Electronics Logic Gates: AND Gates

A two-input AND gate is an electronic logic gate that has two inputs and one output. The output is HIGH only if both of the inputs are HIGH. Any other combination of inputs results in the output’s being LOW.

Input A Input B Output
0 0 0
0 1 0
1 0 0
1 1 1

The standard symbol for an AND gate shows the inputs on the left and the output on the right.

image0.jpg

Notice that you can create AND gates with more than two inputs. For each additional input that you add to a gate, the number of possible input combinations doubles. A two-input gate has four possible input combinations; for a three-input gate, there are eight possible combinations; a four-input gate has 16 possible input combinations, and so on.

In a three-input AND gate, the output is HIGH only if all the inputs are HIGH. Any other combination of inputs produces LOW output.

Input A Input B Input C Output
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1

You can combine gates in a circuit to create logic networks that are more complicated than a single gate could produce. You can create a three-input AND gate by using two two-input AND gates, for example.

The first AND gate produces a HIGH output only if the inputs A and B are true. Then the output of the first AND gate is used as one of the inputs to the second AND gate; the other input is the input C.

image1.jpg

Because the output of the second gate will be HIGH only if both of its inputs are HIGH, and because the first input to the second gate is the output from the first gate, which is HIGH only if both of its inputs are HIGH, the output of the entire circuit (designated as X) is HIGH only if all three inputs (A, B, and C) are HIGH.

AND gates can be used in a home alarm system. Here, the inputs for the various sensors placed on the home’s doors and windows are processed by the sensor circuit, which sends a HIGH signal to one of the inputs of the AND gate if any of the sensors indicates an intrusion anywhere in the house.

Then the arming circuit sends a 1 to the other input of the AND gate if the system is armed. Finally, the alarm circuit sounds an audible alarm if the AND gate’s output is 1. As a result, the alarm will sound if an intrusion is detected and the system is armed.

image2.jpg

When used in this way, an AND gate is often called an enable input, as one of the inputs to the AND gate enables the other input to be processed. When the enable input is HIGH, the controlled input is allowed to pass through the AND gate. When the enable is LOW, the controlled input is inhibited.

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