COMPONENTS

Component	Notes
Raspberry Pi	The server. Talks to the Nano by USB (/dev/ttyUSB0)
Arduino Nano	ATmega328P, old bootloader (cpu=atmega328old). Powered by the USB
ESC	Driven by the servo signal on D9. Its BEC (5 V) is used to detect the battery
Brushless motor	—
LiPo battery, 3S	Powers the ESC. Balance lead used to measure the voltage
Potentiometer	Throttle in POT mode
Push button	Unlock (hold 2 s) / lock (one tap)
3 LEDs	Red, green, yellow; each with a 330 Ω resistor to GND
Buzzer	Beeps (driven with tone())

PINS

Pin	Name in pins.h	Connected to	Mode
D2	PIN_RED	Red LED (bench state)	output
D3	PIN_GREEN	Green LED (bench state)	output
D4	PIN_YELLOW	Yellow LED (battery)	output
D6	PIN_BAT_DETECT	ESC BEC (battery plugged in)	input (external 100 kΩ pull-down)
D7	PIN_BUZZER	Buzzer	output
D8	PIN_BUTTON	Push button to GND	input pull-up
D9	PIN_ESC	ESC signal	servo PWM (Timer1)
A0	PIN_BATTERY	Battery voltage divider	analog
A6	PIN_POT	Potentiometer wiper	analog (input only)
GND	—	Common to the ESC, battery divider, LEDs, button, pot	—

CIRCUITS

ESC signal (D9)

Servo pulses, 50 per second: 1000 µs = 0 % (stopped), 2000 µs = 100 %. The firmware never goes above MOTOR_MAX_PERCENT = 50 % (1500 µs). The ESC ground must be connected to the Nano GND.

Battery detection (D6)

ESC BEC red wire (5 V) --- 10 kΩ --- D6 --- 100 kΩ --- GND

Battery plugged into the ESC: the BEC gives 5 V and D6 reads HIGH (~4.5 V). Unplugged: the 100 kΩ pulls D6 to LOW. Never connect the BEC to the Nano 5V pin.

The Nano does not see the battery itself: it sees the BEC 5 V that come through the ESC cable. So ESC NONE means "the ESC is not powered": the battery or the ESC cable was unplugged (tested: both give EVT ESC NONE).

D6 LOW for more than 50 ms (BAT_DISCONNECT_MS) = battery unplugged. After plugging in, the ESC needs 3 s (BAT_ARM_MS) to arm.

Battery voltage (A0)

battery + (balance lead) --- 10 kΩ --- A0 --- 4.7 kΩ --- GND

V(A0) = V(battery) × 4.7 / (10 + 4.7). A full 3S battery (12.6 V) gives 4.03 V on A0. The highest voltage it can measure is VREF × 14.7 / 4.7 ≈ 14.5 V.

VREF = 4.64 V: the real voltage on the Nano 5V pin (measured with a multimeter; calibrate it for better readings). Each measurement is the average of 16 readings (BAT_SAMPLES), every 100 ms.

State	Battery	Per cell	Meaning
NONE	< 6.0 V	—	no balance lead
LOW	< 10.2 V	3.4 V	the motor cannot turn; 2 s below this while UNLOCKED locks the bench
WEAK	< 10.8 V	3.6 V	finish the test and charge (hysteresis 0.2 V)
GOOD	≥ 10.8 V	—	—

Push button (D8)

D8 → button → GND, with the Nano internal pull-up (INPUT_PULLUP): released = HIGH, pressed = LOW. Debounce 20 ms (DEBOUNCE_MS).

Potentiometer (A6)

Ends to GND and 5V, wiper to A6. With pin 1 to GND and pin 3 to 5V, turning clockwise increases the value (0..1023). A6 is analog input only on the Nano.

LEDs (D2, D3, D4)

Pin → 330 Ω → LED → GND (HIGH = on).

LEDS AND BEEPS

LEDs

What you see	Meaning
Red, green, yellow rotating (100 ms each)	ESC arming: wait
Red on	LOCKED
Red blinking (500 ms)	UNLOCKING: hold the button
Red blinking fast (200 ms)	UNLOCKING, button being held
Green on	UNLOCKED: the motor can turn
Yellow off	No battery
Yellow on	Battery good
Yellow blinking slowly (500 ms)	Battery weak or low

Beeps

Beep	Meaning
2000 Hz short	UNLOCK accepted: hold the button
2000 Hz long	Unlocked
1000 Hz short	Locked (button tap or battery unplugged)
1000 Hz long	Locked: battery low
3000 Hz long	Locked: server lost (heartbeat)