Wiring the digital DS18B20 temperature sensor over the 1-Wire protocol, and why the pull-up resistor matters.
Wiring the BMP180 sensor to measure atmospheric pressure, temperature, and altitude over the I2C protocol with Arduino.
An explanation of how an LED works via electroluminescence, with the method for calculating the current-limiting resistor it needs.
An explanation of how a Zener diode works in reverse breakdown, and its core role in voltage-regulation circuits and component protection.