Unit CardKB2 is a card-sized 42-key portable keyboard input unit. Compact and lightweight, it is designed for everyday carry and embedded integration. Powered by the ESP32-C61HF4, it supports 2.4 GHz Wi-Fi 6. The preloaded firmware supports four communication modes — I2C, UART, BLE HID, and ESP-NOW — for flexible connectivity with host devices. An onboard HY2.0-4P connector enables communication in I2C/UART mode, while a USB Type-C port provides power supply and firmware flashing. Additional features include an RGB LED status indicator, a Reset button, a Boot button, and built-in input overvoltage protection, making it well-suited for wireless input, interactive control, and a wide range of portable applications.