Question about Leds and Resistors

[Pancra85]

I know that each Led connected in parallel needs it's own Resistor (I don't understand why but I know it does) but what if only one of the Leds will turn on at a given time?

I am using a Shift Register that will send a byte with only one "1" in it, for example: "00100000" or "00000010"... etc, and each output of the Shift Register is connected to it's own Led, so I know only one of them will turn on at a given time.

Now that I think about it, it could happen that while I'm sending a new byte two "1" could appear temporarily, for example if I have "10000000" and want to send "00000001" those two "1" will be next to each other while sending the data, even if it's only a matter of micro seconds. Could that be a problem?

Can I use 1 Resistor for all 8 Leds given this case? What about any other case?

Thanks!!!!

[oric_dan]

I believe the problem with parallel Leds is that, even if the same part# and from
the same batch, they each have slightly different characteristics [I-vs-V curves]
and one of the Leds will likely hog more of the current and glow more intensely.

That being said, for the case you describe, I should think you'll be ok. One R
shared by all 8 Leds will set the maximum current flow possible, so you cannot
hurt any of the Leds.

Also, when you send pulses down the line, all that'll happen is the intermediary Leds
will pulse on for a short period of time, and you'll probably not even notice it. Even
if you do notice the flicker, all it will do is alert you that something is happening.

Be interesting to experiment with. The flicker-fusion frequency for human
vision is about 30-40 hz, so you'll want the pulses moving a lot faster than that.

[marco_c]

If you use one resistor you will be ok. The point is that the resistor setes the maximum current that is allowed to flow in the circuit. If onlky one LED at a time is on then you will get the same light intensity from each LED. If you ever want to change to use more then on e LED then you will find the current will be shared between the LEDs (parallel circuit) and the LEDs will be correspondingly dimmer.

[Pancra85]

Oh, so if two leds share the same resistor what will happen is that if both are on at the same time the only difference is that, as they share the current, they will be less bright.
Using two resistors of the same value will make them brighter, right?

[marco_c]

One resistor per LED will control the exact same current going through the LED, hence brightness will be the same when they are all on. One or more make no difference here.

Sharing one or more LEDS per resistor will make them less bright IF they are on at the same time.

It is basic electric circuit theory - serial and parallel circuits and the current that flows through the parts of the circuit.

I am not sure I understand what you mean by more than one resistor - on one LED or for each LED?

[Pancra85]

I am not sure I understand what you mean by more than one resistor

I meant not sharing the Resistor.

I understand now, thank you!!!

[Pancra85]

I was thinking... and what about having multiple buttons sharing only one resistor all connected to inputs of a CD4051 multiplexer?

As the state of only one of the buttons are read at a given time I think sharing a resistor should work, right? Well... I am not sure that's why I am asking, hehe

[marco_c]

Best is to try it but I can't see why it would not work properly, as long as the resitor was on the Arduino input side of the mux.

[Pancra85]

You are right, I will try it when I have time and will report back. I know everybody says that and never comes back but I will try, hehe.