Burned 3.eV on Nano?

hello, I'm building a stopwatch for my kart, I use a magnetic sensor and it looks it works fine.
I use a Nano and a 5110 screen: I used it and it was marvelous, but the other day I read that the 5110 can be retroilluminated if connected the LED pin to the 3.3v ( I never new this!).
so I take another nano, I connect it to the usb (also tried with the 9v battery), with a Dupont I connect the 3.3 to the LED (in my case the 8th hole): it lights on very brightly! so happy I go to my other nano (the one I use for the project) and see that the 3.3v is already used for the VCC hole on the 5110. then I just think to make a bridge and connect both the VCC and the LED to the 3.3v.
That's when the bad happens: the stopwatch stops working, but when I press the reset button it starts back on. the screen is not lighted, so I unplug everything and connect only the LED to the 3.3v, but nothing happens anymore!
I then try to change both the Nano and the 5110, plug the LED to the 3.3v and it works. try the bridge with both the LED and the VCC, and happens the same as before: stopwatch stops working and the screen doesn't light on.

what have I done? is there a way I can solve this?

What logic level shifting are you using between the 5V logic pins of the Nano and the 3.3V logic pins of the 5110 screen ?

Its usually a good idea to post a schematic of how you have things wired up to help the forum help you .......................................

this is what I've been using before adding 3.3 to 8
4 RST -> D9 3 CE -> D2
5 DC -> D10 6 DIN -> D13 (*)
7 CLK -> D11 1 VCC -> 3.3v
8 LCD -> D7 (+3.3v) 2 GND -> GROUND

I don't know if I understand your question, but I'm using no logic level shifting. this is a similar scheme to what I'm using (taken from the internet, but the numbers on my 5110 are different)

A Nano is a 5V device; you should not connect the 3.3V 5110 directly to it. See e.g. Graphic LCD Hookup Guide - learn.sparkfun.com for ideas (2nd and 3rd fritzing diagram).

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@nbonettz, your topic has been moved to a more suitable location on the forum.

I'm sorry, but I thought that I could connect it directly to the 3.3v pin of the Nano. am I wrong?

The output signals of the Nano are 5V so there is a risk of blowing up the 5110; might be dying slowly.

You don't have to be sorry except for the possible loss of your 5110.

Hmm, yep you should have used voltage divider resistors on all I/O pins plus power pins if powered from 5V
R1 1K8 Ohm and R2 3K3 Ohm
Take a look here :
https://ohmslawcalculator.com/voltage-divider-calculator

but then what does the 3.3v pin on the nano means?

Its a pin that supplies 3.3V perhaps ?

If you use that pin as a supply for a device that has to be 3.3V, such as your display, it does not magically turn all the pins on the Nano to 3.3V logic.

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I see….thanks

Dont worry, we all have smoked things before we learned...

I just bought a 8 channel logic level swifter. hope it'll arrive soon.
but then, how do I make the connection?
4 RST -> D9 3 CE -> D2
5 DC -> D10 6 DIN -> D13 (*)
7 CLK -> D11 1 VCC -> 3.3v
8 LCD -> D7 (+3.3v) 2 GND -> GROUND
can I keep this, with the 3.3v with 2 outputs?
obviously putting the 3.3v5v swifter

OK, we seem to be becoming a bit muddled here! Does that diagram you posted actually show how you have connected the display? Never mind the reed switch for the moment.

This appears to be the connections for the 5110:

It seems you have ground correctly connected to pin 8 and the 3.3 V from the Nano also correctly connected to the "Vcc" for the LCD on pin 6.

You show pin 7 which is the backlight pin, connected to an I/O pin on the Nano. I do not know whether the (?)LEDs (are they?) on the 5110 have series resistors or how much current they draw, so I do not know how appropriate this is but suspect you are overloading the I/O pin.

Pins 1 to 5 should have the voltage dividers explained above to drop the 5 V from the Nano to 3.3 V logic levels on the display.

You need to explain whether you have actually done the same as the diagram before we can figure out what is happening. :woozy_face:

at the moment the 5110 is connected to the nano like this:
RST -> D9
CE -> D2
DC -> D10
DIN -> D13 (*)
CLK -> D11
VCC -> 3.3v
LCD -> D7 (I don't know if I should connect this to the +3.3v too)
GND -> GROUND

You meant "LED". :grin:

It seems the suggestion is that you connect it to 3.3 V, not 5 V, and through a 330 Ohm resistor. I haven't been able so far, to find a proper characterisation of the LED to say what resistor value would suit operating it from 5 V.

And use level shifters (if not resistors) for the five control pins.