Simple 3.3v question

Hi, i always used 5v arduinos and components, but for more advanced projects it will be inevitable to use the newer ones like the mkr1000 or other 3.3v arduinos.

I know that the 3.3v pins on the newer models can supply only a small amount of power, so to not destroy a valuable arduino i am playing with my lab bench supply, some leds, some transistors, and two lines, one 5v line and a 3.3 line.

I used transistors before for powering heavier loads so i set up a led with it's resistor connected to the transistor emitter and controlled on the base from the 3.3v

I noticed that i can't get the full brightness of the led unless i 5v in the base, i searched for a guide on this topic but strangely i can't find any.
I am not an expert in hardware, and specifically transistors are a bit of black box, i know the basics, but i really can't read the data sheets apart from some simple infos.

I think this must be a common issue for people switching from 5v to 3.3v.


  1. Schematic?
  2. Resistor values?
  3. Transistor type or part number?
  4. LED type?

Here is the most common method to connect. Change the reference of "Pin 5V" to "Pin 3.3V" and it applies to your situation.

Here is the source:

The AVR processors in standard Arduinos run on 1.8 to 5 V, and you can safely draw 20 mA from the digital pins, regardless of supply voltage.

For LEDs at, say, 3.3V, to get the same brightness you will need to use a lower value of the current limiting resistor.

I noticed that i can't get the full brightness of the led unless i 5v in the base, i searched for a guide on this topic but strangely i can't find any.

That is because it is not actually a problem.
What you are doing is.
But unless you tell us what you are doing we can't tell you what you are doing wrong.

Schematic and photograph please.

I am actually using a bc547, for "newer 3.3v arduino's" i intend something like mkr1000 or mkrgsm1400 in the tech specs it says that each output pin can supply 7ma at max, so if you want to use a bright led you must control it via a transistor wired as @avr_fred showed, but with 3.3v on the base, with this setup the led does not light up with full 5v power, i think this can lead to some problems driving motors or controlling relays.

If you want help, please answer the questions.

  1. What value resistors are you using for RB and RL?
  2. What color is the LED?
  3. What value resistor are you using when connecting the LED to "full 5v power"?

And also how have you connected up the transistor, common emitter or common collector mode?
If you use common collector mode, sometimes called an emitter follower, then you will not get more than the voltage you put in at the base. In fact it will be 0.7V less. It sounds to me like that is what you are doing.

but with 3.3v on the base,

You can not put 3V3 on the base. If that indeed is what you are measuring on the base then you have the incorrect emitter follower configuration.

What color LED? The forward voltage drop of an LED is related to its color. Some LEDs may not even work at all at 3.3V.

Also, the brightness of an LED is directly proportional to the current through it.

@avr_fred sorry i’ll make a scheme tomorrow, it’s ok breadboard style in fritzing?
I have only the resistor on the led it’s 220ohm and the led is green, with the bench supply i have seen that the current flow on the 3.3v is zero, it’s only a signal, on the green led i read 2ma ma in that configuration and like 9ma if i drive it directly with 5v bypassing the transistor. (i may be wrong, i have it in the office, not here at home, so i am working on memory)

@grumpy_mike sorry i don’t know what common emitter or common collector means :o hardware side i don’t have much experience.

However turning on the led is not the final application, it’s just a “concept” thing about driving correctly 5v stuff interfaced to the 3.3v logic of the arduino.

Tnx to all for your time.

sorry i don't know what common emitter or common collector means

it's ok breadboard style in fritzing?

No, we need a schematic.