Rpibot: A Low‑Cost, Expandable Mobile Robot for Embedded Systems Learning

This simple, low‑cost mobile robot platform was created to give developers a practical, hands‑on way to learn embedded systems.

As an embedded software engineer at a German automotive company, I started this project as a learning platform. Although the original project was canceled early, my enthusiasm carried it forward as a hobby, resulting in the Rpibot.

I had a clear set of requirements:

• Minimal, software‑centric hardware
• All‑in‑one €100 budget
• Built‑in expandability for future modules
• Single 5 V power source (e.g., USB powerbank) for all components

The platform serves multiple purposes—education, surveillance, and robotics competitions—but it’s not a beginner tutorial. Basic knowledge of Python programming, electronics, and PID control is expected.

Challenges were inevitable, but a curious and persistent mindset helps overcome them. The source code is intentionally concise, with critical sections commented for guidance.

Full source code and hardware schematics are available on GitHub: https://github.com/makerobotics/RPIbot

Supplies:

Mechanics

• 1 × A4 plywood board, 4 mm thick
• 3 × M4 × 80 screws and nuts
• 2 × gear motors with secondary output shaft for encoders (wheels)
• 1 × free‑wheel
• 1 × pan‑and‑tilt camera mount (optional)

Electronics

• 1 × Raspberry Pi Zero (with header and camera)
• 1 × PCA 9685 servo controller
• 2 × optical encoder wheels and circuits
• 1 × female jumper wires
• 1 × USB powerbank
• 1 × DRV8833 dual motor driver
• 2 × SG90 micro‑servos for camera pan/tilt (optional)
• 1 × MPU9250 IMU (optional)
• 1 × HC‑SR04 ultrasonic sensor (optional)
• 1 × perforated board, soldering wire, headers, etc.

Step 1: Build the Chassis

The chassis is intentionally simple, lightweight, and fast to assemble and disassemble.

• Affordable materials
• Quick assembly
• Room for future sensor additions
• Lightweight to reduce power consumption

Plywood is ideal—easy to cut with a fretsaw and drill. Glue small wooden pieces to create mounting points for sensors and motors. Critical parts should be secured with screws so they can be replaced or debugged without disassembling the entire frame. While a hot‑glue gun is convenient, it may not hold up during long‑term use. 3D printing is an alternative but can be costly and time‑consuming.

The free wheel is lightweight and friction‑free, enabling smoother motion. A simple wooden spacer levels the rear wheel after the main wheels are mounted.

Wheel properties (for software calculations)

Circumference: 21.5 cm
Encoder pulses: 20 pulses/rev.
Resolution: ≈1 cm per pulse (simplifies distance calculations)

Step 2: Electronics and Wiring

The following modules are interconnected as shown in the diagram.

Raspberry Pi Zero – the central controller. It reads sensors and drives motors via PWM. It connects to a remote PC over Wi‑Fi.

DRV8833 – a dual H‑bridge motor driver that supplies sufficient current to the motors (the Pi’s GPIO pins cannot).

Optical encoders – deliver a square wave every time the wheel’s light path is interrupted. Hardware interrupts on the Pi capture each transition.

PCA 9685 – an I²C servo controller providing PWM signals and 5 V power for pan‑tilt servos.

MPU9250 – a 3‑axis accelerometer, gyroscope, and magnetometer, primarily used for heading information.

All components are wired with jumper cables. A breadboard serves as a distribution hub, providing 5 V, 3.3 V, and ground rails. The connection table (see attachment) details every link. Never connect 5 V directly to a 3.3 V pin; double‑check wiring before powering. Measure the main supply voltages with a multimeter after setting up the breadboard.

Modules are secured to the chassis with nylon screws, making them both stable and removable for maintenance.

Soldering was limited to the motor terminals, breadboard, and header pins. Jumper wires are convenient but can become loose; software diagnostics can help identify intermittent connections.

Source: Rpibot – About Learning Robotics