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There are 4 red leds and an arduino in the picture. Each led has forward voltage 2V. My friend says that just digitalWrite(9,HIGH) and the leds will light without resistor and won't be burnt. Is this true?

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    The LEDs will be fine. The Arduino won't be happy though.
    – Majenko
    Jun 27 at 19:27
  • It doesn't hurt to add a cheap resistor, at least it will prolong the life of your LEDs and Arduino
    – Coder9390
    Jun 28 at 6:22
  • That can work, if you don't set pin 9 to OUTPUT (using pinMode). Calling digitalWrite on an INPUT pin will enable the internal pull-up resistor (around 30kOhm). So in that case there is still a resistor limiting the current. The leds won't be very bright due to the high value of the resistor.
    – Gerben
    Jun 28 at 15:27
  • Another option would be to have 3 leds in series, instead of only 2. That way the combined forward voltage will be 6V. Supplying 5V will result in less current going through the leds. Though it may not be enough for the leds to light up, or at least not light up very brightly.
    – Gerben
    Jun 28 at 15:32
  • if you want the pro answer, you'll need the datasheet for the LEDs. Look for the v/i curve and see how much current 2.5v causes. If it's more than half the UNO's max pin current, you're out of spec and bad things may or may not happen. It's like hanging 60kgs on a rope rated for 50kg; it might work, maybe even probably will work...
    – dandavis
    Jun 29 at 4:40
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I am assuming you only have 5VDC available. Your friend is almost correct, it will work for a while however in the long run it will probably fail unless you are one of the lucky ones. What is the Vf (forward voltage drop) range of the LEDS (you state 2V), and are they matched by pairs, if not they will not illuminate evenly. You are forcing the arduino to output its maximum on port D9. I believe the current will not be within the microprocessor specifications. You are assuming a LED is a voltage device, it is not it is a current device. A resistor or constant current source is normally used to drive LEDs. The LED driving circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED and driver. Typically, the forward voltage (Vf) of an LED is between 1.8 and 3.3 volts. It varies by the color of the LED. A red LED typically drops around 1.7 to 2.0 volts, but since both voltage drop and light frequency increase with band gap, a blue LED may drop around 3 to 3.3 volts. across an LED is approximately constant over a wide range but can vary from LED to LED.

LEDs are diodes and have a

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Depending on the LEDs, it might or it might not, who knows?
You'll be both safe and in control by adding a small resistor in series (either with each individual LED or, if the LED pairs are balanced, with the whole LED circuit). The resistor value depends on the LEDs and the Arduino's supply voltage.

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