# Resistor question

Hi there, I am another person that is NEW to arduinos, programming, and even the laws of …well…electronic stuff…

Im using a Diecimila, with a red LED using that special pin that allready came with a built in resistor - so far so good.

Now ,my professor wants me to calculate waht sort of additional resistors I need him to oder to use this red one, and also ayellow and a green LED.

I did -so far- look up some values, the Voltage that these 3 LEDs usually require.
Im using the plain USB port as a power supply so itake it the circuit has 5Vs to use?

Im lets say, completely overtaxed when it comes to mathematics and physics, so I am wondering waht numbers I have to come up with the next time my prof. walks in.

Is there a special formula. And how do I actually use that one?
I tried to calculate teh “needed resistor” though I dont know how to actually use its outcome aor understand waht is supposed to tell me.
Any ideas on how to get ahold of an answer real quick or waht kind of additional information i need to provide in order to help you helping me?

HE says that well prolly just use 100 kOhm devices, though he wants me to calculate an exact number, saying that different colors require ifferent resistors.

Well, I did by now manage to understand these facts, using wikipedia concerning Diodes, I do however NOT know the exact resistor values ((…

EDIT: According to some online calculators:
Is it possible that 3 LEds could be kinda hard to use with only 5 V?
They woulc be somewaht close to the Limit, as from how I understand (…) it!

I have trouble believing that your professor would give you this task without also giving you the resources you need to figure it out. Or is it that he assumes you should already know this stuff? At any rate, it would seem this task he gave you is for your benefit and giving you the answer would be pretty much akin to doing your homework for you. Or am I reading the situation wrong?

At any rate, the only formula you need is V = IR. You know (or should know) all the important Vs (e.g. your source voltage and the voltage drops across each of your LEDs), you should have a desired I that you are trying to achieve (as current determines brightness), and what’s left is to solve for R.

• Ben

Source Voltage = 5 V (USB thingy)
LED Drops:
red =1,2–1,8 V
yellow =2,1V
green = 3,0–3,4 V

If i think about these values, I dont even know what it tells me.

Adding the three LED drop values together shows that “my source aint cappable” of providing enough power in the first place?

Or is it that you dont add them together?

If I look at his sketches I get the impression that the LEDs would pe in a “paralel circuit”
Looking at the Arduino, it looks more like a simple seriell circuit.

Not knowing which one of the two cases apply, I dont even know what formulas to use at that point,
he has however written down “digital I/Os 5+6+7 to be used” maybe you know what kind of curcuit that makes for me.

Then theres that “I” value.
Reading the manual it says 40 mA per pin,
Though again if oyu dont know what kind of circuit you are looking at I dont even know whether I should use 40 mA for my calculation or a total of 120 mA.

I hope this post helps you understanding my rather -from the perspective of a more ttechnical person- trivial problem.

I spend the entire morning reading thru manuals and goole results concerning LEDs and Ohms laws, and yet I dont even know how to calculate the restistor value, even though I have done that in 7th grade or so.

…But then there is a reason for me being an art/languages student, and not an IT guy…

Update: I am SO GLAD you made it clear to me that I need a specific “I”, I finally used my balls and asked him for the ammount that he wanted and that was when he went like “OH, ohh sorry, ill take care of this myself ))”

You just saved a young mans morning there, I appreciate it, I also know alot more about circuits now

http://www.theledlight.com/LED101.html

Source Voltage = 5 V (USB thingy)
LED Drops:
red =1,2–1,8 V
yellow =2,1V
green = 3,0–3,4 V

If i think about these values, I dont even know what it tells me.

current ^= water flow, think liters/second
voltage ^= water pressure in your pipeline
resistor ^= diameter of your valve (more OHM → smaller diameter)

U = R*I (volt= ohm/ampere) (higher pressure → more water / narrower valve → less water )

need a specific amount of water per minute? use a “calculated” valve.

→ U/I = R

i can’t think of an analogy to the voltage drop (anyone?), but effectively it reduces the voltage that you use for the calculations.
if the voltage drop is 1V use 4V instead of 5V to find the right valve.

it sounds as if you’re going to drive your LEDs in parrallel in order to light them up individually. so you can calculate your values independently from each other.

best, kuk

Also remember that the some of voltage drops in a series circuit equals the supply voltage, so what doesn’t drop across an LED drops across the resistor. And current is constant in a series circuit, so the current through the resistor will equal the current through the LED.

The current you should be using for your calculation is what controls the brightness of the LED, but for normal LEDs, you probably want to aim for about 15mA for full brightness.

The 40mA is the physical limit of the microcontroller pin. Basically if you use a value higher than than, you could burn out the pin on the chip (you’re safe because the 15mA you need is lower than the 40mA limit).

Thanks so far, my tutor tried a slightly different path with the breadboards or how theyre called and now everything is ok with 100 kOhm resistors ,)

everything is ok with 100 kOhm resistors

I suspect that it is 100 Ohm resistors and not 100 kOhm as this value is 100 000 Ohms and putting that in series with an LED would not produce very much light at all, if any.

Quite so, its plain 1 kOhm, blame the coffee