LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.
Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the page on logic gates. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).
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).