Logic Gates and Circuits

Each gate has a standard circuit symbol. In the exam these are drawn quickly, so it helps to be able to recognise them by their outline.

• AND: a flat-back D-shape. Inputs enter the flat side, the rounded tip is the output.
• OR: a curved-back D with a slightly pointed tip. The back curves outward.
• NOT: a small triangle pointing right, with a circle (the "NOT bubble") on the tip.
• NAND: an AND symbol with a NOT bubble added to its output.
• NOR: an OR symbol with a NOT bubble added to its output.
• XOR: an OR symbol with an extra curved line behind the back, forming a doubled curve.

The NOT bubble (a small circle on a gate's output) always means "invert what comes out". That is how NAND, NOR and many circuit fragments mark a negation.