Unit A - Foundations

01. Safety, Electricity & Breadboards

Work safely with ELV DC, use Ohms law for LED resistors, and read a breadboard correctly.

Estimated time 2-3 hours

Learning outcomes

Explain voltage, current and resistance and state Ohms law
Calculate and choose a safe series resistor for an LED
Describe breadboard terminal rows and power rails (including mid-rail splits)
Use a multimeter for voltage, and avoid dangerous current/resistance mistakes
Build and test a blinking LED on D8 with common GND

Parts and preparation

Arduino Uno, USB data cable, breadboard, red LED, 220-330 ohm resistor, jumper wires and a digital multimeter.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Safety, Electricity & Breadboards instructional connection diagram

Why this lesson comes first

Before programming, you need safe habits and a working first circuit. This lesson introduces the three electrical quantities you will use constantly, shows how to protect an LED and an Uno pin with a resistor, and explains how a breadboard connects holes.

Flow from Uno D8 through 330 ohm and LED to GND — Safe path: pin -> resistor -> LED -> GND.

Voltage, current and resistance

Voltage (V) is electrical potential difference - how hard the supply pushes. Current (I) is the flow of charge. Resistance (R) opposes that flow.

Ohms law links them: V = I * R. Rearrange as I = V / R or R = V / I when you know two values.

Ohms law triangle with V I and R — Know two quantities, calculate the third.

Quantity	Symbol	Unit
Voltage	V	volt (V)
Current	I	ampere (A) - often mA in labs
Resistance	R	ohm

Choosing an LED series resistor

An LED needs a series resistor so current stays within safe limits for the LED and the Uno pin (design around 20 mA or less per pin on classic Uno R3).

Course example: 5 V pin, red LED drop about 2 V, target 10 mA (0.010 A). Voltage across the resistor is 5 - 2 = 3 V. R = 3 / 0.010 = 300 ohm. Use the next common value 330 ohm - slightly safer and a little dimmer.

Never connect an LED directly from a pin to GND.

Steps from 5 V and LED drop to 300 ohm then choose 330 ohm — Leftover volts across R, divided by target current.

// Target ~10 mA with ~2 V LED on a 5 V pin
// R = (5.0 - 2.0) / 0.010 = 300 ohm -> use 330 ohm

LED polarity and course wiring

LEDs only conduct usefully in one direction. The long leg is usually the anode; the short leg / flat side is usually the cathode.

Course wiring for this lesson: D8 -> 330 ohm -> LED anode; LED cathode -> GND.

D8 to 330 ohm to LED anode with cathode returning to GND — Pin, resistor, LED, ground return. Reverse the LED once if it never lights.

Breadboard connections

On a typical solderless breadboard, the five holes in one terminal strip letter-row group are connected. The next letter row is a different connection - that is how you place a resistor so each end is in a different node.

Long side rails distribute power lengthwise. Many boards split those rails in the middle - use continuity mode to check before you assume + or GND runs the full length.

Power rails and a five-hole terminal row on a breadboard — Five holes per node. Rails may be split - verify on your board.

Multimeter without drama

Voltage: measure in parallel across two points (for example D8 to GND while HIGH). Current: measure in series - you must break the circuit; never put the meter across a supply on current mode. Resistance / continuity: power off first; useful for checking breadboard rails and dead jumpers.

Three cards for voltage parallel, current series, resistance power off — Voltage parallel. Current series. Resistance only when unpowered.

Safety rules for this course

Use only extra-low-voltage DC. Switch power off before rewiring. Never measure resistance on a powered circuit. When measuring current, place the meter in series - never directly across a supply.

Keep fingers and metal tools clear of shorting adjacent header pins while powered.

Do	Do not
Power off before moving wires	Rewire live circuits
Share GND with the Uno	Float sensor grounds
Use a series resistor with LEDs	Connect LED pin-to-GND bare
ELV DC only	Connect mains electricity

Wiring and safe build sequence

Uno D8 -> 330 ohm resistor (one end)
Other resistor end -> LED anode (long leg)
LED cathode (flat / short leg) -> GND
Confirm resistor and LED share connected breadboard rows correctly (different nodes for each lead)

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

const byte ledPin = 8;

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

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

How the code works

pinMode configures pin 8 as an OUTPUT before you drive it.
HIGH is about 5 V on an Uno; LOW is about 0 V.
The resistor limits LED and pin current - it is not optional.
delay(500) makes a clear blink for this first build; later lessons replace long delays when many tasks must run.

Test and record evidence

Expected result: The LED flashes about twice per second. With a multimeter on voltage mode, D8 to GND is about 5 V while HIGH.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Reverse the LED if it never lights (polarity).
Confirm the resistor and LED meet in a connected breadboard node, and return to a real Uno GND.
Dim or dead: try a known-good LED and check for a mid-rail power split.
Board not responding: use a USB data cable and select board/port (Lesson 03).

Extension challenge: Calculate a resistor for a 3.0 V blue LED at 8 mA from a 5 V output, then choose the next higher common resistor value and explain why you rounded up.

Check your understanding

Q1. State Ohms law.

Show answer

V = I * R (voltage equals current times resistance).

Q2. Why does an LED need a series resistor?

Show answer

To limit current through the LED and the Arduino pin.

Q3. How are breadboard terminal holes usually connected?

Show answer

In groups of five along a strip; power rails run lengthwise and may be split.

Q4. How should you measure voltage with a meter?

Show answer

In parallel across the two points of interest.

Q5. Why is current mode across a supply dangerous?

Show answer

The meter is nearly a short - it can blow a fuse or stress the supply.