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Please help. I'm busping the home cockpit, and Arduino's output to led diode is 1.8v. I also tried restarting arduino and also restart MF and still on pins is 1.8V. That diode will not turn on. I need 3.5V. Thank you for your advice ....Daniel
My English is not so good...

Probably you don´t use a series resistor, or one with a wrong value. If the current increase, the voltage degrease - up to a short-circuit. Use a resistor to limit current. You can calculate this by knowing your LED specifications. Mr. Ohm had said R = U / I.
We expect that a pin deliver 5 V - it will be less, but we can calculate with 5 V. If a LED need may be 2 V and 0.02 A then formula is 5 - 2 = 3; 3 / 0.02 = 150. Then use a resistor with 150 ohm, or better the next higher with 180 or 210.
If your LED need 3.5 V and a current of 20 mA, resistor should be 5 - 3.5 = 1.5 ; 1.5 / 0.020 = 75. You can try a resistor with 150 ohm. If LED it´s to dark use 100 ohm. Mostly it´s not nessesary to drive LED´s on it´s upper limits.

Hi.
You can use calculation software for this, too.
For example https://www.elektronik-kompendium.de/sites/bau/1109111.htm
Or simply google for "resistor led calculator" Here you get all data that is needed. You only need to know the color of LED and the Power of Arduino.

But maby you get a problem in your board !
If possible you can check this with a other arduino..... Is same problem there, too ??

to help you we need more information !
1. What board do you use ? Mega or another
2. How do you power it.... Via USB or via external suply.
3. Are there other Boards in the system or do you test only with one board ?
4. Are there hig Power needed devices on this board ( for example lots of stepper motors or 30+ LED same time) ??
5. Can you check the Input Power of your board ( whatever of USB or External)... What voltage arrive your Arduino ?

1. Arduino uno
2. Usb no exter
3. 1board 1 led diode
4. No only one led diode
5. No. Output arduino pin is 4,95V. But when i connect the led with resistor (100ohm) so the voltage drops to 1.85V

I have 100 and 120 ohm resistor...

The resistance is too low!

With 100 ohm you have a current of 30mA. Too much for a standard-LED. With 120 ohm you have a current of 25mA. Also to much current.

Calculate as follows: (Voltage (total) - Voltage (LED)) / Resistance = Current

Normally a LED has a operation voltage. These voltage you have to substract from the total operation voltage to get the voltage over the resistor.
The you calculate the resistor with the formular: voltage over restistor divided by the current through the LED. Result ist the value of the needed resistor
Formular signs are: U in Voltage, I in current and R in Ohm - to calculate as R=U/I. You can also calculate I=U/R or U=R*I

Normally usual LEDs have a bright light an a value of the mcd (millicandellar) up to 30000mcd. Here you need only a current between 6 and 9 mA (0,006 -0,009A) - This is for 5mm LEDs. 3mm-LEDs needs more current and should not be used for an Arduino.

1. Arduino uno
.....
5. No. Output arduino pin is 4,95V. But when i connect the led with resistor (100ohm) so the voltage drops to 1.85V

**********
1. UNO ist not recommend 100%. I stop testing with UNO...... Just only MEGA is recommend. But normaly that should be not the nature of your problem !
2r. I already not understand your problem completly.....

You say your Arduino support 4.95 ( so nearly 5v) of Power.
After you set the resistor you only get 1.85. Thats why we use resistors. we WANT downgrade the voltage so it´s good for the LED.
Or do you mean the hole board ( for example a unused 5v Pin on the Arduino) THEN just support 1.85v ??

If the hole Board is down to 1,85 then Stepahn must comment this.... If you mean just the circut from this single LED is down to 1.85... Thats aboslutly ok i think

but the diode with this plug-in illuminates normally in arduino blink. Just when we get involved, the MF will not shine