Speed gun

Portable speed detection device.

Author: Rîpeanu Constantin-Adrian
GitHub Project Link: https://github.com/UPB-PMRust-Students/proiect-RipeanuAdrian

Description

A portable radar device that uses a Doppler sensor to measure the speed of moving objects. If the detected speed exceeds a configurable threshold, it activates a buzzer, lights up a red LED, and captures a picture using an OV2640 camera module.

• The measured speed and speed limit are shown on an LCD 1602 I²C display.

• The reference speed can be adjusted using physical buttons.

• When speed is within the safe range, a green LED is lit.

• When overspeed is detected, the red LED lights up and a buzzer sounds.

• The image is stored on a microSD card or sent over WiFi via a Telegram bot or HTTP request.

Motivation

This project allowed me to explore embedded systems and real-time hardware control using Rust. I wanted to build something educational and practical — a compact device that detects motion and responds automatically.

I chose this topic because fixed speed cameras are a current subject, especially with the rollout of systems like “e-Sigur”. Inspired by how real traffic radars work, I aimed to recreate a simpler, portable version that can measure speed and take photos.

Arhitecture

The Raspberry Pi Pico 2W acts as the core of the system, coordinating all modules:

The HB100 Doppler sensor detects motion and outputs a frequency that reflects the speed.

An LM358 op-amp amplifies the Doppler signal to be read via ADC.

The LCD 1602 I²C displays the measured speed and configurable speed limit.

OV2640 camera captures an image when overspeed is detected.

The microSD card module stores the image, while WiFi can send it to a remote server or Telegram.

Buttons allow manual adjustment of the speed threshold.

LEDs indicate if the detected speed is within or above the limit.

A buzzer sounds on overspeed detection.

Log

Week 1 – May 6–12

• Chose the project topic: a speed measurement system using the HB100 radar sensor.
• Studied documentation for the HB100 module and its compatibility with the RP2040 Pico W board.
• Created the initial wiring plan (breadboard layout and power distribution).
• Began the schematic documentation using KiCad.
• Set up the development environment: Embassy + probe-rs + defmt logging.
• Wrote a simple test program to verify GPIO functionality and LCD output.

Week 2 – May 13–19

• Purchased the following components:
  • HB100 radar sensor
  • LCD 1602 with I²C interface
  • 5V active buzzer
  • Tactile push buttons
  • LEDs and resistors (220 Ω, 10 kΩ)
  • Capacitors (10 nF, 220 µF)
  • Jumper wires and large breadboard
  • OV2640 camera module (for future use)
• Tested the HB100 sensor in both digital and analog modes using RP2040 and defmt logging.
• Partially completed the KiCad schematic:
  • Added components: RP2040 Pico, HB100, LCD, buzzer, buttons, and LEDs.
  • Learned how to add, rename, and manage components in KiCad.
• Analyzed HB100 behavior with and without filtering (resistor + capacitors).
• Clarified the power supply layout: +5V on RR, GND on RB, common ground with Pico.
• Documented all necessary pin connections for the Pico board in the project setup.

Week 3 - May 20-27

• Implemented the software for the hardware setup.

