NOR Gate:

Figure shows a further example of a 2 input digital gate, again consisting of two NPN transistors, TR1 and TR2, in a different configuration. When input A and input B are both at the 0 state (0v), both transistors are biased OFF and the output will adopt the 1 state (+ 5v). If input A only is given the 1 state, transistor TR1 will be biased ON and current will flow, making the output take up the 0 state. Similarly, if input B only is given the 1 state, transistor TR2 will be biased ON, taking the output to the 0 state. Finally if both input A and input B are at the 1 state together, the output will again adopt the 0 state.