A NOT gate can only have one input. The o on the output means not. The output of a NOT gate is the inverse (opposite) of its input, so the output is true when the input is false. A NOT gate is also called an inverter.
Zener diode A zener diode can be used to maintain a fixed voltage.
Voltmeter Measures voltage. The proper name for voltage is potential difference but voltage is more widely used.
Earphone A transducer which converts electrical energy to sound.