DIGITAL ELECTRONIC

DIGITAL ELECTRONIC

AND GATE

The AND gate is a digital logic gate that implements logical conjunction - it behaves according to the truth table to the right. A HIGH output (1) results only if both the inputs to the AND gate are HIGH (1). If neither or only one input to the AND gate is HIGH, a LOW output results. In another sense, the function of AND effectively finds the minimum between two binary digits, just as the OR function finds the maximum.

There are three symbols for AND gates: the American (ANSI or 'military') symbol and the IEC ('European' or 'rectangular') symbol, as well as the deprecated DIN symbol. For more information see Logic Gate Symbols.

MIL/ANSI Symbol	IEC Symbol	DIN Symbol

The AND gate with inputs A and B and output C implements the logical expression .

Circuits

The 4081 integrated circuit provides four 2-input AND gates which can be used independently. Like all 4000 series devices, the 4081 works with power supply voltages in the range 3-16 V. In your final design, any unusedinputs should be connected either to 0 V or to the +V power supply. (You don't need to do this for circuits on prototype board.) Unused outputs are left unconnected.

In the 74LS series, the 74LS08 provides four 2-input AND gates. As you can see, the pin connections are different!

74LS series devices usually require a regulated 5 V power supply. The LM7805 and LM78L05 are suitable power supply regulator devices.

The 74HC08 is a pin-compatible replacement for the 74LS08, with improved electrical properties and a wider range of power supply voltage.

Although 74HC series devices are designed to work with a 5 V power supply, voltages between 3 V and 6 V are allowed and most circuits work well using 4.5 V (3 x 1.5 V cells). Don't use a 9 V power supply!

Details about these integrated circuits are given elsewhere in the Beastie Zone. Click the diagrams to follow these links.

There are other ways of making AND gates. In the diagram below, an industrial machine is represented by a motor:

The machine won't start unless the safety guard is closed.

If a single AND gate is needed, think about using this diode/transistor circuit:

It's not immediately obvious how this arrangement works as an AND gate. Run the simulation by clicking on the diagram and follow through the truth table to confirm that the output goes HIGH only when both inputs are HIGH.

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