery position as initially we put the battery on one side of the rotating board. As the battery is definitely the heaviest part on the board, we put it to the center hoping to improve. But as the battery module itself was not symmetric, we still cannot balance the system. We searched several solutions on Internet, and removed our power supply system for the propeller clock off-board (explained in power redesign module), then the board itself can be balanced.

Then the led strip was added, the rotating clock could not maintain balance again. One possible reason may be that the LED strip is too soft to keep in position when the motor was rotating at a high speed(around 1800 rpm). As for this factor, we thought of using structure wood to support the led strip. Another possible reason for the unbalance introduced by the led strip is that the LED strip was mounted only on one side, we need to add one similar structure to the other side to balance during rotating. In the end, we found a more stiff material to hold the LED strip and use four wires to hold the whole system, structured like a bridge. We also added some coins to balance the weight and adjust carefully the distance. Finally, the whole system was successfully balanced with all components.


Charging system re-design

The redesign of the charging system has gone through a Li-ion battery to a slip ring, then to inductive wireless charging module. At first, a rechargeable Li-ion battery will be used to charge the propeller clock module. Following our initial charging system design, we successfully recharge the Li-ion battery using the lab power supply equipment, and modified the Li-ion battery boost module, added a switch to control the power and added a charging circuit, in the end, we successfully have 5V output. But as the cumbersomeness of the battery made balancing rotating board so difficult, we removed it and tried slip ring to power the rotary part.

We made a slip ring to replace the Li-ion battery with the following steps:

  • Cut copper and aluminum slice to make a metal roll.
  • Cut the AA battery to get a perfect metal roll.
  • Cut the copper to get a perfect electrode.
  • Design its mechanical structure to make it more reliable.
  • Testing its reliability on high speed.

The finished slip ring worked mostly OK except the motor became hot after some time when rotating. We suspect it’s mainly because of the insulation(the tape and glue) when fixing the slip ring on the motor. Also the slip ring will increase the resistance of the rotation part, thus the rotating board will become somewhat unstable.To combat this problem, we experimented inductive charging module to power the propeller displayer system, and tested the set with oscilloscope and assembled it on the system. This is a lightweight and stable solution that we finally choose to use for our project.


Bluetooth to uart communication module design

The Bluetooth to UART communication module is an essential part of the system communication and controlling. We initially used the module HC-06 to test the communication between the two microcontrollers, but we cannot get the desired reply from the serial to the Raspberry Pi monitor. To debug, we first checked all the hardware and made sure that the circuit is correctly connected. Then, we checked the connection between our computer Bluetooth connection and the module. The HC-06 module could connect to the computer correctly. We then did research on the internet and found similar bugs related to the HC-06 module, so we switched to HC-05 module instead. Using HC-05 module, the testing program could be uploaded successfully. We were also able to achieve the bi-directional communication between Raspberry Pi 3 and Arduino pro mini: the command could be sent to Arduino from Raspberry Pi 3; Raspberry Pi 3 could receive and display the reply information from Arduino.Finally, we redesigned the connector between the Bluetooth module and the Arduino pro mini to make it more compact and easier to balance on the rotating board.


Display design

Initially when Raspberry Pi 3 communicated the with Arduino pro mini, the reply from the Arduino was not correct. After some research we figured out that a time lapse need to be added between sending the command and receiving the reply. Otherwise, the reply will not be prepared, then the receiving buffer would not have the full reply. That’s the reason that the reply displayed on the screen was not correct.

At first, the character displaying effect on the propeller displayer is similar with the picture shown below(left): the shortest distance between the led light spot was separated around 4-5 centimeters. To improve the displaying effect, we reduced the delay time and finally delete the delay time between the two status(light on and off) entirely, but it didn’t work.

So, we suspected that the effect of the fastest updating rate may just be like this. We then searched the document of the dotstar led strip on internet and found that the updating rate could be super fast: for Arduino, the PWM rating could be 8MHZ, which is the clock rate. If that is true, we were supposed to get a better POV effect than the displayed effect. We then figured it might be the software problem. As we were using the Adafruit library previously, twe tried the Fastled library to see the display effect. It worked! The displaying effect with the new library is much better, shown in the picture below(middle).

With the new library, we could display the characters on screen better, but we want to display messages more precise and mimic the real-world display. So we tried to change the software SPI to hardware SPI by changing the connection within the system and got an even better display effect as shown in the picture below(right).

fix_display image fix_display image fix_display image

Based on the third solution, we successfully displayed the character “ECE5725” on our system.

Thank you for creating this project, infinity. Let me know if you need help promoting it.

Using rotation as a method of display is a really ingenious idea, and describing the challenges you encountered in trying to implement it really shows the work you put into it! Would it be possible to see some kind of video demonstrating what the finished product looks like and what it can do?

Hi, this is a really cool project! Was there a specific reason you chose to use an Arduino Pro Mini in conjunction with the Raspberry Pi?

Hi Vaidehi,

This is the project for ECE 5725. We used raspberry pi as the platform to learn embedded operating system. So I choose raspberry pi. And the size and parameters like the Clock frequency Arduino pro mini is fast enough to drive the led strip. So I choose to use Arduino mini pro.