Earphone A transducer which converts electrical energy to sound.
An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
Microphone A transducer which converts sound to electrical energy.
Preset variable resistor A preset is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than standard variable resistors so they are sometimes used in projects to reduce the cost.