Gate circuits: AND gate, OR gate, NOT gate circuits and examples

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Gate circuits: AND gate, OR gate, NOT gate circuits and examples [Copy link]

The gate circuit is the most basic logic unit in the digital circuit. It can create a certain logical relationship between the output signal and the input signal. In digital circuits, signals are mostly represented by two states of potential (level), high and low. The logical relationship of the gate circuit can be used to realize the conversion of signals.

The most basic gate circuits include AND gate, OR gate, NOT gate, etc.

AND Gate Circuit

An AND gate circuit means that something will only happen when all the conditions for it are met.

The basic structure and logic symbol of the AND gate circuit are shown in the figure below:

In the functional diagram of the AND gate circuit, the light will only turn on when both switches A and B are closed. If either A or B is in an open state, the light will not turn on.

The truth table of the AND gate circuit is shown below:

The AND gate circuit composed of a diode and a resistor is shown in the figure below:

In the figure, A and B are two input variables, and F is the output variable. When A and B are both high, F is high. As long as one of A and B is low, F is low.

OR Gate Circuit

An OR gate circuit means that something will happen as long as one or more conditions are met.

The basic structure and logic symbol of the OR gate circuit are shown in the figure below:

In the above picture, as long as one of the switches A and B is closed, the current can pass through the switch into the lamp and the lamp will light up. Only when both switches are open will the lamp not light up.

The truth table of the OR gate circuit is shown below:

Like the AND gate circuit, the simplest OR gate circuit is also composed of diodes and resistors.

See below:

In the figure, A and B are two input variables, and F is the output variable. When A and B are both low, F is low. When one of A and B is high, or both are high, F is high.

NOT Gate Circuit

The NOT gate circuit is also called the "negation" operation, also known as the "inversion" operation, so the NOT gate circuit is also called an inverter.

The basic structure and logic symbol of the NOT gate circuit are shown in the figure below:

In the NOT gate circuit, when switch A is closed, the circuit is short-circuited and lamp F is off; if the switch is open, the lamp is on.

The truth table of the NOT gate circuit is shown in the figure below.

The most basic NOT gate circuit is constructed using the switching characteristics of a transistor, which can achieve non-logical relationships.

The NOT gate circuit composed of a crystal triode and peripheral components is as follows:

In the figure above, A is the input variable and Y is the output variable. By using the inverting amplification characteristics of the transistor, when A is low, the transistor is cut off and the output terminal Y is high. When the input is high, the transistor is in the saturation region and the output terminal Y is low.

The difference between national standards and foreign standards in gate circuit symbols is shown in the figure below:

The NAND gate and NOR gate are composed of AND gate + NOT gate; OR gate + NOT gate respectively, and are also very common in digital circuits.

Gate Circuit Application Examples

Siren signal generator circuit

Siren signal generator circuits are generally used in alarm circuits, and they produce warning sounds by exchanging high and low tones.

The siren signal generator circuit composed of six NOT gate circuits is shown in the figure below:

Non-gates 1 and 2 form a super low frequency pulse oscillator, Non-gates 3 and 4 form a high frequency oscillator, and Non-gates 5 and 6 form a low frequency oscillator. The output of the super low frequency pulse oscillator controls the high and low frequency oscillators to sound in turn through diodes VD1 and VD2. The oscillation signal is amplified by semiconductor transistor VT1 through VD3 and VD4 respectively, and then drives the speaker to sound.

Touch key control circuit

The touch key control circuit is a kind of circuit that uses the human hand to touch the metal touch key to control the on and off of the circuit.

See below:

The circuit is mainly composed of a touch metal plate M, NAND gate circuits 1-4, a relay K, a battery and the like.

The touch metal plate is used to input commands, the four NAND gate circuits are used to identify and process the input commands, and the relay is the command output control end load.

When the hand touches the metal plate, the charge on C1 will be added to the input of NAND gate 2 through the human body resistance, making it a high level, and finally making NAND gates 3 and 4 output high level, VT1 and VT2 turn on, the relay is energized, and the load is controlled to work.

Since NAND gate 1 and NAND gate 2 are connected through resistor R2, the high level provided by C1 to the input of NAND gate 2 will be maintained. Even if the hand leaves the metal plate, the circuit will maintain this state until the metal plate is touched again.

When the relay is energized, the input of NAND gate 1 is at a low level, and C1 is discharged to about 0V through R1 and NAND gate 1.

When the metal plate is touched again, the 0V voltage on C1 is added to the input end of NAND gate 2 through the human body resistance, so that the circuit returns to its original state, that is, VT1 and VT2 are cut off and the relay is disconnected.

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