# Externally powering a series of LEDs at maximum brightness with 470 ohm resistors and fading capabilities

I'm trying to power a project where I have 14 red LEDs lighting fiber cables controlled by an Arduino Nano. I want to make them pulse at maximum brightness to resemble veins.

I have the LEDs set up in parallel, each with a resistor. Due to time constraints, I used 470 ohm resistors that came with some other LEDs I bought (to avoid ordering more.) I realize now this was dumb, and I can no longer use the Arduino PWM to fade up to maximum brightness.

I understand that I will probably have to use a transistor to make it fade.

I could really use help with the following:

• Math help calculating the voltage and current parameters that would make the LEDs shine bright while also being convenient to power with common battery types.
• What kind of battery to use.
• What transistor to buy, and the best technique to use to fade the LEDs.

Here is the datasheet for the LEDs.

• Current - 20 mA max
• Forward voltage = 2.0 to 2.4 Volt

## 1 Answer

From the datasheet

LED Imax = 20 mA.
LED Vf (forward voltage) = 2.0 to 2.5 Volt

Assume the Arduino and LEDs operate on 5V.
This assumption can be altered.

For a series LED resistor Vr = 5V - Vf = 2.5 to 3V.
The resistor will drop this voltage when LED current flows through it.
R = V/I = (2.5 to 3V)/I
Assume 20 mA and 3V to start.
R = V/I = 3/0.020 = 150 Ohms.

Here is a basic circuit.
You can use 1 LED or many per circuit - each with its own resistor.
You can have a number of these circuits - each with its own PWM pin.

Q1 can be almost any cheap bipolar transistor.
I've shown a BC337-40 which is superb and usually low cost = but many others work OK.
R1 at 10k will work for multo LEDs per transistor at 20 mA.
With large numbers it could be smaller - eg 3k3.

When PWM input is high Q1 is turned on and the LEDs light.
PWM low - LEDs off. simulate this circuit – Schematic created using CircuitLab

• However, if the LEDs all have 470 ohm resistors on them, then the power supply for the LEDs needs to be 12V to get 20mA per LED. – Simon B Jun 27 '20 at 13:12
• @SimonB B Yes, it's proportional to volatge. But it's always better to put several LEDs in series to come close, but not above the supply voltage and then add a requiered resistor with less impedence. – Fredled Jun 27 '20 at 13:34