Arduino nano, Nidec 24H motors, MPU6050, 3S 1000 mAh LiPo battery.
Balancing controller can be tuned remotely over bluetooth.
Example (change K1):
Send p+ (or p+p+p+p+p+p+p+) for increase K1.
Send p- (or p-p-p-p-p-p-p-) for decrease K1.
Remote control over Joy BT Commander app.
About schematic:
Battery: 3S1P LiPo (11.1V 500-1200mAh).
Buzzer: any 5V active buzzer.
Transistor: 2N2222 or similar.
Servo: TowerPro MG995 or similar size.
The voltage regulator 7805 is not the best choice. Use any 5V switch type small regulator.
More about this: