Boolean algebra, A AND 1 = 0 WTF ? - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 06:15:12 pm
content: so my BTEC L3 course states that one of the 12 laws of boolean algebra is that:A and 1 = 0How can they claim this if A is not defined because as i read it:1 x 1 = 10 x 1= 0if A was the first digit in those two the so called rule does not work.....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2014, 06:15:45 pm
content: Could be a typo.

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 06:18:44 pm
content: So i'm not going mad.....When will education go back to where it used to be, this course is a pile of crap, it's a good job i already know what I've learnt so far or I'd be real confused.......

author: Bored@Work
author_rank: Super Contributor
timestamp: May 17, 2014, 06:21:03 pm
content: A and 1 = A

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 06:27:25 pm
content: Quote from: Bored@Work on May 17, 2014, 06:21:03 pmCould be what they meant, I'm starting to wonder who is the teacher and who is the learner here. I'm going to keep a log of all inaccuracies, what worries me is what happens when i do get to stuff that I don't know and need to learn and they are making errors.The guy who wrote the course was supposedly part of the HNC/D qualifications specifications team but then he just did the examples and questions so probably learnt as he went along.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2014, 06:33:31 pm
content: He could've meant A AND 0 = 0

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 06:39:09 pm
content: He could have meant anything, if there are going to be typos where fundamentals like this are explained it throws the whole thing out of the window, they should paying extra care to write these theorems correctly because they underpin any further discussion on logic networks and calculations and simplification of boolean expressions. Fortunately the text book that comes with the course also covers the 12 basic theorems infact they give 18.

author: IanB
author_rank: Super Contributor
timestamp: May 17, 2014, 08:24:13 pm
content: Typos often happen in worksheets and handout materials.Although you find it slightly annoying, having such mistakes to find is actually good for you. You should test everything you are taught in your own mind to see if it makes sense. If you find something that doesn't seem to make sense it will make you think harder about it and will reinforce your learning.As and when you cover more advanced material you can also think to yourself, "What are the assumptions made here?" "What are the limitations?" "When might it not apply?"So be cool about it, and smile to yourself when you spot something that seems to be wrong, and later find out you were correct.

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 08:32:35 pm
content: no I'm sorry but this course is just crap all over. The explanation of using a current limit resistor on an LED was incredibly stupid, apparently you need the resistor to make up for the differences in voltage between LED's even though made in the same batch (manufacturing tolerances), they totally omitted the fact that a diode will not control current but allow as much through as you try and push until it blows and that is why you need the resistor.On a more advanced level yes you could regulate the voltage in the LED and leave out the current limiting resistor for efficiency sake but that is an advanced topic beyond this level of course. If it wasn't for the fact that this is the only course available in the UK I'd have sent it right back! The explanation of the zener was also ridiculous, they didn't even mention that a minimum amount of current is required to guarantee the voltage across it.......

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 08:35:59 pm
content: and as soon as they want to make a more detailed explanation they refer you to a web page that could be taken down any day....... there is distance learning and there is learning attached to the internet, they could at least refer me to the text book they included with the course.

author: IanB
author_rank: Super Contributor
timestamp: May 17, 2014, 08:39:05 pm
content: Quote from: Simon on May 17, 2014, 08:32:35 pmThere you are, you have understood the range and limitations of what you were taught and put it into context with other things you already know.But I have to say, the current limiting resistor does not seem as stupid as you think. It is very common in electronics to use series resistors to provide for even current sharing between parallel devices. Without such resistors small variations in manufacturing tolerances can produce large imbalances in current flow.

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 08:49:49 pm
content: Quote from: IanB on May 17, 2014, 08:39:05 pmYes but the explanation implied that you would be expected to run an LED straight off a voltage source but for manufacturing tolerances, they failed in the explanations of diodes, zeners and LED's to explain that a diode based device cannot be connected to a non current regulated voltage source or it would short out and blow, the explanations were of no practical use and need more information before I would be able to use the components but for the fact that I know this stuff already.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 17, 2014, 08:54:59 pm
content: Simon has a point though - if you know enough about the basics to spot fundamental errors in the easy stuff it undermines your confidence in the tutors and makes it harder to trust their explanations of the more advanced topics.

