Hardware

Bill of Materials

Kit-Supplied Components

Component Model Qty
Microcontroller ESP32 DevKit v1 1
Water turbidity sensor SEN0189 (mandated) 1
Soil moisture sensor Capacitive v2.0 (mandated) 1
DC gearmotor JGA25-370 12V 300RPM (no encoders) 2
Motor driver L298N 1
Servo MG996R 1
Buck converter LM2596 1

Additional Components

Component Model Qty Role
Ultrasonic sensor HC-SR04 1 Forward wall detection
Voltage regulator AMS1117-3.3V 1 3.3V rail
Resistors 220 Ω + 470 Ω (SEN0189 divider) + 1kΩ/2kΩ (ECHO) — SEN0189 output divider + HC-SR04 ECHO 5V→3.3V divider
Battery LiPo 7.4V 2200mAh 1 Main power

The robot has no wheel encoders and no IMU — heading is open-loop dead reckoned from commanded wheel speeds (path log only). There is no I2C device on the robot at all; the single HC-SR04 ultrasonic is a collision guard on two plain GPIOs.

Power Architecture

LiPo 7.4V 2200mAh
│
├── L298N VM (7.4V direct → ~5.4V at motors after L298N drop)
│     └── expect reduced speed/torque on turf and sand
│
├── LM2596 Buck → 5V ──→ SEN0189 (5V supply)
│                          MG996R servo
│                          L298N logic
│
├── LM2596 Buck → 5V ──→ HC-SR04 ultrasonic (5V device; ECHO returns 5V → divider)
│
└── AMS1117-3.3V (from 5V) ──→ ESP32
                                Capacitive moisture sensor

Critical: the SEN0189 output swings to ~4.5V. The ESP32 ADC pin maximum is 3.3V — the signal must pass through a voltage divider. As built: signal → 220 Ω → tap → 470 Ω → GND, with the ADC reading the tap. TURBIDITY_DIVIDER_RATIO = 690/470 = 1.4681, defined in hardware_pins.h. Direct connection clips every clean-water reading and can damage the pin.

Pin Assignments (hardware_pins.h — authoritative)

GPIO Function
13 L298N IN1 — Motor L direction
14 L298N IN2 — Motor L direction
4 L298N ENA — Motor L PWM (LEDC ch0)
18 L298N IN3 — Motor R direction
19 L298N IN4 — Motor R direction
23 L298N ENB — Motor R PWM (LEDC ch1)
27 MG996R servo PWM
21 free (was I2C SDA — the colour sensor and the whole bus are gone)
22 free (was I2C SCL)
16 HC-SR04 TRIG
17 HC-SR04 ECHO — via 5V→3.3V divider
34 SEN0189 ADC — via voltage divider
35 Capacitive moisture ADC — direct (3.3V safe)
0 BOOT button — flashing only; the firmware no longer reads it
2 Status LED — solid ON once boot completes

GPIO 21/22 used to be reserved for I2C and are now free — the colour sensor was the only device on the bus. They remain unassigned rather than reshuffling a working motor pinout.

GPIO 34/35 are ADC1 — safe while WiFi runs. Never move analog inputs to ADC2 pins (0, 2, 4, 12–15, 25–27): ADC2 is unusable with the WiFi AP active.

Obstacle Sensor Wiring

There is no I2C bus. The HC-SR04 is the only sensor that is not a plain ADC read, and it uses two ordinary GPIOs — TRIG 16, ECHO 17. ECHO drives 5V, so it must pass through a divider to 3.3V before the ESP32 pin (e.g. 1kΩ series + 2kΩ to GND → 0.66×). TRIG accepts the ESP32's 3.3V directly.

Double-Arm Seesaw Mechanism

One MG996R servo drives two rigid arms on the same shaft. One servo replaces two linear actuators — simpler, lighter, fewer failure modes. This is the headline innovation.

          SERVO SHAFT (pivot)
                 │
   ┌─────────────┴─────────────┐
   │                           │
 Arm A                       Arm B
 SEN0189                Cap. Moisture v2.0
 (Water probe)           (Soil probe)

 Servo  90° → NEUTRAL: both arms horizontal (travel position)
 Servo   0° → Arm A DOWN: water sensor deployed
 Servo 180° → Arm B DOWN: soil sensor deployed

Rules:

  • Robot must be stationary before the servo moves.
  • Always return to 90° after every sampling sequence.
  • Arms are physically rigid — both move together on every rotation.

Physical Compliance

Constraint Limit
Length ≤ 400mm
Width ≤ 300mm
Height (arm at 90°) ≤ 400mm
Weight (battery included) ≤ 1.5kg

All dimensions measured in starting configuration (servo at 90°, arms horizontal). Deployable parts may not exceed the limits.

Mounting Requirements

  • HC-SR04 forward-facing, at a height where it sees the arena wall squarely. It is a collision guard only: it cannot see down into a recessed water slot, so nothing in the firmware protects the wheels from a slot — that stays the operator's job.
  • HC-SR04 forward-facing at wall height, both transducers unobstructed.
  • Servo arm pivot at the front so both probes reach past the front bumper.
  • Battery secured with an accessible power switch — officials can order an immediate power-down.
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