ESP32 Key Fob

GitHub: Carputer-project/esp32_keyfob

2-button ESP-NOW key fob for carputer remote keyless entry. ~3-year battery life on CR2032.

Hardware

Component	Cost
ESP32 DevKit (or ESP32-C3 for lower power)	~$3-5
2× momentary push buttons	~$1
CR2032 battery holder	~$1
Total	~$5-7

Buttons

Button	GPIO	Function
BTN1	GPIO 0	Lock / Unlock (toggle)
BTN2	GPIO 2	Remote start (hold 2s)

Protocol

Uses ESP-NOW (no WiFi router needed) — direct peer-to-peer communication with the body controller.

Action	Button	Payload
Lock/Unlock	BTN1 (press)	{"cmd":"lock","hmac":"..."}
Remote Start	BTN2 (hold 2s)	{"cmd":"rstart","hmac":"..."}

Authentication

HMAC-SHA256 challenge-response:

• Key derived from VIN + module secret (set at bind time)
• Nonce included to prevent replay attacks
• Body controller validates HMAC before executing commands

Build & Flash

cd esp32_keyfob
pio run              # Build
pio run --target upload  # Flash

Configuration

Edit platformio.ini to set:

• WIFI_CHANNEL — ESP-NOW channel (must match body controller)
• Button GPIO pins in build_flags

Dependencies

• ESP32 Arduino core (built-in ESP-NOW support)
• ArduinoJson — JSON serialization (v7)

Battery Life

Deep sleep between button presses. Wake on GPIO interrupt. Estimated ~3 years on CR2032 with typical use (10 presses/day).