# Wiring / Resistors to Light 14 LEDs Over 7 Digital Pins

Hey Guys -

I'm working on a project which will need to control the lighting of 7 pairs of LEDs (14 LEDs total) via 7 digital pins (each pin powers/controls 2 LEDs) If it makes a difference, only 1 pair of LEDs could be flashing or all 7 pair at any given time.

Given that each LED is 1.8-2.2V / 20mA and I'm powering the Arduino Nano via 5V, what would be the most efficient way to wire this & what resistors to use? I have limited space for wiring, so if possible to use a common ground wire for all 14 and place a resistor in between each Nano pin & each pair of LEDs, that would be preferred.

Thanks!

On a 5V Arduino, one pin can power two low voltage (red or yellow) LEDs in series. A 100 Ohm series resistor will keep the current to about 10 mA for the pair. You can use a common ground wire.

Other types of LEDs typically have forward voltage 3.2 to over 4 V.

So just to confirm, the LEDs I'm using are yellow and as mentioned are...

• 1.8-2.2V
• 20mA
• 5mm

Given that, are you suggesting that I could set it up with a common ground, no resistors, & splice a wire from each pin to the anode of each pair? Didn't know if sending ~2.5V to each would be ok or not (assuming that's what would occur)

Thanks again

You must use a current limiting resistor, or you will damage the port pin.

Didn't know if sending ~2.5V to each would be ok or no

No. LEDs (like all diodes) have an extremely nonlinear current/voltage relationship. I suggest something like this on each of the seven pins.

Thanks -

1. To verify, is example A (see attached) what you are describing (with 100ohm resistor & 5V battery)?
2. If so, would it be possible to instead use something like example B? The only reason I ask is because my space for wiring is limited and based on where the LEDs are located on the model, it would be easier to wire. Should still be able to make A work, but B would be easier.

Thanks again!

1: "Port" means 5V digital output pin, not battery.

1. Circuit "B" wastes almost twice as much power, for the same amount of light, as "A" and requires a different resistor.

Circuit B is Bad. You should not wire leds in parallel. Even leds from the same batch from the factory won't have identical forward voltages. The one with the lower voltage will draw more current, making the brightness uneven and shortening the life of that led. Once that led fails (which could be years, depending), the resistor won't drop enough voltage, causing the failure of the other led.

So in circuit B you need a resistor for each led. But in your case circuit A is more efficient and uses fewer components.

PaulRB:
You should not wire leds in parallel. Even leds from the same batch from the factory won't have identical forward voltages. The one with the lower voltage will draw more current, making the brightness uneven and shortening the life of that led.

The thing is, you say that, and Mike frequently says it, but many commercial products do exactly that, paralleling LEDs in pairs or up to ten or so. And most high power LEDs use multiple dies in series-parallel.

The fact is that LEDs have substantial internal resistance, and LEDs from the same batch from the factory do generally match their internal resistances, so paralleling generally works quite well.

Note this graphic:

Or this one:

Are you saying they are bogus?