Understand the difference between a bare photoresistor and an LDR module, build the divider, read A0, and calibrate a stable switching threshold.
Trigger the HC-SR04, measure echo time, and calculate distance with a timeout, while accounting for target angle, surface and air temperature.
Compare Uno, Nano, Mega, Leonardo, Pro Mini and LilyPad by MCU, I/O count, memory, voltage, size and programming requirements.
Build local phone control with an HC-05 and isolated relay, with correct 3.3V signal handling, safe startup logic, and mains precautions.