blue led resistor

[coolnumber0129]

so the brightness is controled not only by the PWM of the arduino but by the current as well?
if i were to use the same resistor on all leds will the PWM be reliable on controlling the relative brightness?
also, whats the units on the V=IR?
is it Voltage=Amp*Ohm?
or Voltage=mA*Ohm?

[aarg]

1. Yes.
2. Yes. Assuming you mean the same value resistor on all LEDs, not the same actual resistor.
3. Amp.

[DrAzzy]

1. Brightness is (roughly speaking) a function of the average current going through the LED. If you PWM it, you're turning it off for part of the time, thus changing the average current going through the LED.

2. Each LED needs it's own resistor (if you share resistors between LEDs, when two are on, they'll each be half the brightness). To make two LEDs have different brightness with the same PWM duty cycle, if the LEDs are different colors or from different manufacturers, you may need to use different resistor values on the different types of LEDs. Regardless, PWM will still adjust their brightness.

[coolnumber0129]

so the brightness is affected not only by the pwm but by the current as well?
if i use the same resistor on every leds will the relative brightness from the pwm be reliable?
what are the units in the V=IC?
is it voltage=mA*Ohm?
the current doesnt "go anywhere" right? like it doesnt shrink after passing an led?
if i connect 15 leds in parallel to one pin, will the amperage be enough to power them all?

thanks for the awesome comments!

[outsider]

Have some blue LEDs from a bag of 100 that are extremely bright, Vf is about 3.4 volts, right now I'm using one with a 1k resistor and its plenty bright enough for an indicator, current is only 1.6 mA.

[ChrisTenone]

so the brightness is affected not only by the pwm but by the current as well?
if i use the same resistor on every leds will the relative brightness from the pwm be reliable?
what are the units in the V=IC?

is it voltage=mA*Ohm?
the current doesnt "go anywhere" right? like it doesnt shrink after passing an led?
if i connect 15 leds in parallel to one pin, will the amperage be enough to power them all?

thanks for the awesome comments!

V=IR

V -= voltage in volts, I = current in amperes, R = resistance in ohms. So:

 Volts = ma/1000 * ohms

[coolnumber0129]

sooo.... 200ohms should work with 3.3V for about the maximum brightness?
to achive about 6mA i need to use a 600ohm resistor?

[ChrisTenone]

sooo.... 200ohms should work with 3.3V for about the maximum brightness?
to achive about 6mA i need to use a 600ohm resistor?

I often try it out on a solderless breadboard first, and fine-tune the resistor to give me the brightness and current I want.

[coolnumber0129]

thank you all!
your posts really helped!

[jrdoner]

"No, transformers only work on AC."

Actually, there's no problem running an LED on AC, as long as you have an adequate current limiting resistor, and place a blocking diode in series with the LED  to stop the reverse current flow.  Of course, the LED is only lit half the time.  I have a diode on a 110 v. AC mains line that's been running fine for years.

[aarg]

"No, transformers only work on AC."

Actually, there's no problem running an LED on AC, as long as you have an adequate current limiting resistor, and place a blocking diode in series with the LED  to stop the reverse current flow.  Of course, the LED is only lit half the time.  I have a diode on a 110 v. AC mains line that's been running fine for years.

Actually, the leakage current in your series blocking diode may be sufficient to induce a reverse voltage on the LED. It's better to place a reverse diode in parallel with the LED.