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.
Light Emitting Diode A transducer which converts electrical energy to light. Usually abbreviated to LED.
Diode A device which allows current to flow in only one direction.
A potentiometer has 3 contacts and is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.