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.
Galvanometer A very sensitive meter used to measure tiny currents, usually 1mA or less.
Zener diode A zener diode can be used to maintain a fixed voltage.
An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).