Circuit Doctor Catches an LED With No Resistor

Everyone leaves a resistor out at least once. The LED goes in, the wires look right, and then the circuit does something odd, or nothing at all, or something worse. Mokxi has a checker for this called the Circuit Doctor. It reads your wiring and your code together and tells you what looks wrong, in plain words. In this post you will build an LED circuit on an Arduino Uno on purpose without its resistor, see what the Circuit Doctor says, and fix it.

What the Circuit Doctor is

The Circuit Doctor is the Doctor tab in the editor. It looks at the circuit as wired and the sketch as written, and lists findings, ranked, with fixes where it has one. Tapping a finding lights up the parts, wires and pins it is talking about on the canvas. A fix applies at once and the panel checks again, so you watch the finding disappear.

The checks run in your browser. They are fixed rules, not a guess from an AI model, so the same circuit gets the same findings every time.

What you need

• An Arduino Uno
• One LED
• One 220 ohm resistor (you will add it at the end)
• Jumper wires

First, the wrong circuit

Wire the LED with no resistor at all. This is the mistake we want the Circuit Doctor to catch.

From	To	Note
Uno pin 9	LED anode (long leg)	Straight to the pin, which is the mistake.
LED cathode (short leg)	Uno GND	Completes the loop.

Here is the sketch. It is the classic blink, on pin 9.

const int LED_PIN = 9;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Blinking the LED on pin 9");
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(500);
  digitalWrite(LED_PIN, LOW);
  delay(500);
}

I compiled this sketch for the Uno with Mokxi's own build path. The code is fine. The wiring is the problem, and the sketch does not know that.

What the Circuit Doctor says

Open the Doctor tab. Because the LED is the only LED in the circuit and sits on pin 9, a finding reads:

Your LED on pin 9 has no resistor.

Underneath it, the detail says: "Real LEDs burn out without one, and they can take the pin with them. Put a 220 Ω resistor in series with it." The finding is marked as an error, not a gentle suggestion, because this is the kind of mistake that damages hardware.

Here is why that matters. An LED is not like a light bulb. Once it starts to conduct, its resistance drops, and it will pull as much current as the supply allows. The resistor is what limits the current to a safe amount. Mokxi has no burnout model, so nothing breaks on screen, and the mistake costs nothing. The habit is worth building anyway, because it comes with you to the bench. Our LED resistor guide shows the math: subtract the LED's own voltage drop from the supply, then divide by the resistor. It works out (5 V minus 2 V) divided by 220 ohms, which is about 13.6 mA.

Fix it with one tap

The finding comes with a button: Add a 220 Ω resistor. Tap it and the Circuit Doctor inserts a 220 ohm resistor into the wire from the LED, then checks again. The finding is gone.

You can also fix it by hand, which is a good exercise:

From	To	Note
Uno pin 9	One end of the 220 ohm resistor	Pin drives the resistor.
Other end of the resistor	LED anode (long leg)	Resistor in series.
LED cathode (short leg)	Uno GND	Unchanged.

Press Check again in the Doctor tab, and the finding should clear. Press Run and the LED blinks as before, now through a resistor.

The order does not matter, by the way. The resistor can sit on the anode side or on the cathode side of the LED. What matters is that the current has to pass through it.

What "Explain more" does

Under the findings there is an optional Explain more button. It asks an AI model for a short, friendly explanation of why your project probably does not work and what to try first. A few facts about it:

• It is optional. The findings above it are the fixed rules and are always there.
• It is capped per day. The cap is a limit on how many explanations you can ask for, set by your plan, and there is a site-wide daily cap too. When you hit one, the panel says so, and the findings keep working.
• It sends only the circuit and the code: the list of parts, which pins are joined together, the open sketch with line numbers, the findings and any compiler messages. It does not send your project's name, the serial monitor output, or any account details.

If you have set up your own key in the editor's Ask panel, Explain more uses that instead.

Line by line

• const int LED_PIN = 9; names the pin once.
• pinMode(LED_PIN, OUTPUT); lets the pin send out a signal.
• digitalWrite(LED_PIN, HIGH); turns the pin on, which sends current toward the LED.
• delay(500); waits half a second.
• digitalWrite(LED_PIN, LOW); turns it off, and the second delay(500) waits again before the loop repeats.

Try next

• Take the resistor out again and change the LED to a different color. Does the Circuit Doctor say the same thing?
• Add a second LED on pin 10 with no resistor. The Doctor names each LED and its pin in the finding, so you can tell the two apart.
• Wire a button to pin 2 and leave out the pull-up on purpose. The Circuit Doctor has a check for floating inputs too.
• Try the Fade an LED project and break it in a new way.

Try it on Mokxi

The LED resistor guide explains why the resistor is there and how to pick one, and the Arduino simulator is where to wire the circuit and open the Doctor tab.

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