[Solved] Easy Question: Output voltage on Leonardo

Hi everyone,

I'm a little confused here and I hope that you can help me with this. On the main page it says that the IO pins of the Leonardo operate at +5V. I'm using many of these on my board and one thing I find confusing:

I've connected several IO-pins to a motor controller, one to a led and some others to a BT chip (HC05).

Now, I measure +5V on the L293D, thats good. But I only measure 2,75V on the led. How is it possible that I do not burn the LED when setting the pin to high? It should apply 5V and thus the led should be fried, right? Instead I measure 2,75V and the led is working. Wouldn't I need a resistor to operate the led?

Besides my curiosity, I'm asking since I want to bring the BT device to the at mode with the key pin. This pin needs 3,3V and not 5V. I do not want to try to just connect it and set the level to high, since I'm afraid that it could burn the chip...

Maybe you can help me with this supposedly simple question.

you can run the LED at any voltage, its the current passing though it that kills it, using a resistor restricts that current based on how much you plan on feeding into it

LED's also have a forward voltage that needs to be met for illumination, sounds like your LED is using roughly 2.2 volts hince the drop

for voltage conversion you need to have some plan in place, a 74HC4050 is ideal since its a high to low level shifter, but one might could use a resistor based voltage divider

That's good to know :) So the Level is indeed 5V and a resistor is necessary. Would 100Ohm be okay? Sadly I got the leds from china and I have no datasheet...

I did not completely understand the drop though.. So if I apply 5V, does that mean, that the forward voltage is how much of the current is dropping on it and hence this is also the minimum voltage that needs to be applied?

Elarion: that the forward voltage is how much of the current is dropping

Voltage and current are totally different.


The forward voltage of the LED is the voltage it drops. All other voltage needs to be dropped on something else, like the current limiting resistor. That's why you use them. The resistor is linear while the LED is not. Whatever voltage the LED doesn't drop, drops across the resistor. The forward current is determined by the remaining voltage drop of the resistor, through ohm's law.

Sorry... I know the difference very well... I was in a little hurry when replying and thus my fingers were typing something else than I was thinking :) Gonna insert a 100Ohm resistor now :)