NOT Gates

The NOT gate has a single input and one output.

The little bubble on the output indicates that the output goes LOW when the input goes HIGH.

We can say that the output goes LOW when the input is ACTIVATED.

The opposite happens when the input is LOW. The output goes HIGH.

The TRUTH TABLE shows that the output is the opposite of the input.

The NOT gate is also called an INVERTER. It inverts the input.
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The Boolean expression is A = Z

Which is read as, NOT A EQUALS Z

or IF A IS LOW THEN Z IS HIGH

or BAR A = Z

Exclusive OR (EXOR)

Before reading this page read the one on LOGIC GATES.

If you look at the truth table for the OR gate not only does the output go high when A OR B are high but it goes high when A AND B together are high.
This can cause problems in some applications.

Therefore the EXCLUSIVE OR or EXOR is used.

The output of the EXOR only goes high when A or B on its own goes high.
That is when A or B exclusively goes high.

When A and B are both high the output stays low.

This is shown in the truth table.

OR Gates

Before reading this page read the one on LOGIC GATES.

The OR gate has two or more inputs and one output.

The output voltage goes high only when one or more input voltages are high.

In the switch diagram the lamp lights up when A OR B (or both) are operated.

In the truth table Z = 1 when A or B = 1.

The Boolean expression is A+B = Z which translated says, A or B high makes Z high.
The plus sign + translates as OR.

NOR Gates

Before reading this page read the one on LOGIC GATES.

The NOR gate has two or more inputs and one output.

The output voltage goes low only when one or more input voltages are high.

In the switch diagram the lamp goes out when A OR B (or both) are operated. (A short circuit is placed across the lamp)

In the truth table Z = 0 when A or B = 1
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The Boolean expression is A+B = Z which translated says, A or B high makes Z low.
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The plus sign, + translates as OR. Z (called Z bar) means Z is low.

NAND Gates

Before reading this page read the one on LOGIC GATES.

The Nand gate has two or more inputs and one output.

The output voltage goes low only when all input voltages are high.

In the switch diagram the lamp goes out when A and B are operated. (A short circuit is placed across the lamp)

In the truth table Z = 0 when A and B = 1
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The Boolean expression is A.B = Z which translated says, A and B high makes Z low.
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Z (called Z bar) means Z is low.

US symbol

UK symbol

switches simulating NAND

truth table

AND Gates

Before reading this page read the one on LOGIC GATES.

The AND gate has two or more inputs and one output.

The output voltage goes high only when all input voltages are high.

In the switch diagram the lamp lights up only when A and B are operated. If only one is switched then the lamp stays off.

In the truth table Z = 1 only when A and B = 1

The Boolean expression is A. B = Z which translated says, A and B both high, makes Z high.

Multivibrators

Multivibrators (sometimes called FLIP-FLOPS) have two "cross coupled" transistors, say TR1 and TR2. This causes TR1 to be on (conducting) and TR2 off. They can be made to reverse states, with TR1 turning off and TR2 turning on. The states of the transistors can be indicated using a lamp for each transistor.

THE ASTABLE is continually changing state. The lights flash alternately on and off. This is because it has two UNSTABLE states and is changing automatically from one to the other. The duration of these states is determined by two RESISTOR/CAPACITOR TIME CONSTANTS. Changing the values of the time constants alters the flashing rate.

THE MONOSTABLE has one unstable and one stable state. It rests in the stable state, waiting for an external pulse. When a pulse arrives it changes to the unstable state. It stays in the unstable state for a time determined by an RC time constant. It then falls back into the stable state and waits for another pulse. This means that the lights wait in the stable state, one on, the other off. When the pulse arrives they change state for a while and then fall back to the waiting state.

THE BISTABLE has two stable states. It waits in one state for an external pulse. When the pulse arrives it changes into the other stable state waiting for another pulse. If another pulse never arrives it will wait forever. The lights change over on the arrival of each pulse.

Logic Gates

Logic gates usually come packaged as integrated circuits which have type numbers such as 7400 or 4001.

They belong to semiconductor families such as TTL (transistor, transistor logic) or CMOS (complementary metal oxide semiconductor). The names describe their internal construction.

They are DIGITAL devices not ANALOGUE.

A thermometer is an analogue device because it can record an infinite number of values such 100 degrees, 0.1 degrees or 34.354 degrees etc.
Other analogue devices are a car speedo and a Hi Fi amplifier (which can handle lots of different frequencies and loudnesses.)

A digital device or system uses only two values. These can be expressed in several ways.

high or low
true or false
white or black
on or off
1 or 0

etc.

A light switch and a rat trap are digital devices.

Most gates usually have two or more inputs and one output.

The state of the output (high or low) depends upon the combination of the input states.

In the case of the gate shown, the output will only be high if both inputs are high. If either one input or both inputs are low then the output will be low

These characteristics can be shown using a TRUTH TABLE. In the following example 1 indicates a high and 0 indicates a low.
Note that Z is only a 1 when A AND B are both at 1.

There is a form of mathematics associated with logic gates called BOOLEAN ALGEBRA.
It was invented a few hundred years ago by Mr Boole, before the days of electronics. He used it to solve problems in logic.

For example
Some cats are black AND black items cannot be seen against a black wall.Therefore it is TRUE that some cats cannot be seen against a black wall.

Here is a Boolean expression for the gate shown. A . B = Z
Read this as IF A AND B ARE HIGH THEN Z IS HIGH. (The . is read as AND).

The most frequently used gates are AND, OR, NAND, NOR, NOT and EXOR.

An integrated circuit containing 4 AND gates each with 2 inputs is called a QUAD 2 INPUT AND ic. An ic with 6 NOT gates is called a HEX INVERTER ic.