I also used various components I had on hand, such as small prototyping boards, a 5V power regulator, capacitors, resistors, a ULN2803 transistor array, LEDs, a piezo buzzer, single and dual row pin headers, wires, shrink wrap and 2.1mm male and female power jacks: total value less than $10. The LEDs, with a power consumption of 8mA, could have realistically been driven directly from MCU output pins, but the buzzer couldn't, and I wanted to allow switching other devices which consumed more power. There is a two-row pin header to distribute regulated 5V to most of the modules, as well as an unregulated 12V output for the motor driver module.