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1  Using Arduino / General Electronics / Re: A noob resistance question on: January 06, 2013, 07:32:33 pm
Since you brought it up, I was wondering, How does using an LED Driver IC differ from powering the LEDs through a PWM enabled pin? Couldn't I use a shift register IC on a PWM pin to get the same result? Not that I have any immediate plans.
2  Using Arduino / General Electronics / Re: A noob resistance question on: January 06, 2013, 03:53:05 pm
The LEDs voltage tolerances are often more than 0.1V either way, so what voltage do you aim at...?

Well fungus, I would say that I would add the 0.1V to the 4V maximum forward voltage so 5V - 4.1V = 0.9V drop voltage, then to calculate the resistor needed (if I use a resistor vs a chip) it would be 0.9V / .02mA = a 45Ω resistor (or the next larger value), HOWEVER since I am always wrong about these things my gut is telling me to SUBTRACT 0.1V from the 4.0V BECAUSE it is the MAXIMUM forward voltage and if it is maximum then I can't go beyond it right? So 5V - 3.9V = 1.1V and 1.1V / .02mA = a 195Ω resistor (or the next larger value). I am aware that you may have been asking a rhetorical question, but I wasn't sure.
3  Using Arduino / General Electronics / Re: A noob resistance question on: January 05, 2013, 10:09:46 pm
The moment I hit post I knew I screwed up, but the dog asked to go out and you caught it before I could fix it smiley-red.

so it should be a 50Ω resistor... Do you know HOW I know I want to learn this? Because I keep on trying even though I have been wrong 100% of the time!
4  Using Arduino / General Electronics / Re: A noob resistance question on: January 05, 2013, 09:56:30 pm
Thanks JimboZA,

Although I got a great deal of information before your post, you gave me the piece of the puzzle I was lacking. I hadn't understood just WHERE the current I needed to power, in this case, an LED was coming from. Having looked briefly at the datasheets for different chips and sensors I wasn't making the now obvious connection that the items current requirements was in there. I was under the assumption that 15mA (right?) was the rule of thumb for ALL LEDs, but now it's so obvious I could face-palm.

So using the following datasheet:

I would subtract 4V (maximum forward voltage) from the 5V the Arduino is putting out to get a drop voltage of 1V which using V = r * I would be 1V = r * 20mA or 1 / .2 = a 5 Ω resistor? I combed the datasheet for any other values that looked right but theses were the only ones that seemed to fit the bill.
5  Using Arduino / General Electronics / Re: A noob resistance question on: January 02, 2013, 04:23:25 pm
Grumpy_Mike, Telecommando, and oric_dan(333),

Thank you all for your help. I would like to say that I believe you have helped me to understand this subject better. I have read and reread your responses, followed your links, and bit by bit I THINK I may be getting a handle on this.

I know I can go to different websites and read what other people have done to set up, let's say an RTC, and I can mimic what they have done but I really want to understand WHY you put a capacitor here or need a resistor there or why the oscillator should be 32Mhz (forgive the capitalized M if it is incorrect please) instead of 16Mhz. Is there a good resource for electronic components that explains 1)this is what it is 2) this is what it does 3) this is how it works and 4) this is why you would want to use it? Not a cookbook as much as a compendium of ingredients and a cooks insight on how you might want to combine them.
6  Using Arduino / General Electronics / Re: A noob resistance question on: January 02, 2013, 02:32:00 pm
Explained like that it makes sense to me. Believe it or not I used to understand ohms law but over the years spent in graphic design and asset management I have forgotten it. Once you reduced it to a 'solve for x' equation (voltage/current=resistance; voltage/resistance=current; resistance*current=voltage) it made sense. So if I were running my Arduino at 3.3V and needed to pull 15Ma from a pin I would need to use a 220Ω resistor BUT if I were running the board at 5V and needed 15Ma then I would need a 334Ω (or the next larger value). Also, I am assuming (incorrectly no doubt given my track record) that 15Ma is the amount of current needed for an LED to turn on. Okay NOW I think I understand why you would need a LOWER resistance for 10 LEDs than for 1 (3.3V=x*150Ma(10 LEDs*15Ma each)). Therefore x=a 33Ω  (or the next larger value)resistor.

Still wrong?
7  Using Arduino / General Electronics / Re: A noob resistance question on: January 02, 2013, 01:15:46 pm
Well, I can see I am going to have to do a LOT of work on the hardware side of this. From your link:

Like all math we can make the "proportion" bit into an equals if we put in a constant of proportionality, like this:-

V = k * I

The constant of proportionality is called "resistance" and so we write :-

V = R * I

In the first equation I am guessing 3.3V=(?)*40Ma? I have no idea what k stands for but the voltage should be the 3.3V the board runs at and the current is the 40Ma the pin puts out? If I thought I would have had any interest in electronics later in life I would have paid more attention in my high school electronics elective.

As for the second if it is 3.3V=220Ω*40Ma, and I am only guessing, how do you multiply ohms by Milliamps and come out with an answer that makes sense? Isn't that kind of like comparing apples to oranges or combining a tractor with a reading lamp?

This probably doesn't require a reply, what the link has illustrated for me is that I am going to have to take a class or something to figure out the electronics end.

Thanks for the reply.
8  Using Arduino / General Electronics / A noob resistance question on: January 02, 2013, 12:18:58 pm
I think I understand that the reason you put a 220 Ω resistor on the ground leg of an LED (such as in the 'blink' tutorial) is to dump any excess current as heat before it goes to ground, although if you are grounding it I'm not entirely sure why that is necessary. My question is if you use a 220 Ω resistor on one LED could you use a 2.2k Ω resistor and tie 10 LEDs to it? I'm not asking because resistor are so expensive or that I would prefer to use a rats nest of wires to accomplish the same task but just out of curiosity. It seems to make sense mathematically but I never see anyone doing it so if you CAN do it there must be some reason why you shouldn't and I just wondered why that is.
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