author: IanB
author_rank: Super Contributor
timestamp: May 17, 2014, 09:00:08 pm
content: Quote from: Simon on May 17, 2014, 08:49:49 pmTheir explanation is essentially correct on the whole. I don't think you should be so ready to dismiss it. You are being presented with information from a slightly different perspective than you have thought about before. Rather than dismissing it, you should consider whether your current understanding is incomplete.You can, of course, connect an LED to a non-current regulated voltage source. An LED has a voltage/current curve just like any other device. If you apply the right voltage to it the expected current will flow and it will light up. It will not short out and blow...it will simply pass as much current as the applied voltage induces.In reality an LED does have variations in manufacturing tolerances and other causes of variation in its voltage/current curve and that makes it difficult to reliably apply the right voltage. That is exactly why you need to stabilize the current with a series resistor in a practical circuit.

author: IanB
author_rank: Super Contributor
timestamp: May 17, 2014, 09:07:47 pm
content: Quote from: grumpydoc on May 17, 2014, 08:54:59 pmOne should also be careful about rejecting taught material without due consideration. In my first year of university study I was occasionally indignant about being presented with "wrong" information when it challenged what I thought I already knew. Later on I understood the lecturer was correct, but I was not calm and patient enough to see it at the time.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 17, 2014, 09:24:30 pm
content: QuoteFair point but in that case a good tutor should be able to explain things to you, or at least show you enough for you to realise that you need to wait to get the full picture.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 17, 2014, 09:35:06 pm
content: Quote from: Simon on May 17, 2014, 06:18:44 pmDon't worry, there was never a time when textbooks were without errors and instructors were never wrong. There have always been good and bad classes, good and bad instructors. Textbooks have mostly been uniformly mediocre, although bad ones certainly exist. Maybe you've got a bad class, maybe they just had a bad day, but going back to the "good old days" isn't going to fix anything. I am sure any of the guys even older than me around here can attest to that as well. The good thing about technical stuff like logic is that if you are paying attention you will figure out when they are wrong, even when they get to new material. If a history text tells you that the battle of hastings was in 1104, you just have to take their word for it until you look like a total idiot at a dinner party. If a text book tells you that "1 and X = 0" is an identity, you can figure out that it is inconsistent and go figure out or find the correct answer.

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 09:37:24 pm
content: I am afraid Ian that in this case the material is deficient, the explanations given so far have been so vague it beggars belief. BJT transistors were explained with a scarce mention of hfe, it was made to sound like a secondary consideration, there are virtually no equations presented to explain component choices, like i say, if i really knew nothing this material would have taught me nothing and if anything just left me more confused. why am i being asked to take more explanations off the internet than from the course material ? I was given a 2 page wishy washy explanation of JFETS's and MOSFET's and then given addresses to online tutorials that are several pages long for more information but was warned that there was more information there than i needed (the basic maths maybe ??). The idea of having a book is that you read it, not keep ping ponging from book to internet because the book is incomplete.2 of the "activities" tell me to watch "a video" - what video ? none was supplied, none is on the materials check list and no internet resource was cited, i was being asked to take measurements of waveforms in the video off the screen. There is clearly very little will in this course to explain and illustrate stuff, as soon as it means a bit of work there is either a gaping hole or I'm told to use a website that could be taken down tomorrow just so they didn't have to draw a graph or explain something. As it stands i could write the first 50 pages of this course myself and do a damn better job.I'm just going to start the op-amp section, god only knows what I'll not be finding in there!

author: Simon
author_rank: Unknown
timestamp: May 17, 2014, 09:43:11 pm
content: Quote from: ejeffrey on May 17, 2014, 09:35:06 pmI see more of a tendency now to write books just to make a buck with little intention of helping the reader (Don't touch McGraw Hills books with an effing barge pole unless it's highly recommended by someone, been burnt too often with their unchecked and non proof read garbage). We used to have standards and writers were more honest, there is making an honest mistake and there is being negligent and lazy, these days we just want people to make a token gesture to get a grade, I don't want the damn grade, i want the knowledge. the guy that wrote this garbage was supposedly on the team that did the HNC/D course but then he only produced the exercises for it so probably learnt from his colleagues and then went and wrote this crap.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 18, 2014, 12:25:44 am
content: Quote from: Simon on May 17, 2014, 09:37:24 pmhfe is a secondary consideration. It is usually introduced first due to historical reasons and because a current gain model is simpler than the Ebers-Moll model, but it is not accurate enough for most calculations other than as an estimate of the current required for a saturated switch.

author: Marco
author_rank: Super Contributor
timestamp: May 18, 2014, 01:01:14 am
content: Could be in Hex

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 07:09:40 am
content: Quote from: Marco on May 18, 2014, 01:01:14 amAt the moment that would not make it any worse. started on the op-amp section, they sorry he has not bothered to explain the virtual earth between inputs just said that the output is gain x (+in + -in) the input resistance is infinite and the output resistance 0 in an ideal opamp, then give the inverting and non inverting configurations and formulas wasting time explaining that he has called one resistor R1 and the other R2 and that is you reverse the resistor notation in the diagram you reverse it in the formula, i mean duh ? this is a level 3 course not a kindergarten lecture.......And apparently the opamp symbol is supposed to look like the not gate symbol for some reason........

