Lab 7.2: External LED

~60 min. Wire an external LED to a digital pin on the R4. Modify the Blink sketch to control it. Demonstrate that you can apply weeks 1-6 to a microcontroller project.

Goal: bridge from the built-in LED (which has its own internal resistor) to an external LED that needs its own current limiter; verify your week-5 resistor-sizing skill

Estimated time: 60 minutes

Prerequisites: lab 7.1 complete; lab 5.1 complete (LED current control)

Steps

Step 1: Plan the circuit (10 min)

You want an external LED to blink when the Arduino sets a pin HIGH. Pick:

• Pin: 8 (a digital-only pin; not PWM-capable, which is fine for blinking)
• LED color: red (low forward voltage; common kit color)
• Resistor: at 5 V supply (which is what the pin outputs when HIGH), with 2 V LED drop and 15 mA target: R = (5 - 2) / 0.015 = 200 Ω. Round up: 220 Ω (a standard value)

Sketch the schematic: pin 8 → 220 Ω → LED anode; LED cathode → ground

Step 2: Wire on the breadboard (15 min)

From R4 pin 8: jumper to breadboard tie-strip. From that tie-strip: through the 220 Ω resistor to another tie-strip. From that tie-strip: LED anode (long lead) to that tie-strip; LED cathode (short lead) to a different tie-strip; that tie-strip to R4's GND pin via jumper

Triple-check: the LED's anode is the longer lead and goes on the +5V (pin 8) side; the cathode goes to ground

Step 3: Modify the Blink sketch (10 min)

Open the sketch from lab 7.1. Change LED_BUILTIN to 8 in both pinMode and digitalWrite calls. The sketch becomes:

void setup() {
  pinMode(8, OUTPUT);
}
void loop() {
  digitalWrite(8, HIGH);
  delay(500);
  digitalWrite(8, LOW);
  delay(500);
}

Save as blink-pin8.ino. Upload

Step 4: Verify (10 min)

The external LED on the breadboard should blink at 1 Hz. The built-in LED should be off (since the sketch no longer drives LED_BUILTIN)

If not working:

• Is the LED polarity correct? (Long lead toward the pin)
• Is the resistor value reasonable? (220 Ω works at 5 V; 1 kΩ also works but is dimmer; 0 Ω is dangerous and damages the LED)
• Is the ground connection good? (Probe with multimeter: pin 8 should be 5 V when HIGH and 0 V when LOW)

Step 5: Add a second LED (15 min)

Wire a second LED + resistor to pin 9. Modify the sketch to blink both LEDs alternately (one on while the other off). Upload. Confirm both work

void setup() {
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
}
void loop() {
  digitalWrite(8, HIGH);
  digitalWrite(9, LOW);
  delay(500);
  digitalWrite(8, LOW);
  digitalWrite(9, HIGH);
  delay(500);
}

Expected output

• External LED blinking on pin 8
• Second LED alternating on pin 9
• Confirmation that week 5 resistor-sizing applies to Arduino projects

Common pitfalls

• Forgetting the resistor: an LED directly on a 5 V pin will burn out quickly. Always include the current-limit resistor. The R4's pin can source up to 8 mA per pin reliably; an unlimited LED can easily exceed that
• Wrong pinMode: forgetting pinMode(8, OUTPUT) means the pin is in default INPUT mode. digitalWrite has no effect. Always set the mode in setup()
• Short between pin and ground: a wire that accidentally shorts a HIGH-driven pin to ground draws too much current and can damage the pin. Verify your breadboard before powering on

Stretch (optional)

• Wire three LEDs on three pins; cycle through them in a chase pattern (one at a time, moving left)
• Wire an RGB LED (if your kit has one) on three pins. Cycle through red, green, blue colors. Combine for white (all three on simultaneously)
• Replace one of the LEDs with the kit's buzzer (with its current-limit resistor if any). The buzzer beeps in sync with the blinking. Loud; consider time of day

Lab 7.2 v0.1. The first time your microcontroller controls external hardware that you wired yourself.