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Final Report Gilliland

Page history last edited by abbygill@... 4 years, 11 months ago

Final Project: Sneaky MP3



Teachers across America scream “Pay attention! Put away your music!” However, music is a major part of creating culture, life vibrancy, and connections between many people. 



Hidden MP3 player, with hidden control to change the song.



The MP3 player is hidden beneath one's clothing. For sake of demonstration I removed the cover from the MP3 so the lights go through the clothing in the video. The cover would be replaced so no lights would go through clothing, and the product would stay sneaky. A wire with a button at the end runs up one's sleeve to their palm, where it is hidden from the teacher. When the button is pressed, the song playing on the MP3 changes. 






#include <SD.h>

#include <SPI.h>

#include <EEPROM.h>


#include <mp3.h>

#include <mp3conf.h>


// include the adafruit pcd8544 & gfx libraries for a nokia 5110 graphic lcd.


#include <Adafruit_GFX.h>

#include <Adafruit_PCD8544.h>


// setup microsd and decoder chip select pins, and the decoder-specific pins.


#define sd_cs         17        // 'chip select' line for the microsd card


#define mp3_cs        A0        // 'command chip select' connect to cs pin

#define mp3_dcs       A1        // 'data chip select' connect to bsync pin

#define mp3_rst       -1        // 'reset' connects to decoder's reset pin

#define mp3_dreq      A2        // 'data request line' connect to dreq pin


// now assign pins for the graphic lcd (carried over from the etch-a-sketch).


#define lcd_dc        A4        // 'data/command input' connect to d/c pin

#define lcd_cs        A3        // 'slave chip select' connects to  cs pin

#define lcd_rst       -1        // 'reset' connects to graphic lcd rst pin


// 'read_buffer' is the amount of data read from microsd and sent to decoder.

// it's probably best to keep this a factor of 2, up to about 1kb (2kb is the

// max). you might change this if you experienced skips during song playback.


#define read_buffer  512        // size (bytes) of the microsd read buffer

#define mp3_vol      200        // default volume. range min=0 and max=254


// file names are 13 bytes max (8 + '.' + 3 + '\0'), and the file list should

// fit into the eeprom. for example, 13 * 40 = 520 bytes of eeprom are needed

// to store a list of 40 songs. if you use shorter file names, or if your mcu

// has more eeprom, you can change these.


#define max_name_len  13

#define max_num_songs 40


// id3v2 tags have variable-length song titles. that length is indicated in 4

// bytes within the tag. id3v1 tags also have variable-length song titles, up

// to 30 bytes maximum, but the length is not indicated within the tag. using

// 60 bytes here is a compromise between holding most titles and saving sram.


// if you increase this above 255, look for and change 'for' loop index types

// so as to not to overflow the unsigned char data type.


#define max_title_len 60


// ---- global variables ----------------------------------------------------


// instantiate a graphic lcd object using the pins that we #define'd earlier.

// comment out the graphics lines to save memory if you're not using the lcd.


// Adafruit_PCD8544 lcd = Adafruit_PCD8544(lcd_clk, lcd_din, lcd_dc, lcd_cs, lcd_rst);

Adafruit_PCD8544 lcd = Adafruit_PCD8544(lcd_dc, lcd_cs, lcd_rst);


// 'File' is a wrapper of the 'SdFile' data type from the sd utility library.


File sd_file;                   // object to represent a file on a microsd


// store the number of songs in this directory, and the current song to play.


unsigned char num_songs = 0, current_song = 0;


// an array to hold the current_song's file name in ram. every file's name is

// stored longer-term in the eeprom. this array is used in 'sd_file.open()'.


char fn[max_name_len];


// an array to hold the current_song's title in ram. it needs 1 extra char to

// hold the '\0' that indicates the end of a character string. the song title

// is found in 'get_title_from_id3tag()'.


char title[max_title_len + 1];


// the program runs as a state machine. the 'state' enum includes the states.

// 'current_state' is the default as the program starts. add new states here.


enum state { DIR_PLAY, MP3_PLAY, PAUSED };

state current_state = DIR_PLAY;


//---- module functions -----------------------------------------------------


// you must open a song file that you want to play using 'sd_file_open' prior

// to fetching song data from the file. you can only open one file at a time.


void sd_file_open() {  

  // first, find the file name (that's stored in eeprom) of the current song.




  // then open the file using the name we just found (stored in 'fn' global).


  sd_file = SD.open(fn, FILE_READ);


  // find the current song's title tag (if present) then print it to the lcd. 





