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.
Earphone A transducer which converts electrical energy to sound.
Zener diode A zener diode can be used to maintain a fixed voltage.
Diode A device which allows current to flow in only one direction.