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.
A NAND gate can have two or more inputs. The o on the output means not showing that it is a Not AND gate. The output of a NAND gate is true unless all its inputs are true.
A variable capacitor is used in a radio tuner.
The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.