Calculating necessary transistor for LED switching

Question

I have only just started to play with electronics (literally less than a week ago) and have purchased myself some cheap Arduino Nano clones, as well as some LEDs, resistors (only 10 ohms at the moment, but more varieties are coming), a breadboard and some jumper cables.

The reason I've got into this is that I also make scale models and want to light them up with LEDs and fibre optics. I may play with Bluetooth connectivity etc as a later date, but first, my issue with multiple LEDs and their power requirements.

Please excuse and correct me if any of this is wrong, its all based on my current (and probably incorrect) understanding at this point...

The Arduino Nano has 15 digital I/O pins and 6 of them use PWM. These pins output 5 V at a maximum of 40 mA (20 mA is recommended).

I want to run 4 LEDs from one of these pins. The LEDs I have are 3.3 V and 20 mA.

This means that I'll either not be able to light them (9.9 V / 20 mA if I run the LED's in series) or I'll damage the Arduino if I run them in parallel (3.3 V at 60 mA).

I came across a video (Here's the link) that discussed this issue, and suggested the use of a transistor to get around it, and the video makes sense, but it never mentioned how to make sure I get the correct transistor for my project.

Is there a simple way to calculate which transistor I want/need? For instance in my case, I have 4 LEDs, each one needs 3.3 V and 20 mA. What should I look for in a transistor to handle this, and how do I calculate it for myself?

Answer 1

Start with a type of transistor, for your application a N chanel Mosfet is the best to use

To chose a transistor you have to define some parameters like

Vth : in your case you have to chose one around 1 - 3 volt (to be sure that the Arduino 5V will be enogh to sature it)

Vgsmax : you will use Arduino to have margin, take a transistor with at least 8V

Rdson : Not critical in your case (just to minimise loses take the one with the minimum Rdson)

Id : the current in your circuit, in your case it is 20mA so you have to chose one with support a greater current (attention do not take the value in the first page, generally it is given for a specific Vgs and Vds, verifyyour condition, or go and find this value in the abacus )

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

You just need a transistor circuit. you need a PBNO (push button normally open) configuration. so select a NPN transistor

some common part numbers are BC547, 2N2222,

The maximum current of your leds load should be less that Ic (collector current) of transistor.

the maximum volatge should not exceed transistor collector-emitter voltage.

base current to turn on.

frequency response of transistor