Here it is the complete schematic. The PIC 16F84 is on its classic configuration: a reset circuitry, oscillator section and output GPIOs. The reset section is made of a switch button, current limiting resistor (R2) and pull-up resistor (R1). The oscillator is a simple 4MHz quartz with two 10pF capacitors.
GPIOs RB1-RB7 (PORTB) and RA2,RA3 (PORTA) drive the LED bus (9 lines). GPIOs RA0,RA1 and RA4 select respectively the bottom, middle and upper LED layer. Note that RA4 is an open-drain GPIO when in output mode so a 4.7 Kohm pull-up resistor is required; the other GPIOs don’t require such resistor.
The actual beat detection is done by three main parts:
- MAX9812H linear amplifier (IC3)
- RC low-pass filter (R17, C7)
- Signal squarer (T1,T2)
The square wave signal is directly fed to RB0 which is programmatically set as interrupt pin. On each signal pulse (i.e. music beat) the LED Cube selects and shows a different LED pattern.