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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

hi please help me how to balance redox reactions?
Brian Reply
Which equation do u wanna balance
Rifumo
i tried to write it but my phone just can't write it Cr2 O72-(aq) + H2S (aq) Cr3 (aq) + S(s)
Brian
show is the equation
Pride
us
Pride
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
Christina
I hope you're answered
Christina
thanks Christina
Baningi
My pleasure
Christina
Thank you Christina. This is very helpful 😀👍👍💯
Thokozani
💯💯
Thokozani
Anytime Thokozani
Christina
How to calculate pH?
Thabo
thank you guys i didn't know about the primary, secondary and tertiary alcohols
Brian
thank you Christina
Brian
Hey guys which topic does AC nd DC generator falls
Phumelele
electrodynamics
Christina
thank you
Phumelele
can somebody help me with functional isomers
Baningi Reply
function isomer has the same molecular formula but different functional group
Thokozani
what's momentum
Luvelo Reply
the product of an objects mass times velocity. it is mainly prevalent in collisions.
tyrique
momentum
Sesethu
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
tyrique
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
Reunity
thanks
Elizabeth
what is different between Dependent variable and independent variables
Elizabeth
difference between A.c and D.c
Gerrard
AC has slip rings DC has split rings commutator
Mohludi
to avoid rust and to improve strength and conductivity
Shubham
the variable whose value doesn't depend on other is known as independent variable ex time
Shubham
are UV rays dangerous?
Khathutshelo Reply
no
Lianda
what's this?
Lianda
if there is extensive exposure, it can lead to skin cancer
tyrique
yes depend upon their intensity
Shubham
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
I have a problem with calculation in Newton's law of motion
Vuyokazi
me also VuyokAzi
Amahle
what's the question
Christina
Which area in particular?
Kevo
just get the hang if free body diagrams,know the directions.
Muregu
I know how to draw free diagram I have a problem with calculation in second law
Vuyokazi
what is the wave model of atom ?
Trazy Reply
guys what is momentum
Luvelo
@Luvelo... Momentum is the product of an object's mass and its velocity.
Christina
the model based on wave nature of electron
Shubham
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
Nombuyiselo
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Cassiey
the change in frequency of the sound detected by a listener because sound source and listener have different velocity
Elizabeth
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
Tumelo
yes that's true
Mphoko
Always know Newton's second law of motion. It appears to be in every question paper
Botho
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
Courtney
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
Sphesihle
how many bones are in the human body?
Asali
it's the rate at which the reactants are able to change to products
Courtney
206 bones
Elizabeth

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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