Electronics Logic Gates: NOR Gates
A NOR gate is a combination of the electronics logic gates OR and NOT. As with NAND, the name NOR is a contraction of NOT and OR. The output of a NOR gate is LOW when any of its input are HIGH. Otherwise, the output of the NAND gate is HIGH.
|Input A||Input B||Output|
The standard symbol for a NOR gate is the same as the symbol for an OR gate, with a negation circle added to the output. The circle simply indicates that the output is inverted. A NOR gate is essentially a combination of an OR gate and a NOT gate
Like the NAND gate, the NOR gate is a universal gate. Any logic network that consists of NOT, AND, and OR gates can be recreated with just NOR gates.
Just as a NAND gate is like an OR gate for LOW inputs, a NOR gate is like an AND gate for LOW inputs. In other words, the output of a NAND gate is HIGH when both of the inputs are LOW.
Some versions of alarm circuits use a NOR gate instead of an AND gate to feed the audible alarm circuit. In this circuit, the alarm sounds when the output from the sensor circuit is LOW and the output from the timer circuit is also LOW.
A NOT gate isn’t needed for the 555 timer output, but now a NOT gate is required on the output from the AND gate to invert its signal, so that it emits LOW when the system is armed and the alarm is tripped.