attachInterrupt to pin7

[kd-chicago]

Code uses digital pin 7 to attach interrupt: arduino UNO can only use pins 2 & 3 for this interrupts;
other pins can be assigned as interrupt source through PCINT

Also, keep in mind MAX7219 controller board with SMD chip is missing filter capacitor, so it will only work well at the minimal intensity settings. One has to solder 10-100uF capacitor between controller VCC and GND to make it work consistently.