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pull-up and pull-down resistors with IC's

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mstechca

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Its amazing how one lousy resistor can affect the entire operation of a circuit :lol:

I don't know exactly how to ask this, but here is the scenario:

Say that I have a push button connected from VCC to the input of a NOT gate (or the input to any pin of any IC) Also, the input is connected to ground through a resistor.

Lets assume I decide to do the reverse (resistor connected to VCC and button connected to ground.

The question is what is the best resistor (or best range of resistance values) I should use in the above scenarios and why?

I'm asking this because I notice that a very high value resistor has almost no effect, and a low value resistor (about 330 ohms) makes the IC malfunction while the resistor is used.

I am mainly using CMOS 4000 series IC's, and I use some 74HC series IC's as well.
 
There's no 'best' value - but there probably is a point where it's too low, and a point where it's too high - but there's a HUGE range in between.

Personally I usually choose values between 10K and 100K, the actual value depending on what's to hand - anywhere in this range should be fine for CMOS (like PIC's). TTL can require substantially lower values, as you have to provide current for the chip - check TTL datasheets for details.

Incidently, the problems with 'too low' are likely to be excessive current consumption, and potential port damage (on a PIC the pin might accidently be set to be an output, which could damage it).
 
Hello!

At school we always use 330 ohms with switches. I asked my teacher why, because I was finding this really low, and he told me that it's for cleaning the switch contacts with the current.
 
JFDuval said:
At school we always use 330 ohms with switches. I asked my teacher why, because I was finding this really low, and he told me that it's for cleaning the switch contacts with the current.

It's one point of view, but if you look at any commercially produced equipment you won't see them anywhere near that low - most low current switches have gold contacts, and don't require cleaning (which would damage the coating).

No disrespect to your teacher (who might be really good?), but generally electronics teachers in schools have VERY little elecronics knowledge (obviously there are the very occasional exceptions!).
 
Using a switch on the input of some devices without debouncing can cause problems also. A mechanical switch has a charastic known as contact bounce. This usually occurs in milliseconds. If connected to the clock input of a counter one might get multipule clock pulses for on depression of the switch. There are ways to debounce the contacts.
One way is to use a contact debounce circuit like a schmitt trigger.
 
Nigel Goodwin said:
No disrespect to your teacher (who might be really good?), but generally electronics teachers in schools have VERY little elecronics knowledge (obviously there are the very occasional exceptions!).
LMAO :lol:

I don't understand why you say that. In fact, my mom knows absolutely nothing about electronics, and her embedded wish is for me to go into school to take electronic courses and get a diploma.

Here's the funny thing. I have taken some courses, and found out I almost learned nothing, and even that doesn't fizz on her. She believes a diploma is the ONLY ticket to success. In my books, it isn't.

Who says the old times doesn't exist anymore? We can still get a few decent jobs without a diploma if we try hard enough (a.k.a. pound the pavement and show experience).

In fact, I already have an employer wanting my electronic skills even though I have no diploma, and I don't compare to audioguru :lol:
 
That piece of paper means more than you think. Sure, some employers are adept enough to see your true value and skills, but most of the time you must go thru a Human Resources department that has absolutely no concept of how to measure your skills other than looking at that piece of paper. And this is the problem, how to get past HR to the real interview with the those who know the job.
Take it from me. I have over 30 years computer & digital telecom experience and 2500 classroom hours of training that most HR departments cannot translate if it equates to the job they are trying to fill. A piece of paper seems to be all most understand.

Dialtone
 
Dialtone said:
That piece of paper means more than you think. Sure, some employers are adept enough to see your true value and skills, but most of the time you must go thru a Human Resources department that has absolutely no concept of how to measure your skills other than looking at that piece of paper. And this is the problem, how to get past HR to the real interview with the those who know the job.
Take it from me. I have over 30 years computer & digital telecom experience and 2500 classroom hours of training that most HR departments cannot translate if it equates to the job they are trying to fill. A piece of paper seems to be all most understand.

Very true!.

My wife works as a classroom assistant in a local primary school (in the reception class - 4 to 5 year olds). A while back the mother of one of the young kids mentioned she was applying for a similar job (but not in reception) at another school. Now the woman was a complete idiot, doesn't have the slightest idea of bringing her own child up, and as a result her child was (is!) in serious danger of being the first child to be expelled from the school.

So the headmaster contacted the headmaster of the other school, advising him of the difficulties of this woman, and not to take her on under ANY circumstances. Amazingly the governors at the school over rode the headmasters objections and employed the woman because she was so well qualified on paper!.

They have since SERIOUSLY regretted it!.

A LOT of companies will employ based purely on qualifications, regardless of the capability of doing the job, which is why there are so many people (usually in highly paid jobs!) who haven't got a clue what they are doing!.
 
