Loudspeaker A transducer which converts electrical energy to sound.
Diode A device which allows current to flow in only one direction.
The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.
Relay An electrically operated switch, for example a 9V battery circuit connected to the coil can switch an AC mains circuit. The rectangle represents the coil. NO = Normally Open, COM = Common, NC = Normally Closed.