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2014, 07:13:28 am
content: A teacher with tenure, risen to the top level of his incompetence.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 07:18:11 am
content: Well I hope that the rest of the HNC/D team are not like him, I am doing this BTEC L3 so that I know i have all of the basis covered before going for a HNC or a HND which is as near to a degree as you can get without getting a degree, a HND will certainly be equivalent to an undergraduate degree so I'm hoping this clown did not do too much damage to that course as again there is only one HNC or HND course by distance learning in the UK and they estimate it takes 5 years to do and i have been told that I'll need solid grounds in maths, they want £487 for a "bridging module" in maths so it sounds like it may be a more serious course.

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2014, 07:22:43 am
content: Go to your local community college and see what they have as maths in night courses. Even if you only can do a little you will learn more face to face than by correspondence. It will help you a lot, the maths is steep at times but they will bring you up to a better level quite fast. Scary thing is once you start learning it it makes many things a lot easier all of a sudden, and teaches you better on things like logical deduction and why things work.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 07:27:03 am
content: The problem is that this country is an educational shit hole it is easier to find courses on makeup and hair styling than engineering and serious subjects. My local college only does wishy washy courses their branch in a town 20 miles away used to do electronics but that has gone. There are far fewer courses in electronics than any other subject and engineering courses as a whole seem to have been reduced, hence i could hardly find one by distance learning. I've got a maths book on order that amazon are making me wait 2 weeks for just because i didn't use them as the carrier that I'll use to get a head start but maths in itself is boring and to understand it it helps to have it related to something practical.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 07:35:17 am
content: Ah yes of course, what does the short lived section on opamps tell me to do at the end ? go to this address (wikipedia) to read all about opamps and read about all the things we just pretended to teach you !

author: vk6zgo
author_rank: Super Contributor
timestamp: May 18, 2014, 09:36:52 am
content: Quote from: IanB on May 17, 2014, 09:00:08 pmIn many circuits an LED needs to be operated from a supply much higher in voltage than that which appears across it in operation.AnidealLED would not have the variations referred to above,but would still need a series resistor to limit current flow.In practice,it is seldom necessary to match the current through multiple LEDs in such simple circuits,as they are usually panel indicators.As the human eye is quite insensitive to small variations in light level,the difference between say,20mA,18mA,& even 12mA,is vanishingly small.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 09:42:25 am
content: my point exactly, the whole explnation given was redundant and should have been a different and correct one.I am now reading through 25 pages of opamp explanations on wikipedia as directed by the course that spent a mere 5 wishy washy pages on it.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 18, 2014, 10:01:38 am
content: Wikipedia is not really a trustworth source. Tryhttp://www.ti.com/lit/an/slod006b/slod006b.pdf

author: GeoffS
author_rank: Unknown
timestamp: May 18, 2014, 10:01:59 am
content: After watching Dave's videos on op amps, I've been browsing throughOp Amps for Everyonefrom TI, I've even been understanding some of it!I've had to take a few breaks to brush up on my maths over at theKahn Academy.EDIT: Beaten to the punch

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2014, 10:04:18 am
content: Quote from: vk6zgo on May 18, 2014, 09:36:52 amWhy would it need a series resistor? If you apply the correct voltage you will obtain the desired current flow.Quote from: Simon on May 18, 2014, 09:42:25 amThe explanation givenwascorrect.Physical systems can often be looked at in different ways, and that means that there can be different, but equally correct, ways of explaining their behaviour.There seems to be some misconception that a diode without a series resistor will turn into a short circuit, pass infinite current and blow up. This is not the case. A forward biased diode will pass current according to its I/V curve. Since the slope of the I/V curve is very steep in the normal operating region, apracticalcircuit needs some means of stabilizing the current to keep it at the value desired, but this does not undermine the fact that in principle you could apply the correct voltage and get the current desired.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 10:05:07 am
content: Quote from: Bored@Work on May 18, 2014, 10:01:38 amTell that to the asshole that wrote the course

