Arduino Electronics: Voltage, Current, Resistance, and Breadboards

Before you begin

Upload and modify the blink sketch in ARD02. This lesson adds an external circuit powered only from an Uno R3’s USB connection.

• Your First Arduino Program: Blink an LED

Three quantities describe different things

A light fails to turn on. Should you increase voltage, remove resistance or add another wire? Without a model, every change is a guess. Three electrical quantities give you a useful starting point.

Voltage, measured in volts, is electrical potential difference between two points. Current, measured in amperes, is the rate of electric charge flow. Resistance, measured in ohms, relates voltage across a resistive component to the current through it. For an ordinary resistor, Ohm’s law is V = I × R.

A water analogy can help: pressure difference resembles voltage, flow resembles current and a narrow pipe resembles resistance. The analogy stops being reliable for semiconductor devices, switching circuits and many time-dependent effects. An LED is a semiconductor diode, not an ordinary resistor.

Ground is the reference node that we call zero volts in this circuit. It is not a magical drain where electricity disappears. Current needs a complete path from the supply through components and back to the supply.

Calculate before connecting

An LED emits light when current flows through it in the intended direction. Its forward voltage depends on its color, construction, current and temperature. Our worked estimate uses a red indicator LED at approximately 2 V, driven from an approximately 5 V output.

With a 330 Ω resistor, the estimated resistor voltage is 5 − 2 = 3 V. The current is 3 / 330 amperes, approximately 0.009 A or 9 mA. This is an estimate, not a measured guarantee; actual output and LED voltages vary.

The resistor dissipates roughly I²R, approximately 0.027 W using that estimate. A standard 0.25 W resistor comfortably exceeds the estimate in this small circuit. Increasing the resistance to 1 kΩ lowers the current and generally reduces brightness.

The Uno R3 pinout specifies 20 mA per I/O pin; chip-wide limits also matter. Our single LED aims well below that figure. Maximum ratings are boundaries to avoid, not design targets. Never replace the resistor with a wire. See the Uno R3 pinout and datasheet for the reference board.

Wire the circuit while unplugged

A solderless breadboard connects groups of sockets internally. On a common board, each group of five holes in a numbered row connects together on one side of the center gap. The two sides do not connect across that gap. Power rails may be split halfway along their length; their colored lines are labels, not proof of continuity.

Disconnect USB. Connect Uno D8 to one end of a 330 Ω resistor. Put the resistor’s other end in the same connected row as the LED’s anode. Connect the LED’s cathode to Uno GND. The longer lead is commonly the anode and the flat edge commonly marks the cathode, but confirm the actual LED if its leads have been trimmed.

Place the two LED legs in different connected rows. If both land in one row, that row electrically bypasses the LED. The resistor can appear before or after the LED in this series path; the same current flows through both.

Check the complete path with your finger and board diagram before restoring USB power. Do not use a multimeter’s current mode directly across a power supply; that creates a low-resistance path. If measuring voltage, use DC voltage mode across the two points of interest.

D8 → 330 Ω resistor → LED anode → LED cathode → GND

Drive the external LED

Change the pin from the onboard LED to D8. The program still uses only the Arduino core, and no extra library is required.

const byte indicatorPin = 8;

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

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

Digital signals and analog quantities

The physical world is not divided neatly into two values. A room’s brightness changes continuously, but a digital input interprets its voltage according to electrical thresholds. An analog input instead converts voltage into a number with finite resolution.

On the Uno R3, analogRead() normally returns an integer from 0 through 1023. That number is not a voltage unit until you apply the reference relationship, and it is not evidence that the measurement is perfectly accurate. ARD05 uses those readings to compare light levels.

Some digital pins support pulse-width modulation, or PWM. They switch on and off quickly with an adjustable duty cycle. On the Uno R3, analogWrite() uses PWM on supported pins rather than producing a smooth adjustable voltage directly. Keep this distinction in mind before attaching a component that expects a true analog signal.

For this project, the staged test is visual: inspect the unpowered circuit, upload with the circuit attached, verify the period and then disconnect power before swapping the resistor. If the LED stays dark, check polarity and row placement before editing correct code.

Important terms

Voltage

Potential difference, measured in volts.

Current

Rate of charge flow, measured in amperes.

Resistance

Opposition to current in a resistive component, measured in ohms.

Series circuit

A path in which the same current passes through successive components.

Anode and cathode

The two terminals of a diode; polarity matters.

PWM

Fast switching with an adjustable fraction of time spent on.

Mini project: Compare two resistor choices

1. Build and verify the D8 LED circuit with 330 Ω.
2. Calculate its approximate current using a 2 V LED drop.
3. Disconnect USB and replace 330 Ω with 1 kΩ.
4. Predict the new current and compare the visible brightness.
5. Record that brightness comparison is qualitative, not a calibrated current measurement.

Common mistakes and debugging

• Putting both LED legs in one connected row: separate them across distinct rows.
• Omitting the resistor because the LED briefly works: excess current can damage the LED or controller.
• Treating ground as optional: the circuit needs a complete return path.
• Assuming analogWrite means a smooth voltage: on the Uno R3 it provides PWM on supported pins.

Independent challenge

Predict the approximate current through a 680 Ω resistor using the same 5 V supply and 2 V LED estimate. Explain why a real measurement may differ.

Check your understanding: 10 questions

1. What units measure voltage, current and resistance?

2. What approximate current flows through 330 Ω with 3 V across it?

3. Can a breadboard’s colored rail marking guarantee continuity?

4. Does resistor position before versus after an LED matter in this series circuit?

5. Why is analogWrite not a direct analog-voltage command on Uno R3?

6. In your own words, what does “Voltage” mean?

7. In your own words, what does “Current” mean?

8. In your own words, what does “Resistance” mean?

9. In your own words, what does “Series circuit” mean?

10. In your own words, what does “Anode and cathode” mean?

Quiz answers

Summary

Electrical quantities and connection paths explain the LED’s behavior. A resistor limits current, correct polarity permits conduction and a complete return path allows the circuit to work.

Continue learning

ARD04 adds a button so a physical input can control the LED.

• Read a Button and Control an LED
• Arduino Sensors: Light, Temperature, Distance, and Motion
• Control Raspberry Pi GPIO Pins Safely

Sources and further reading

• Arduino Uno R3 board documentation
• Arduino built-in examples

Prepared 2026-09-18. Editorial draft; primary documentation consulted. Hardware build not bench-tested; code has not been executed on the reference board.