// read a number of bytes from the microsd card, then forward them to the Mp3

// library's 'play' function, which streams them out to the decoder chip.


void mp3_play() {

  unsigned char bytes[read_buffer]; // buffer to read and send to the decoder

  unsigned int bytes_to_read;       // number of bytes read from microsd card


  // first fill the 'bytes' buffer with (up to) 'read_buffer' count of bytes.

  // that happens through the 'sd_file.read()' call, which returns the actual

  // number of bytes that were read (which can be fewer than 'read_buffer' if

  // at the end of the file). then send the retrieved bytes out to be played.


  // 'sd_file.read()' manages the index pointer into the file and knows where

  // to start reading the next batch of bytes. 'Mp3.play()' manages the index

  // pointer into the 'bytes' buffer and knows how to send it to the decoder.


  bytes_to_read = sd_file.read(bytes, read_buffer);

  Mp3.play(bytes, bytes_to_read);


  // 'bytes_to_read' is only smaller than 'read_buffer' when the song's over.


  if (bytes_to_read < read_buffer) {



    // if we've been in the MP3_PLAY state, then we want to pause the player.


    if (current_state == MP3_PLAY) {

      current_state == PAUSED;





// continue to play the current (playing) song, until there are no more songs

// in the directory to play. 2 other sd library methods (that we haven't used

// here) can help track your progress while playing songs: 'sd_file.size()' &

// 'sd_file.position()'. you can use these to show say, the percent of a song

// that has already played.


void dir_play() {

  if (sd_file) {



  else {

    // since 'sd_file' isn't open, the recently playing song must have ended.

    // increment the index, and open the next song, unless it's the last song

    // in the directory. in that case, just set the state to PAUSED.


    if (current_song < (num_songs - 1)) {





    else {

      current_state = PAUSED;





void switch_song()


  if(current_song != num_songs - 1)









// ---- setup and loop ------------------------------------------------------


// setup is pretty straightforward. initialize serial communication (used for

// the following error messages), mp3 library, microsd card objects, then the

// graphic lcd. then open the first song in the root library to play.


void setup() {

  // if using a graphic lcd, initialize with contrast, then setup the screen.

  attachInterrupt(0, switch_song, FALLING);








  lcd.print("Barebones Mp3!");




  // initialize the mp3 library, and set default volume. 'mp3_cs' is the chip

  // select, 'dcs' is data chip select, 'rst' is reset and 'dreq' is the data

  // request. the decoder sets the 'dreq' line (automatically) to signal that

  // its input buffer can accommodate 32 more bytes of incoming song data.


  // the decoder's default state prevents the spi bus from working with other

  // spi devices, so we initialize it first.


  Mp3.begin(mp3_cs, mp3_dcs, mp3_rst, mp3_dreq);



  // initialize the microsd (which checks the card, volume and root objects).




  // putting all of the root directory's songs into eeprom saves flash space.




  // the program is setup to enter DIR_PLAY mode immediately, so this call to

  // open the root directory before reaching the state machine is needed.





// the state machine is setup (at least, at first) to open the microsd card's

// root directory, play all of the songs within it, close the root directory,

// and then stop playing. change these, or add new actions here.


// the DIR_PLAY state plays all of the songs in a directory and then switches

// into PAUSED when done. the MP3_PLAY state plays one specific song and then

// switches into PAUSED. this sample player doesn't enter the MP3_PLAY state,

// as its goal (for now) is just to play all the songs. you can change that.


void loop() {

  switch(current_state) {


    case DIR_PLAY:




    case MP3_PLAY:




    case PAUSED:





void pause_play()


  int last = 0;

  int time = millis();

  if(time - last > 80)


    if(current_state != PAUSED) 


      current_state = PAUSED;




      current_state = DIR_PLAY; 



  last = time;




Comments (1)

Dongao Yang said

at 11:30 am on Aug 18, 2015

Hi, Abby,

We like the idea you have to make a hidden mp3 player. You have a clear user need from your life experience, didn't you? We wish you can have Verplank diagram in your report or have a link in your report to it. In the design report, we usually focus more on the process of design instead of the result of the design. So Verplank diagram is definitely also a part of your report.

We like you have included your mind map in your project, it did help us understand better on how you convert your idea into the real. Same here, we wish you can include the picture inside your report.

We would like to see pictures of your paper prototypes. To design a good product, the prototypes established the bridge to achieve that. Your final project today can also be recognized as a prototype for your future design. The prototype makes a product real and solid. It is milestone and we wish we can see it.

We like your code. It is clean and formatted well. We would be really glad if you can have your state diagram here. In report, sometimes code just have to much details to review. In that case, a state diagram can always remind you what you did in your code and work as a reference for your future programming.

We like your project, and hope you enjoy this class! Have a great summer.

Best wishes,

Dongao, David, Praveen, Samyuktha, Tian, Xiangyu, Zahra

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