author: vk6zgo
author_rank: Super Contributor
timestamp: May 18, 2014, 10:05:50 am
content: Quote from: Simon on May 18, 2014, 09:42:25 amI would suggest you go to your local University & have a look through their secondhand bookshop.You may well find some "Treasure trove" in the form of old Electronics books.Boolean algebra has been around forever,as have Op Amps,so you should find something useful.The very best pickings is when the Uni has a mass sell off of books,which may occur every few years.Misprints & "glossed over" subjects in Math & Technical Books have been around for years,but if you have another reference you can check whether it's you,or the text.Unfortunately,Internet sites,including Wikipedia although they have an aura of authority,are equally vulnerable to typos & errors.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 10:09:08 am
content: Quote from: IanB on May 18, 2014, 10:04:18 amfirstly we are talking about a simple course, what you refer to which I understand is somewhat advanced for this course and seldom used in practice. Take a look at your current source driving an LED, what is being used to sample the current for making adjustments ? oh let me guess a resistor......

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 10:11:14 am
content: Quote from: vk6zgo on May 18, 2014, 10:05:50 amYou are under the misapprehension that there are universities around here that teach electronics, that is the problem, there is very little available in the Uk in the way of electronics courses of either type. Even the renown open university does not do electronics.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 18, 2014, 10:14:01 am
content: Quote from: IanB on May 18, 2014, 10:04:18 amIt would be nice if you quoted the complete paragraph:"In many circuits an LED needs to be operated from a supply much higher in voltage than that which appears across it in operation.An ideal LED would not have the variations referred to above,but would still need a series resistor to limit current flow."The second paragraph follows directly from the first.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2014, 10:24:42 am
content: Quote from: vk6zgo on May 18, 2014, 10:14:01 amI'm sorry about that, but I'm trying to address Simon's point here:Quote from: Simon on May 17, 2014, 08:32:35 pmThe series resistor is to make up the difference between the higher supply voltage and the required operating voltage of the LED.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 18, 2014, 10:29:12 am
content: Quote from: Simon on May 18, 2014, 10:05:07 amNo, I am telling that to you. You want to learn something.

author: Simon
author_rank: Unknown
timestamp: May 18, 2014, 10:35:58 am
content: I think have that one but thank you and I can't take a 400+ page aside as part of this course as I have 11 months to complete it. aim here is to learn what there is to learn from it and get the bit of paper so my employer is happy. Then I can worry about going further on my own.

author: GreyWoolfe
author_rank: Unknown
timestamp: May 18, 2014, 09:35:56 pm
content: When I went to ITT Technical Institute in the late 90's, over the course of the 2 year AS-EET program, we found 3 or 4 errors in the textbooks we were using. The class teachers had no issue with us finding the mistakes as long as we proved why there was an error and what the correct information was. Then we were allowed to use our answers.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2014, 10:42:14 pm
content: Quote from: Simon on May 18, 2014, 10:09:08 amLet me illustrate with an example. Here is a test that I just ran, with avoltage sourcedriving an LED. There is no current regulation and no resistor in the circuit. I am simply placing 2 V across the LED and it is quite happily lighting up and passing 7 mA. There is no magic smoke.So why don't we simply run all red LEDs with a regulated 2 V supply? It's because of variations in LEDs, due to manufacturing tolerances, design variances, ambient temperature, and so on. So how do we allow for that? We use a series resistor to compensate for the differences.This is not advanced, it is simply what you need to know to design a practical, working circuit.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 19, 2014, 04:20:50 am
content: Quote from: IanB on May 18, 2014, 10:42:14 pmI'm sorry,but that's just plain silly!We use series resistors as it is a far more economical alternative when the LEDs are incidental to the operation of the circuit,& the operating voltage of that circuit is higher than that which could directly power a LED.Apart from anything else,when LEDs first made an appearance,a regulated 2v supply was as rare as hen's teeth.I don't know anyone who selects series resistors so the LED has exactly the voltage required for that individual device.They have a look at what voltage normally appears across a generic LED of that colour,subtract it fromthe supply voltage,& using Ohm's Law,determine what resistor is needed for around 20mA or so through the LED,Or,if it is a 12v supply, they try 1kOhm,& if that isn't bright enough,go to 560 Ohm,or 470 Ohm.

