LED - Diode Test and Signal Analysis

The multimeter was set to diode mode to verify several LEDs. Most illuminated, confirming polarity and basic functionality. One red LED did not turn on as expected, so we checked conditions more carefully.

LED Forward Voltage by Color

LEDs have a forward voltage (Vf) depending on color/material. Red/orange/yellow often work around 2.0–2.2V, while blue/white usually need 2.8–3.7V. Even if voltage is enough, the LED may not light if current is too low. A series resistor is required to keep current safe and stable.

Simulation Exercises

These simulations were done first to confirm how the microcontroller reads inputs and controls outputs. It helped avoid mistakes before designing the final PCB.

Exercise 1 – Push Button Controlling an LED

Basic test: when the button is pressed, the LED turns ON; when released, it turns OFF.

const int led = 9;
const int boton = 10;

void setup() {
  pinMode(led, OUTPUT);
  pinMode(boton, INPUT);
}

void loop() {
  int estado = digitalRead(boton);

  if (estado == HIGH) {
    digitalWrite(led, HIGH);
  } else {
    digitalWrite(led, LOW);
  }
}

Exercise 2 – Displaying Button State on an I2C LCD

Added an I2C LCD to show the button state and LED state. This demonstrates I2C communication and user feedback on a display.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int led = 9;
const int boton = 10;

LiquidCrystal_I2C lcd(0x27,16,2);

void setup() {
  pinMode(led, OUTPUT);
  pinMode(boton, INPUT);

  lcd.init();
  lcd.backlight();
}

void loop() {
  int estado = digitalRead(boton);
  digitalWrite(led, estado);

  lcd.setCursor(0,0);
  lcd.print("Boton: ");
  lcd.print(estado ? "ON " : "OFF");

  lcd.setCursor(0,1);
  lcd.print("LED: ");
  lcd.print(estado ? "ON " : "OFF");

  delay(100);
}

Exercise 3 – Potentiometer Controlling LED Brightness

Used a potentiometer as an analog input. The read value is mapped to PWM to change LED brightness.

const int led = 9;
const int pot = A0;

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

void loop() {
  int valor = analogRead(pot);
  int brillo = map(valor, 0, 4095, 0, 255);
  analogWrite(led, brillo);
}

Custom PCB Shape Design (Mjolnir)

I created a custom PCB outline inspired by Mjolnir. The decorative lines are part of the visual design, while all electronic components remain inside the functional area.

Components used: XIAO SAMD21 (main microcontroller), LED (output indicator), push button (user input), R1 (current limit for LED), R2 (button stability / avoid floating), and power nets (5V, 3V3, GND).

Design Rule Check (DRC)

When running DRC, warnings appeared because some decorative lines were not connected to any net. The tool reads them like traces, so it reports them as “not connected”.

After confirming the real circuit connections were correct (LED + button + power), the warning list was cleared using Clear All.

3D Visualization

Finally, I used the 3D view to confirm the final size and check that the components fit well inside the outline.

Initial PCB Design and Issues

My first idea was a larger board to use leftover copper (less waste). But I changed to a smaller board because the first version still removed too much copper during milling.

I had two main problems: (1) I confused some pin header assignments, and (2) while labeling nets, I accidentally deleted one label, which caused missing connections.

PCB Layout and Detected Errors

Because of the wrong pin mapping and the missing label, the PCB showed airwires (unrouted connections) and multiple DRC messages. These errors basically mean: “there are nets that should be connected, but they are not routed or not recognized”.

Correcting the PCB Connections

I fixed the pin assignments and restored the missing net label. After that, I routed the traces again until all required nets were connected properly.

Dimension Verification

In the 3D environment, I measured the board to confirm the final dimensions and verify it fits fabrication constraints.

Final PCB Design

The final result is a compact development board that reduces copper waste while keeping all required connections for the XIAO and external headers.

Potentiometer Test (Resistance and Voltage Range)

A potentiometer was tested to understand how its resistance changes with rotation and how it can be used either as a variable resistor or as a voltage divider.

Resistance Measurement (Ohms)

When the wiper was rotated fully to one side, the measured resistance dropped close to 0 Ω. When rotated to the opposite extreme, the reading reached approximately 250 kΩ.

At intermediate knob positions, values around 140 kΩ and 110 kΩ were observed, confirming continuous adjustment across its full range.

Voltage Divider Behavior (Max = 3.3 V)

When used as a voltage divider, the output at the wiper varies from 0 V up to the supply voltage. Since the system was supplied with 3.3 V, the maximum measured output was also 3.3 V.

Video Demonstration

Simulation – LED Voltage/Current vs Supply Voltage

A series of simulations were performed to observe how LED current and voltage behave when changing the power supply and resistor conditions.

Case 1 – 2×AA Batteries (≈3.0 V)

Case 2 – 3×AA Batteries (≈4.5 V)

Case 3 – Regulated Bench Supply (5.0 V)

Regulated 5V: stable readings and controlled LED current.

Case 4 – High Voltage (30 V) Without Proper Current Limiting

30V without proper current limiting: overcurrent and LED failure in simulation.

Case 5 – High Voltage (30 V) With Current Limiting (2 kΩ)

30V + 2 kΩ: current reduced to safe values and LEDs operate normally.