Arduino Uno R3

Build a protected LED output

Wire and control an external LED while verifying polarity, resistance, pin mapping, and a shared ground.

CSTA 2026 mapping candidatesNot reviewed by CSTA

MS-ALG-PS-03Algorithms & DesignMS-PRO-TR-19Programming

Students will

Identify LED polarity
Use a series resistor to limit current
Control and verify an external digital output

Evidence of success

The LED lights without bypassing its resistor
The circuit and sketch use the same output pin
The student can trace the complete current path to GND

Hardware

Arduino Uno R3
5 mm LED
220 Ω resistor
Breadboard and jumper wires

Prerequisites

Upload, blink, and observe
Identify a breadboard row, digital pin, and GND

Before class

Confirm LED polarity and the resistor value
Prepare one known-good circuit for comparison
Use a conservative resistor supported by the actual LED

Reviewed connection map

Protected LED connection

Arduino D9Program-controlled output

220 Ω resistorLimits LED current

LEDAnode toward D9; verify the actual part

GNDShared return path

Connect D9 to one end of the 220 Ω resistor.
Connect the resistor to the LED anode and the LED cathode to GND.
Disconnect USB power before moving components or changing rows.

Build and test

01Identify the LED anode and cathode
02Build the series path from D9 through the resistor and LED to GND
03Inspect breadboard row continuity before applying power
04Upload the external blink sketch
05Change the timing and prove that D9 controls the external LED

Read before uploading

Annotated Arduino sketch

const int LED_PIN = 9;

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

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

Optional engineering record

Record evidence from this lesson

Choose the prompts that help students explain predictions, evidence, debugging, and transfer. Saving creates a new entry in this browser’s Rudi notebook.

Record your thinking — optional4 prompts

Use these prompts if they help students capture evidence, decisions, or questions. You do not need to complete every prompt or create an entry at every step.

Predict: Before you begin, what do you expect to observe when you identify LED polarity?Evidence: What evidence best demonstrates this result: The LED lights without bypassing its resistor?Debug: If you encounter “LED stays dark,” what will you check first, and why?Transfer: Where could you reuse the skill from “Build a protected LED output” in another classroom project?

Nothing is saved until you choose this button.

Troubleshooting

LED stays dark

Reverse the unpowered LED, then inspect row placement, resistor placement, D9, and GND.

Built-in LED changes but external LED does not

Confirm the sketch uses D9 rather than LED_BUILTIN and that the jumper is on D9.

2026 middle school standards

Potential CSTA connections

Not reviewed by CSTA

These are evidence-based crosswalk candidates for curriculum review—not a claim of official alignment.

MS-ALG-PS-03

Verify the accuracy of an algorithm for given inputs.

Students test the same output algorithm with known timing inputs and verify the expected physical output.

Lesson evidence: Observed LED timing record

MS-PRO-TR-19

Use systematic strategies to test, refine, and document changes to a computing technology to meet the intended purpose.

Students isolate variables, compare observed behavior with the intended purpose, and document a revision.

Lesson evidence: Current-path trace and revision

View the official 2026 CSTA standardsStandard text: Computer Science Teachers Association (2026), CC BY-NC-SA 4.0.

Supporting concepts

ohms-law · power-and-signals

Sources and review

Arduino Uno R3 documentation Arduino Projects Book · CC BY-NC-SA 3.0

Starting point, not verified curriculum. Review the actual hardware, circuit, code, power requirements, and classroom conditions.

After teaching this lesson