Nigel Goodwin said:
(who might be really good?)

Yes, he is not that bad :wink:

In fact, he isn't "only" a teacher, he work at the Centre de Terchnologie Physique du Québec as a electronics designer.

For sure at school we don't have gold contacts switch, so is method is probably well adapted to the situation.
 
JFDuval said:
Nigel Goodwin said:
(who might be really good?)

Yes, he is not that bad :wink:

In fact, he isn't "only" a teacher, he work at the Centre de Terchnologie Physique du Québec as a electronics designer.

That's sounds far, far better than most 8)

Often one of the teachers is sent on a one or two day course, this supposedly qualifies them to teach electronics in school :lol:

For sure at school we don't have gold contacts switch, so is method is probably well adapted to the situation.

Almost all small switches have gold contacts, it obviously depends on the types you are using - the small PB types I use in my tutorials are all out of old TV sets, just cheap little switches, but they are gold plated.
 
A LOT of companies will employ based purely on qualifications, regardless of the capability of doing the job, which is why there are so many people (usually in highly paid jobs!) who haven't got a clue what they are doing!.
Then why do the companies just hire based on a diploma?

no wonder why 1/2 the people in the world are NUTS!
sorry, but it is true!

Its a chain reaction! A new guy learns from an idiotic teacher (who probably got the job from the diploma), and he then receives his diploma, eventually forgot everything he learned, and incidentally, he is accepted into a high-end well paying job. Then more and more idiots become employed.

Now I see the picture much clearer now.
Please understand that I'm not commenting about you guys in particular. In fact, I think Nigel, and Audioguru are considered "gems" in this world, because you show you have the intelligence!

I fail to understand. Why don't people hire people based on experience and common sense? :shock:

If I was HR, I wouldn't consider a diploma the only tool to get into the job. In fact, I wouldn't even make a diploma a requirement!
 
mstechca said:
I fail to understand. Why don't people hire people based on experience and common sense? :shock:

If I was HR, I wouldn't consider a diploma the only tool to get into the job. In fact, I wouldn't even make a diploma a requirement!

It's simple, the people doing the hiring probably don't even understand what the job is, never mind how to do it!. So they can only go on qualifications.

When we've done interviews at work for technical jobs, I'm usually consulted once it's down to a small number - to check if they know their stuff or not?. One question I asked a few years ago was "do you have any bowel or wind problems" :lol: he got the job, but he lied :lol:
 
A diploma shows that you have studied the course and understood enough to pass its exam.
It shows that you completed a task in a specified amount of time.
I wouldn't hire anyone without a diploma unless I know him and his ability.

It's true. I've interviewed idiots with a diploma and they didn't know anything! They weren't interested in electronics, they just want any job.

I've had idiots for teachers. I taught them a lot. Usually the textbook defines the course, not the teacher.


Most clock inputs without a Schmitt-trigger have a maximum spec'd clock rise and fall time. A very high value pull-up or pull-down resistor provides a very low current to charge stray capacitance, slowing its rise or fall time when the switch is opened so that clock input doesn't work. The high impedance also picks-up interference.
74HCxx logic ICs have a maximum allowable switching time. While the output transistors are switching, they are both conducting a high current through them from the supply, causing a huge supply dropout if it isn't bypassed properly and overheating the transistors.
Your problem with a low resistance is contact bounce of the switch. How many times do the contacts bounce? 20 times? The counter counts them all.
 
If I may add to this thread about diploma's, I'm not in the electronics field as a job I'm a mechanical fitter/machinist/welder by trade. Now when I get my business up and going the first thing I will ask when Interview a potiential employee, is where the person got his trade, where has he worked in the past and provide some phone references. It's surprising on some of the minesite shutdowns I work on just how many workers have never worked on a mine before but they reckon they know everything about any job. If I ever had to employ an electronic tech for my business a diploma wouldn't be enough I'd expect a uni DEGREE at least. Having said that over here in Oz a diploma is a glorified bit of toilet paper and either a trade certificate or a uni degree is a must to get any real worthwhile position.

Just my 2 cents worth

Cheers Bryan :D
 
Mstecha, experience and common sense are necessary complements to a diploma, but you will be appalled to discover the gaps in your "education" if you forgo the diploma and try to get by on what you learn in the school of hard knocks. Just as an example, I will guarantee you that you will never be able to design a real analog PLL without learning Laplace transform theory. There is so much theory and mathematics behind electronics that is very difficult to learn outside the classroom. If you never want to go into design as a career, then experience can get you a long way. If you want to do serious design, bite the bullet and go for that diploma - which represents much more than just a piece of paper.
 
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