Exam study help?

Discussion in 'Homework Help' started by pfelectronicstech, Jul 26, 2012.

Jan 18, 2012
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Ok guys I'm back again with the need for some study help. I'm having trouble with two questions about 555 one-shot multivibrators. I have found the paragraphs that deal with 55 one-shot multivibrators but am still having trouble with these questions. I'll start with one.

What changes in resistance and capacitance will produce the shortest pulse width in a 555 one-shot MV?

1-increase both resistance and capacitance
2-decrease both resistance and capacitance
3-decrease resistance and increase capacitance
4-increase resistance and decrease capacitance

I believe its #1 increase both resistance and capacitance but I have been reading these pages over and over again all afternoon so my brain is fried. Thanks for the help.

2. justtrying Active Member

Mar 9, 2011
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Jan 18, 2012
178
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Great page I just read the whole thing. Interesting subject to me. In my book it says you can lengthen or short the duration of a pulse by adjusting the values of R and C. ah ha I think I'm getting to the answer. What a dummy, but any other help, pointing in the right direction would help? Thanks again for the help. As much as this stuff confuses me at times, I love it, I like struggling and then finding the answer after sifting through info for a few hours.

4. justtrying Active Member

Mar 9, 2011
329
396
have you looked at a site that derives the formula? That helped me a lot when I struggled with 555 circuits... Also, I do find TI application for 555 quite useful (http://www.ti.com/product/lm555)

going back to the question, since it is a multiple choice, they are looking for the answer to their question - shortest pulse, so there are clearly two choices which will result in a shorter pulse with only one choice giving the shortest possible pulse assuming right numbers.

5. WBahn Moderator

Mar 31, 2012
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Unless they are giving you a circuit diagram, the answer is a bit ambiguous because you could conceivably come up with a circuit in which (pick your answer) is correct.

But, in general, what is the usual operation of a 555 circuit? Usually, the time that it takes to do something is controlled by how long it takes to charge a capacitor. Is you want a 'shorter' pulse, that means that you want things to happen faster. If everything else is the same, will the capacitor charge slower or quicker if the it is made larger or smaller? If everything else is the same, will the capacitor charge slower or quicker if the resistor limiting the current flowing into it is made larger or smaller?

Last edited: Jul 27, 2012
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Jan 18, 2012
178
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Awesome, now that made it make more sense. After having a tough time with the study questions yesterday afternoon, I put the books down. Then later I picked them and boom got 5 or 6 answers. It always seems to work like that for me. Sometimes you need to clear your head, and restart. You can get lost in these books. Its a lot of books too, this course is in serious depth. Thanks again for the help.

7. WBahn Moderator

Mar 31, 2012
18,087
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I know exactly what you mean. This is one of the reasons why I hate short-schedule courses, like a summer course that is 7-8 weeks long or some that are even 3 weeks long. The belief is that someone can take five classes in 15 weeks either by doing them all at the same time over the entire 15 weeks or by doing them one at a time for 3 weeks each. The claim is that since you are in class the same number of hours each day and since you have the same total time to do the same things, that it makes no difference. It's an example of the Mythical Man Month. While it might work for some people with some topics, most engineering courses are such that I think most people need some time to let the thoughts and concepts rattle around in their head while their driving, taking a shower, eating dinner, or laying in bed to gel.

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Jan 18, 2012
178
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Thanks again for the help. Here is another question that I just can't find anywhere in my books. The question is worded very odd so you tell me what you think?

An output of a high from a logic gate having a high on only one input describes which of these?

1-an XOR gate
2-an NOR gate
3-a NOT gate
4-an AND gate

I can't find the answer anywhere, I looked and looked and reread my books, but this one really eludes me. Thanks again for any help.

9. WBahn Moderator

Mar 31, 2012
18,087
4,917
It can be argued whether this description is meant to convey what the output is when neither input is HI or when both inputs are HI, but in this case it doesn't matter given the answer choices.

They are saying that, at a minimum, part of the truth table looks like this (assuming a two input gate):

 A B Y L H H H L H

So which of the gates given is consistent with this?

10. justtrying Active Member

Mar 9, 2011
329
396
... you could also quickly write TT for the choices (AND, NOT, XOR, and NOR gates), and both the question and the answer will become more clear. That is how I approached these questions. Assuming a two input gate is usually safe for these types of questions as it gives an idea of how the gate behaves.

Jan 18, 2012
178
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XOR gate I think based on my books? Anyway I'm gonna take the exam tomorrow, see how it goes. Thanks for the help.

12. WBahn Moderator

Mar 31, 2012
18,087
4,917
Yep, that's correct.

Good luck on the exam!

pfelectronicstech likes this.

Jan 18, 2012
178
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Thanks, I'm gonna need it. I'm getting into the tougher parts of the course everyday. The difference between the basic electronics part and the Electronics tech. part is ENORMOUS! Next I'm going into logic circuits and the numbers system. I'm starting to get nervous.

Jan 18, 2012
178
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OK guys I'm not happy at all, I got a 72 on my exam. I mean this score really shook me, I got the first 4 questions wrong and I never even checked them twice because I didn't think there was a need, I thought they were right. I am very upset over this score, I still have an 85 average, but I wanted to get that up to 90. Its gonna be hard now after this miserable score. I'm just sick over this exam score.
On the good side of things, I think I found what I would like to work on after graduation, Fiber optics. I watched some video on Fiber optic splicing, and it looked extremely interesting. Its definitely something I would like to work on down the line. Are there a lot of jobs in the Fiber optics industry? I don't think I'd like to go the Verizon Fios installation route, but ya never know. I don't want to be crawling around in peoples basements for the most part.
So I got a terrible exam score, and I'm not happy, but thanks for all the help.

15. justtrying Active Member

Mar 9, 2011
329
396
Don't worry about the score, you passed. Just make sure you actually understand the material. Can't comment on fiber optics, looking for work in biomedical

pfelectronicstech likes this.

Jan 18, 2012
178
4
Thanks for the advice, I am certainly glad I passed. Make up exams are nerve racking! I am determined to get a good score of 85 plus next exam.

17. arun the curious Member

Aug 6, 2012
60
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i cant understand why answer clearly isnt XOR gate...Odd no. of high inputs gives high input in XOR gate only.

18. WBahn Moderator

Mar 31, 2012
18,087
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But the question didn't ask for a gate in which an odd number of inputs gives a high output (which is what I think you meant). You are choosing to make some inferences and assume that you have a correct restatement of the question. Yours is a very reasonable restatement, but there are others.

All the question is telling us is the output for the 01 and 10 case (and even then we assuming that we are talking about a 2-input gate). It says nothing about the output for the 00 and 11 cases. Anything we assume for those is just that, an assumption arrived at by inferring information that wasn't given. Think of the slight rewordings that would remove some of that ambiquity:

As given: "An output of a high from a logic gate having a high on only one input describes which of these?"

Alternative #1: "An output of a high from a logic gate having a high ONLY on only one input describes which of these?"

Alternative #2: "An output of a high from a logic gate having a high AT LEAST on only one input describes which of these?"

The original statement was poorly written english, so my attempts to make tiny tweaks make that worse.

The basic issue is whether you interpret the information as defining the only conditions under which the output is HI, or whether you interpret it as defining the minimum conditions under which the output is HI. It is also reasonable to interpret it as having an implied "but not both" in there, but that doens't necessary mean that it is specifying the behavior "when neither".

Depending on you interpretation, and OR or a NAND gate could have been reasonable answers (though if those had been given, in addition to XOR, I would have argued that XOR was the "best" answer).

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