# OR Gate

Discussion in 'The Projects Forum' started by 009Legends, Dec 8, 2013.

1. ### 009Legends Thread Starter New Member

Dec 8, 2013
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can you guys give me more example of OR gate application aside from Doorbell and 2 floors stairway light and pls include the diagram. Thanks

2. ### MrChips Moderator

Oct 2, 2009
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Can you give me an example of where one would use 1+1 = 2?

3. ### 009Legends Thread Starter New Member

Dec 8, 2013
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i dont get your question. theres no 1+1 = 2 in logic algebra. I'm just asking for more example, coz the 2 example i gave are common xD

4. ### MrChips Moderator

Oct 2, 2009
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OR and AND are boolean operators just as + and x are math operators.
They are used everywhere in digital systems and you don't realized they are being used, in the same way + and x are used everywhere numbers are manipulated.

5. ### 009Legends Thread Starter New Member

Dec 8, 2013
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Ok mrchips, can you give me some example of application of OR gate(7432). simple application will do

6. ### MrChips Moderator

Oct 2, 2009
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OMG, you don't get it.

Dec 8, 2013
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8. ### MrChips Moderator

Oct 2, 2009
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Ok, if you insist.

009Legends likes this.

Dec 8, 2013
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10. ### BillB3857 AAC Fanatic!

Feb 28, 2009
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Obviously the OP is just starting out and trying to learn something. The diagram linked in post #9 shows two switches on the left side.
These switches are shown in a position that supplies a ground to the input of the OR gate as well as supplying a current path to ground for the two LEDs that share the OR gate inputs.

The resistors shown in series with the LEDs are to limit the current so the LEDs are not destroyed.
They also provide what is called a PULL-UP function.

As drawn, the LED tied to the output of the OR gate will be ON when the switches are in the state shown since both inputs of the OR gate are at a logic 0,or ground, allowing current through the LED-resistor combination.

When either switch is changed from its current position, the resistor-LED combination associated with that particular switch will allow the voltage on the input of the OR gate to be PULLED UP to a logic 1 state.

The OR gate function will then allow the output of the OR gate to change to a logic 1 state and since the voltage on each end of the LED on the OR gate output will be the same, it will be turned off.

As for the use of an OR gate in digital logic, one is used any time more than one input needs to provide a single output without having any effect on other inputs. It provides isolation between inputs.

I hope the OP can follow this explanation. It is wise to remember that at one time this was all very strange for most of us.

11. ### WBahn Moderator

Mar 31, 2012
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That, or the OP has an assignment due soon and needs someone to answer one of the questions for him. Especially since he insists that we provide a diagram instead of just an idea of where an OR gate would be useful.

Interesting examples he gave, though. A doorbell? Only if there are multiple doors ringing the same bell, which is a bit unusual (how do you know which door to go open?) A 2-story stairway light using an OR gate? That's an example of an XOR gate.

12. ### 009Legends Thread Starter New Member

Dec 8, 2013
9
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Bill, pls check mail

13. ### 009Legends Thread Starter New Member

Dec 8, 2013
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wbahn, yah,my mistake ) 2 story is xor

Dec 8, 2013
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Idea will do

15. ### WBahn Moderator

Mar 31, 2012
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So think of everyday situations in which you want A to happen if either B or C happens. Notice the use of the word "or" in that description? It's called and OR gate for a reason!

I can think of several just as I go from home to work. I open the back door of my car to put some stuff in there and the dome light comes on. I finish and then open the front door to get in and the dome light comes on. If I happen to have both doors open at the same time, the dome light comes on. What is the relationship between the doors and the dome light? Most cars these days have electric windows and door locks, right? How many places can you operate the locks and windows from? What kind of relationship are those.

009Legends likes this.
16. ### 009Legends Thread Starter New Member

Dec 8, 2013
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WBahn can you give more ideas pls

17. ### tshuck Well-Known Member

Oct 18, 2012
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009Legends, perhaps you need to sit and think about this for a while. You are asking for times an OR gate would be useful when most circuits out there will utilize multiple OR functions.

Do you understand the truth table for the OR gate? Do you understand when you might use boolean algebra, or logic, to perform an action?

If any if these are true, you don't need us to tell you applications - you can think of them yourself by examining the output of a function.

18. ### MrChips Moderator

Oct 2, 2009
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4,195
Suppose you want to implement a function such as:

F = A.B + C.D + A'.B.C'.D'

where A, B, C and D are boolean variables.

Where you see a +, this is an OR gate.

Where you see a ., this is an AND gate.

The number of applications for OR and AND gates is infinite.

19. ### WBahn Moderator

Mar 31, 2012
20,064
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No. I already gave you three. Perhaps after YOU put in some effort and sketch up a circuit for all three I will give you a couple more. But at some point YOU have start thinking for yourself.

20. ### Papabravo Expert

Feb 24, 2006
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They were used all the time in the days of early microprocessors to detect that a certain range of addresses was on the address bus (active low out of a decoder eg 74HCT138) AND that the processor was performing a READ or a WRITE cycle. RD* and WR* were active low strobes.

The logic you want is "a LOW AND a LOW is a LOW" which deMorganized is performed by -- ta da -- an OR gate eg 74HCT32.