Palm-Sized RF Activity Indicator (USB‑C)

A compact, palm-sized, receive‑only RF activity indicator designed for road‑safety awareness. The device passively measures nearby radio‑frequency activity and displays relative signal strength on a 10‑segment LED bar. It does not decode, record, or transmit any signals.

⚠️ Important: This project is strictly receive‑only and intended for general RF activity awareness. Always comply with local laws and regulations. Do not use this device to interfere with, evade, or obstruct emergency services.

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📌 Project Overview

• Purpose: Visual indication of nearby RF activity using signal strength as a proximity proxy.
• Form factor: Fits in the palm of your hand.
• Power: USB‑C (5 V) — works with a power bank, car USB port, or wall charger.
• Display: 10‑segment LED bar driven by an LM3914.
• Antenna: Internal (no external mounting required).
• Processing: ESP32 (optional but recommended for calibration and future upgrades).

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🧠 How It Works (High Level)

Internal Antenna
      ↓
RF Detector Module
( RF → Analog Voltage )
      ↓
      ├──→ LM3914 → LED Bar (0–10 segments)
      └──→ ESP32 ADC (optional monitoring & calibration)

USB‑C 5V → ESP32 → RF Detector + LM3914

• Nearby RF energy is picked up by a small internal antenna.
• The RF detector converts RF energy into a proportional DC voltage.
• The LM3914 maps that voltage to a 10‑level LED bar.
• Higher RF power → more LEDs → closer/stronger activity.

This device does not show distance in meters. Signal strength is used as a relative range indicator.

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🧰 Bill of Materials (BOM)

Core Components

• ESP32 development board (USB‑C recommended)
• RF detector module with analog voltage output (e.g. 1–600 MHz or wideband)
• LM3914 bar‑graph driver (chip or module)
• 10‑segment LED bar graph
• Internal antenna element (short whip, helical, or PCB trace)

Power & Passives

• USB‑C power (via ESP32 or USB‑C breakout)
• 0.1 µF ceramic capacitors (decoupling)
• Resistors / trim‑pot for LM3914 reference and LED current

Mechanical

• Plastic enclosure (~90 × 55 × 25 mm)
• Perfboard / standoffs / foam tape
• Hook‑up wire

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🔌 Wiring Summary

• Power:

  • USB‑C → ESP32
  • ESP32 5V/VIN → RF detector Vcc + LM3914 V+
  • All grounds common

• Signal:

  • Internal antenna → RF detector RF‑IN
  • RF detector OUT → LM3914 SIG‑IN
  • RF detector OUT → ESP32 ADC (optional; add divider if >3.3 V)

• Display:

  • LM3914 OUT1…OUT10 → LED bar segments

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🧪 Calibration & Use

• Adjust LM3914 full‑scale so:

  • Background RF = 0–1 LEDs
  • Strong nearby activity = full bar

• Optional ESP32 firmware prints raw and smoothed ADC values for tuning.

• Treat the LED bar as:

  • Low = far / background
  • Mid = nearby
  • Full = very close

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🛠 Assembly Notes

• Use a plastic enclosure (metal will block RF).
• Keep RF detector and antenna away from ESP32 and USB wiring.
• Keep RF wiring short.
• Add decoupling capacitors close to modules.

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🚀 Future Upgrades

• Buzzer alert above a set threshold
• Bluetooth output to phone
• Small OLED numeric display
• Calibration button
• Logging / statistics

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📄 Documentation

A full step‑by‑step build guide is available here:

palm_sized_rf_activity_indicator_build_guide.pdf

This README is intended as the GitHub landing page; the PDF contains detailed assembly, wiring, calibration, and testing instructions.

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📜 License & Disclaimer

This project is provided for educational and safety‑awareness purposes only. The author assumes no responsibility for misuse. Use responsibly and legally.

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Enjoy building — and keep it safe, small, and clean.