Arduino LED not lighting up? Seven checks that find the problem

You wired an LED, you uploaded the sketch, and nothing happens. The LED stays dark. This is probably the most common first-week problem in electronics, and the good news is that the list of causes is short. This post gives you seven checks in the order that finds the problem fastest. I tested the main ones in Mokxi's headless runner, the same simulator the editor uses, so you can reproduce each one yourself without risking a real part.

Open this circuit in Mokxi

The circuit we are fixing

The circuit is the simplest one there is: Arduino Uno pin 9, through a 220 ohm resistor, to the long leg (the anode) of an LED. The short leg (the cathode) goes to GND. The sketch turns the pin on and leaves it on:

const int ledPin = 9;

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, HIGH);
}

void loop() {
}

I built this as a project with the LED wired correctly. After 100 milliseconds of simulated time the test asked whether the LED was on, and it passed. Now we will break it in the ways beginners usually do.

Check 1: is the simulation actually running?

In Mokxi, a circuit that is not running does nothing, and an LED on a stopped canvas is drawn in its off state. Press Run. The readout in the header starts counting when the simulation starts. On a real board the equivalent is checking that the upload finished.

Check 2: is the LED the right way around?

An LED is a diode, which means current flows through it in one direction only. Backwards, it does not conduct at all. This is the single most common cause of a dark LED.

I swapped the two legs in the test circuit so the cathode faced the resistor. The same sketch ran, and the test failed with "led1 is off at 150 ms (brightness 0), expected on". The code was fine. The LED was just backwards.

Fix: the longer leg goes toward the pin and resistor, the shorter leg toward GND. In the Mokxi editor, select the LED and press R to rotate it.

Check 3: is there a complete loop back to ground?

Current needs a way back to where it started. If the resistor goes to the pin and the LED goes nowhere, there is no loop, so the LED stays dark. The usual culprit is a missing ground wire. Mokxi's Checkup button on the toolbar looks for the common wiring mistakes, including a part with no ground and a pin that your code uses but nothing is wired to. Click it before you start guessing.

Check 4: does the code set the pin as an output?

pinMode(ledPin, OUTPUT); tells the board that this pin will drive something. Without it, the pin starts as an input. On an Uno, writing HIGH to an input pin does not drive it. It only turns on a weak internal pull-up resistor.

I removed the pinMode line and ran the same circuit. The LED did not light. The runner reported a brightness of 0.0053 at 150 milliseconds, which the test counts as off. A real LED might show a faint glow. Either way, it is not what you wanted.

Fix: put pinMode(pin, OUTPUT) in setup() for every pin you write to.

Check 5: is the pin number in the code the pin you wired?

The sketch above uses pin 9, and the wire goes to pin 9. It sounds obvious, yet a wire in pin 8 and a sketch writing to pin 9 is a very common slip. Say the number out loud, once from the code and once from the wire. On boards with labels such as D9 and GPIO 9, make sure they are the same thing.

If you use analogWrite() to dim the LED, remember that only some pins can do PWM. On an Uno they are marked with a tilde: 3, 5, 6, 9, 10 and 11. On the other pins analogWrite() only switches fully on or off.

Check 6: is it on the breadboard the way you think?

A breadboard strip is five holes. Two pins in the same strip are connected. Two pins in the same row but on opposite sides of the center channel are not connected. The power rails run along the sides and are not connected to each other unless you add a wire. And a part that is merely near the board, with its pins not in holes, is not connected at all.

Count the holes between the two legs if you are not sure which strip each one is in.

Check 7: did the LED burn out?

Mokxi models damage the way a bench does. An LED with no resistor carries far too much current and burns out. The first time it happens, the editor spares the part and rings it in amber with a message that says the part nearly burned out, how much current flowed, and what its limit is. The next time, the LED is drawn dark with a crack. To start over, fix the cause, press Stop and then Run, and every part starts whole again.

The fix is always the same: add a resistor in series. Mokxi's own message suggests 220 ohms at 5 volts. If you want to work out a value for a different supply, divide the voltage that must be dropped by the current you want. The Ohm's law page on Wikipedia has the formula.

On real hardware, a burned LED may still look normal, so if a new LED works in the same circuit when the old one does not, replace the old one.

One more: a layer you hid

Mokxi has a layers panel, opened with the L key. A hidden wire or part is still there and still simulating, but you cannot see it, and a dimmed layer is easy to mistake for an LED that is off. If everything looks right and still nothing happens, open the panel and check.

A quick order of attack

1. Press Run and look at the readout.
2. Click Checkup.
3. Flip the LED.
4. Read the code for pinMode and the pin number.
5. Check the breadboard strips.

Most dark LEDs are fixed by step 3.

More help

Mokxi's page nothing lights up goes through power, grounds and chips with their own supply pins, and an Arduino LED that will not light, or is dim covers the code side in more depth. To practice, build the circuit from this post in the breadboard simulator with an LED and a resistor, then break it one way at a time. It is a gentler way to learn than wondering whether a real part is dead.

Ready to build it yourself? Create your free Mokxi account and open the editor in your browser. No installs, no parts to buy.