A comparison of UART, I2C, and SPI protocols by wire count, speed, and number of supported devices, with a guide to picking the right one for your project.
Use a ready-made phone voice-recognition app with an HC-06 Bluetooth module to turn LEDs on and off with simple voice commands.
Build an RGB lamp that automatically changes color based on ambient light using three LDR light sensors and the map() and analogWrite() functions.
Wire four servo motors to an external power supply and control them together in full synchronization via the Servo library, with a safety note on power supply.