Wiring the FPM10A fingerprint sensor via serial UART, and explaining the enrollment and matching stages for storing up to 127 fingerprints.
Connect a 1.8-inch ST7735 TFT over SPI, with correct 3.3 V logic guidance, breakout-board checks, library selection, and display testing.
Building an SMS-controlled switch for a 12V lamp using a SIM900 shield and a relay, parsing ON, OFF, and STATE commands.
Using a single-channel relay to control a high-voltage device from one Arduino pin, with a very important safety note on high-voltage hazards.