author: Simon
author_rank: Unknown
timestamp: May 19, 2014, 05:44:47 am
content: Ian, well done but your missing the point entirely. Take that LED off and get another one from a different source that looks the same and wee what happens. Tell me honestly would you propose that solution for a design that was going to be made by the 1000's ? what would happen when the exact model of your LED was not in stock of a different batch came in from the manufacturer, can you stake whatever you like on it still working ? what happens with temperature variation, if we are going to have the advanced version of the discussion which the course material was certainly not trying to have lets put all the factors on the table not just a one off experiment where you fine tuned it for that one single individual LED.I am currently looking at lighting LED's in a battery powered box and i do need to make the most of my power because I will have some AA batteries powering 80mA of LED's for a week, so my plan is to come as close to what you have done without loosing my head: I will use as low a value resistor as i can and run them off a small SMPS with a voltage as close to the LED voltage as i can get, but again this will be a one off and i can afford to make a broad assumption on the parallel supplied LED's being pretty much identical and using just a few ohms of current limit with a fine tuned voltage supply...... but I'd be being more careful if i was getting a manufacturing house down the road to make me a thousand.

author: Kremmen
author_rank: Super Contributor
timestamp: May 19, 2014, 06:30:07 am
content: If you are planning to use an SMPS as part of the solution then may i recommend you check the Supertex HV 9910B. It is a simple buck type _current_ controlled LED driver that avoids the series resistor and its power loss. There is a series current sense resistor that is unavoidable but depending on your exact numbers it could be a near optimal solution. Notably, the chip supports constant-off-time mode as standard thus avoiding certain undesirable buck characteristics when the duty cycle is very low.I have used and recommended the HV9910B earlier with good success.

author: IanB
author_rank: Super Contributor
timestamp: May 19, 2014, 06:33:43 am
content: Quote from: Simon on May 19, 2014, 05:44:47 amYes of course, that's why we use a series resistor. But aren't your words above exactly what your course material was saying, which you rejected as wrong back in post #8?I'm not trying to tell you that you can make a design that runs LEDs off a constant voltage supply. That would be silly. I am trying to tell you that the explanation given in your course material, that variations between LED samples and other factors mean that you need a dropper resistor to control for those variations, is quite a reasonable explanation. It's the same explanation you have given above.Specifically, my experiment was designed to show that an LED without a dropper resistor can in fact control the current flow and it will not necessarily turn into a short circuit and blow up as you suggested. There is quite a range of voltages I can apply to that LED above and below 2.0 V without it blowing up.

author: Simon
author_rank: Unknown
timestamp: May 19, 2014, 07:02:35 am
content: unfortunately i have 4 independently switched LED's so I'll have to use voltage regulation and current limiting resistors but it's only 15mA each so not much sweat

author: Kremmen
author_rank: Super Contributor
timestamp: May 19, 2014, 12:49:06 pm
content: Quote from: Simon on May 19, 2014, 07:02:35 amWell - you could get creative and connect the LEDs in series and short out those you want to blank. That would be perfectly OK for a current source but would need a mechanical contact or a slightly complicated transistor/FET switch for each led.

author: Simon
author_rank: Unknown
timestamp: May 19, 2014, 12:50:14 pm
content: Given the time i have or rather have not I'll stick with what I mostly know, at the rate I'm going it will be batteries and resistors only

author: Simon
author_rank: Unknown
timestamp: May 19, 2014, 12:52:21 pm
content: It would also mean for my 4x2.6V LED's I will end up needing twice the battery power planned before I account for drained batteries.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 20, 2014, 03:38:40 am
content: Quote from: IanB on May 19, 2014, 06:33:43 amI think I can see what you & the original text are getting at:-Because of variations in LED characteristics,it is unwise to do something which may seem intuitive,which is to use six "2.0v" LEDs in series between a 12 volt line & Common.Parallel "2.0v" LEDs across a 2.0volt regulated supply is not a good idea,either.Even having several LEDs in parallel,with one series resistor is problematic.( I have seen it done,though).Although the intention may have been good,the obvious implication in the original text is that the series resistor needs to be "selected on test" to obtain either the correct voltage across the LED,or correct brightnessThe contrary is in fact true,& mass produced equipment uses a fixed value of series resistor with many different batches of LEDs from different suppliers & manufacturers,without any problems.As I pointed out earlier,any difference in resultant brightness is not evident to the human eye..

author: Simon
author_rank: Unknown
timestamp: May 20, 2014, 05:37:57 am
content: The V/I curve of the LED is very steep so it is hard to control the current, the resistor takes some of the slope out and makes it more linear so that minor variations will have virtually no effect, there I daid in one line what my course babbled a whole paragraph about and still got wrong.