<< Chapter < Page Chapter >> Page >

Logic gates

There are five main types of logic gate: NOT, AND, OR, NAND and NOR. Each one makes its decision in a different way.

The not gate

Problem: You want an automatic circuit in your office to turn on the heating in the winter. You already have a digital electronic temperature sensor. When the temperature is high, it sends out a 1. When the office is cold, it sends out a 0. If this signal were sent straight to the heater, the heater would turn on (1) when it was already hot, and would stay off when it was cold. This is wrong! To make the heater work, we need a circuit which will change a 0 (from the sensor) into a 1 (to send to the heater). This will make the heater come on when it is cold. You also want it to change a 1 (from the sensor) into a 0 (to send to the heater). This will turn the heater off when the room is hot. This circuit is called an inverter or NOT gate . It changes 0 into 1 (1 is NOT 0). It changes 1 into 0 (0 is NOT 1). It changes a signal into what it is NOT.

The symbol for the NOT gate is:

The action of the NOT gate can be written in a table called a truth table . The left column shows the possible inputs on different rows. The right column shows what the output (decision) of the circuit will be for that input. The truth table for the NOT gate is shown below.

Input Output
0 1
1 0

When you read the truth table, the top row says, “If the input is 0, the output will be 1.” For our heater, this means, “If the room is cold, the heater will turn on.” The bottom row says, “If the input is 1, the output will be 0.” For our heater, this means, “If the room is hot, the heater will switch off.”

The and gate

Problem: An airliner has two toilets. Passengers get annoyed if they get up from their seat only to find that both toilets are being used and they have to go back to their seat and wait. You want to fit an automatic circuit to light up a display if both toilets are in use. Then passengers know that if the light is off, there will be a free toilet for them to use. There is a sensor in each toilet. It gives out a 0 of the toilet is free, and a 1 if it is in use. You want to send a 1 to the display unit if both sensors are sending 1s. To do this, you use an AND gate.

The symbol for the AND gate is:

Symbol for the AND logic gate.

The truth table for the AND gate is shown below. An AND gate has two inputs (the NOT gate only had one). This means we need four rows in the truth table, one for each possible set of inputs. The first row, for example, tells us what the AND gate will do if both inputs are 0. In our airliner, this means that both toilets are free. The right column has a 0 showing that the output will be 0, so the display will not light up. The second row has inputs of 0 and 1 (the first toilet is free, the other is in use). Again the output is 0. The third row tells us what will happen if the inputs are 1 and 0 (the first toilet is in use, and the second is free). Finally, the last line tells us what will happen if both inputs are 1 (the first toilet is in use and the second toilet is in use). It is only in this case that the output is 1 and the display lights up.

Questions & Answers

an object will remain at rest or move at a constant velocity unless acted upon a net force
Lebogang Reply
thank you
law of inertia
an object resisting the change in velocity.
What is a molecule
Thabiso Reply
a molecule is a simplest structure unite of an elements
thank you
Plz remind me the 1st Newton's law
Define the term functinal group of an organic compound
Kamvelihle Reply
a single atom or a group of atoms which is responsible for the property and function of an organic compound
What's the relationship between intensity and the current?
Lufuno Reply
yu are Spi.ke spanish hola ele
cómo se llama el video donde disquete salgo yo
The intensity doesn't effect the current.
what does the word emitted mean?
Mwinga Reply
to be ejected or released
Ok thanks
what are the hooke laws
Tyriek Reply
what's that
what do really asked in exam
Leiyo Reply
hi please help me how to balance redox reactions?
Brian Reply
Which equation do u wanna balance
i tried to write it but my phone just can't write it Cr2 O72-(aq) + H2S (aq) Cr3 (aq) + S(s)
show is the equation
hello. what is the difference between a primary, secondary and tertiary alcohol
Thokozani Reply
primary the C=bonded to 1 Carbon atom.... secondary=bonded to 2 carbon atoms tertiary=bonded to 3 carbon atoms
I hope you're answered
thanks Christina
My pleasure
Thank you Christina. This is very helpful 😀👍👍💯
Anytime Thokozani
How to calculate pH?
thank you guys i didn't know about the primary, secondary and tertiary alcohols
thank you Christina
Hey guys which topic does AC nd DC generator falls
thank you
can somebody help me with functional isomers
Baningi Reply
function isomer has the same molecular formula but different functional group
what's momentum
Luvelo Reply
the product of an objects mass times velocity. it is mainly prevalent in collisions.
in order to produce an interference pattern, the waves used must be what?
Methane contains C and H. This compound is
Juan Reply
ketones's functional group
Moloi Reply
Why does the carboxyl group have acidc properties?
Evi Reply
carboxylic acids
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
what is different between Dependent variable and independent variables
difference between A.c and D.c
AC has slip rings DC has split rings commutator
to avoid rust and to improve strength and conductivity
the variable whose value doesn't depend on other is known as independent variable ex time

Get the best Siyavula textbooks: gr... course in your pocket!

Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 12 physical science' conversation and receive update notifications?