# Logic probe question?

Discussion in 'Homework Help' started by pfelectronicstech, Sep 18, 2012.

1. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
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I'm just not understanding this question, and can't find the answer.

If a simple logic probe is fed a stream of digital logic data consisting of equal proportion high and low logic pulses,

1 the HI LED glows brightly
2 the LO LED grows brightly
3 both LED's glow dimly
4 both LED's glow brightly

Just an educated guess from reading about logic probes, I would say its 4 Both LED's glow brightly. Am I thinking right? And isn't this an odd question? I guess it could be 3 as well, but I have eliminated 1 and 2. Thanks for any help.

2. ### vk6zgo Active Member

Jul 21, 2012
677
85
I would guess it to be (3),as the duty cycle for both Low & High is reduced,compared to a constant logic level.
Most of my logic work was done with an Oscilloscope,& I would have used a Logic probe less than a dozen times in over 30 years.

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3. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Thanks it was a toss up between the two, but I defer to your expertise. Its a bit of an awkward question, no?

4. ### WBahn Moderator

Mar 31, 2012
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It requires a bit of reading between the lines, but I think it is a reasonable question. Implied is that "brightly" corresponds to just feeding the probe a constant HI, in which the HI LED is on "brightly" and the other LED is off.

If you then changle your stream so that it occasionally (say one-tenth of the time) went LO, then you would still expect the HI LED to stay pretty bright but you would expect the LO LED to be one but very dimly. When you have a stream that if 50/50 (and also that the HIs and the LOs are pretty thoroughly mixed together), then they will both be getting about the same average current, which will be about half of what they would get if they were on solid. So you expect them to be about equal in brightness and to be about 'half' as bright as normal. So, given the choices, both on dim is better than both on bright.

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5. ### t06afre AAC Fanatic!

May 11, 2009
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My logical probe has a yellow LED then the signal is pulsating Kind of awkward question I think. Because the LED may actually blink. But to fast for our human eye to see it. The LED will either be fully on or off. But in average send out less light. And it is this average value we will see

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6. ### JoeJester AAC Fanatic!

Apr 26, 2005
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The number of pulses per second detectible by the human eye is up for discussion. Some say that it's close to 50 Hz. So, depending on the frequency of the pulse train, 3 or 4 could be the correct choice.

1 or 2 assumes a steady state.

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7. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Thank you all, I appreciate the help. WBahn, as always a very thorough, thoughtful answer.

8. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Got another one for ya if you can help.

If you feed the digital values 0101 and 1001 into the arithmetic-logic unit, then give the IC, an AND logic command, what binary pattern will be output?

1- 1100
2- 0001
3- 1110
4- 1101

I just don't get it, even if you point me in the right direction I would be happy because I just don't get this one. Thanks again guys

9. ### t06afre AAC Fanatic!

May 11, 2009
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10. ### WBahn Moderator

Mar 31, 2012
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I'm skeptical of a claim of 50Hz, since movies are displayed at 24 frames per second, though the two aren't directly comparable. But I believe that "in the old days" of physical film reels, the amount of time that a given frame was displayed was fairly brief, since the shutter had to close while the film advanced to the next frame. If that's correct, then the screen was actually dark a significant portion of the time and the persistence of vision was relied on to create the illusion of continuous imagery.

11. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
t06afre, thank you very much for the link! For some reason that clicked for me much better than my study books. Happens that way sometimes I guess. Made me understand the whole process better. Its really just remembering the rules for each type of gate and then applying it. Interesting, thank you very much.

12. ### JoeJester AAC Fanatic!

Apr 26, 2005
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For me I know it's much lower, like 15 HZ or even less. I know 30 frames per second seems fluid to me.

13. ### WBahn Moderator

Mar 31, 2012
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Me, too. I think 10fps causes noticeable chatter for me, but I know 20fps is pretty smooth (for me).

One thing that I thought was clever and unintuitive is how they convert a film from 24fps to a video at 30fps. In the same amount of time that you display 5 frames of the film, you have to display six frames as a video. So what do they do? They simply repeat every fifth frame a second time. If you single frame through a film that has been put onto video (say DVD), then you will see that, sure enough, this is exactly what they've done. Yet I've never heard of anyone serious claiming that they can tell the difference between that and something that was shot natively at 30fps (though I'm sure there are people that claim they can, I would wager a small amount that they've never been put to the test -- successfully, anyway).

14. ### vk6zgo Active Member

Jul 21, 2012
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Yes,the 525 line,30fps system is a bit of a nightmare for telecine equipment.
The 625 line,25 fps is a breeze,they just run the projectors 4% fast,which is not enough to notice.

Remember back in the day,when the only way they could make copies of the very early 1900s film was to run it at standard speed,instead of around 16 fps,so everybody looked like they were in a hell of a hurry?

15. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Is that what caused that? Is that why people looked like they were flying around the sets? Is that why when you see Babe Ruth footage it looks like he runs around the bases in 5 seconds flat? Interesting info.

16. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Do you see how the frequency applies to your initial inquiry?

17. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Thanks, my original question was about logic probes though?

18. ### JoeJester AAC Fanatic!

Apr 26, 2005
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The logic probe is a visual ... the led is on, off, or dim. Your problem alluded to a 50 percent duty cycle, equal on and off times.

Do you see how frequency plays into the question?

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19. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Oh I'm sorry yes, had a brain freeze. Thank you, now I got it. Yes now it makes sense.