 Welcome to Exclusive NOR gates. The Exclusive NOR, or XNOR gate, is a logic device that always has two inputs and one output. The schematic symbol for this device is shown here. The XNOR gate produces a logic one at its output when both inputs are a logic one or when both inputs are logic zero. A logic zero is produced when both inputs are of opposite states. Observe the changes in input and output states of the gate in the truth table and schematic symbol. The circuit can be used to illustrate the operation of the XNOR gate. The lamp, which represents the output, is at a one state on condition only when both switches are in the same zero or one state positions. The output is at a zero state off condition when both switches are in opposite positions. The XNOR gate operation can also be expressed by a Boolean algebra equation. For the two input XNOR gate just described, the equation is shown here. This expression reads X equals not A and not B, or A and B. This means that the output is high when A0 and B0 are both zero or A and B are both one. The XNOR gate can also be represented by the following equation. This diagram shows a heat exchanger tank that is used to warm water as it passes through. If the level becomes too high, the water will overflow. If it becomes too low, the heating element will burn out when it is not covered with water. To alert someone if the water level gets too high or too low, float switches are placed at two locations inside the tank. Whenever a float switch is pushed upward when in contact with water, it will cause the float's lever to break contact with an armature, which causes a logic zero to be applied to the XNOR gate. Whenever the float is not in contact with water, the float lowers and causes the armature to connect to a positive 5 volt contact, which causes a logic one state to be applied to the input of the XNOR gate. The gate output produces a one state and turns the alarm on when positive 5 volts are simultaneously applied to its inputs, or when a 0 volt ground is simultaneously applied to its inputs. Whenever the water level is between the float switches, logic one is applied to the XNOR gate from the high level float and the logic zero is applied to the low level float. Opposite states applied to the XNOR gate will cause its output to be a logic zero and not activate the alarm. You've completed exclusive XNOR gates.