Music cards are not something new. But those that you normally get play annoying tunes that are low quality. Dmitry thought that he can do better. So he started building his own version of music card. His goals were to have full fidelity playback, random songs each time and be small enough to mimic gift card. To accomplish this he had to solve several problems like playing quality sound with Attiny85 microcontroller. It seems that he’s done this pretty well by generating 64MHz on one timer with PLL which his used to generate PWM based analog output. Another 32KHz timer generated actual music sampling. Music has to be stored in SD card in WAV format. Then they are picked by using uFAT library.
Another issue were music volume. Since Attiny85 could source up to 20mA with 3.0V battery is could provide about 60mW power which is really low. One option could be adding power amplifier, but since signal is digital, he ended up with using full bridge driver out of four MOSFETs. This way he was able to reach 5.8W output signal. Player is powered with 180mA Li-Po battery. Having in mind that battery is a limiting part, he built-in small USB powered charger out of MCP73831. The rest was to put everything in to CD case, add switching mechanism that would turn circuit when its opened and take care of card itself.