Hardware

1. Raspberry Pi Pico 2W
   – Acts as the main controller and WiFi client

2. HB100 Doppler Radar Sensor
   – Measures object speed via frequency shift

3. LM358 Operational Amplifier
   – Amplifies weak IF signal for ADC processing

4. LCD 1602 with I²C Adapter
   – Displays speed and settings

5. OV2640 Camera Module
   – Captures images when threshold is exceeded

6. microSD Card Module
   – Stores JPEG images locally

7. Buzzer
   – Audible alert when speed is exceeded

8. Red & Green LEDs
   – Visual status indicators

9. Push Buttons (×3)
   – Adjust speed threshold and trigger capture

10. Logic Level Shifter
    – For safe I²C communication (LCD module at 5 V, Pico at 3.3 V)

11. Breadboard & jumper wires
    – Rapid prototyping and testing

12. External 5 V USB Power Supply
    – Ensures sufficient power for all components

    Schematics

Bill of Materials

Device	Usage	Price
Raspberry Pi Pico 2W	Main microcontroller, handles all components and WiFi	39,66 RON
HB100 Doppler Radar Sensor	Measures object speed via Doppler shift	19,99 RON
LM358 Operational Amplifier	Amplifies low-frequency IF signal from HB100	2,40 RON
OV2640 Camera Module	Captures images when speed exceeds threshold	51,28 RON
LCD 1602 I²C Display	Displays measured speed and configurable speed limit	16,34 RON
microSD Card Module	Stores captured images on microSD card	4,08 RON
microSD Card 16 GB	Storage medium for image files	39,99 RON
Level Shifter 3.3V ↔ 5V (TXS0108E)	Enables safe I²C communication between Pico and 5V devices	3,99 RON
Breadboard 400-point	Rapid prototyping without soldering	4,56 RON
Jumper Wires (Male–Male)	Connects components on breadboard	7,99 RON
Push Buttons 6×6×6 mm	Used for user input (threshold control & manual trigger)	2,16 RON
Red & Green LEDs 5mm	Visual indicators for speed status	2 RON
Buzzer	Audible alert for overspeed detection	1,98 RON
Resistors (10 kΩ, 220 Ω)	Pull-up resistors, current limiting for LEDs and filters	1 RON
Condensator 10 nF ceramic	Filter for Doppler signal smoothing	2,34 RON

Software

Library	Description	Usage
embassy-rp	RP2040 HAL + async executor support	Low-level access to GPIO, I²C, timers and board initialization (init, Input/Output, I2C)
embassy-executor	Async task executor for embedded	#[embassy_executor::main] entry point, task scheduling
embassy-time	Async timers and delays	Timer::after(...) and Delay types for non-blocking waits
embedded-hal	Hardware abstraction traits	Underlying trait layer used by embassy-rp for GPIO/I²C interfaces
defmt-rtt	Real-time logging over RTT	defmt::info! for printing debug/log messages via probe
panic-probe	Panic handler that logs via RTT	Catches panics and reports stack trace over RTT
hd44780-driver	HD44780-based LCD over I²C	Drives the 16×2 character display (lcd.clear, lcd.write_str)

Detailed Design

1. Reference Speed Control

• Libraries used: embassy-rp::gpio, embassy-time
• Responsibilities:
  • Read “+” and “–” buttons with 50 ms debounce (Timer::after)
  • Increment/decrement the ref_speed variable
  • Update the LCD only when ref_speed changes

2. Measurement Control

• Libraries used: embassy-rp::gpio, embassy-executor
• Responsibilities:
  • Toggle measurement on/off with the “Start” button
  • Manage measuring and alerted flags to coordinate measurement cycles and one-time alerts

3. Doppler Sensor Task

• Libraries used: embassy-rp::gpio, embassy-time
• Responsibilities:
  • Sample the HB100 IF pin for GATE_MS = 200 ms, counting falling edges
  • Compute Doppler frequency f_d = pulses * (1000 / GATE_MS)
  • Apply threshold filter (MIN_F_HZ = 80 Hz)
  • Convert to speed in km/h: v = 0.5 * f_d * λ * 3.6

4. Display Manager

• Libraries used: embassy-rp::i2c, hd44780-driver, heapless::String
• Responsibilities:
  • Initialize I²C and the 16×2 HD44780 LCD over address
  • Clear display, position cursor, and write formatted “Ref: X km/h” and “Cur: Y km/h” lines
  • Use heapless::String<16> to build strings without heap allocation

5. Alert System

• Libraries used: embassy-rp::gpio, embassy-time
• Responsibilities:
  • On measuring && current_speed > ref_speed && !alerted:
  • Generate 100 PWM cycles on the buzzer
  • Blink the red LED for 500 ms
  • Clear the current‐speed line on the LCD
  • Stop measuring and set alerted = true

Links

1. PM‑Rust Laboratory Guide – Lab 01 GPIO & Timers
2. PM‑Rust Laboratory Guide – Lab 03 I²C & LCD
3. PM‑Rust Laboratory Guide – Lab 05 SPI & microSD
4. PM‑Rust Laboratory Guide – Lab 06 Wi‑Fi Networking
5. Github