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Electrical Circuits · Measurement and Safety

Breadboard circuits

Reading40mLesson 2 of 3 in this module

Size a resistor for an LED, build the circuit on a breadboard and check it with your meter.

Why it matters

A breadboard lets you build and change a circuit without soldering. Engineers prototype on them, physics labs use them for experiments, and trade schools use them to teach how series and parallel connections look in real life. Building an LED circuit is the classic first build because it combines Ohm's law, polarity and measurement in one small, safe project.

How a breadboard is connected inside

Under the plastic, metal clips join groups of holes:

  • Terminal strips: in the main area, each short row of five holes (often labelled a–e and f–j) is connected. The two halves are separated by a centre channel, so a–e is not connected to f–j.
  • Power rails: the long lines along each edge, usually marked red (+) and blue or black (−), are connected along their length. On some full-size boards the rails are split in the middle; check with your meter's continuity setting.

Two component legs in the same five-hole row are connected. Legs in different rows are not, unless a wire joins them.

The LED

A light-emitting diode (LED) lets current flow in one direction only and glows when it does.

  • The anode (+) is the longer leg; the cathode (−) is the shorter leg, often next to a flat edge on the rim.
  • An LED has a roughly fixed forward voltage (Vf) when lit: about 1.8–2.2 V for red, yellow and green, about 3.0–3.3 V for blue and white.
  • An LED does not limit its own current. Connected straight to a battery, it draws too much current and burns out. Always use a series resistor.

Sizing the resistor

The resistor takes the voltage the LED does not use:

R = (Vsupply − Vf) / I

Typical small LEDs are happy at 5–15 mA (check your LED's datasheet; 20 mA is often the maximum).

Example 1: two AA cells (3.0 V), red LED (Vf ≈ 2.0 V), target 10 mA.

R = (3.0 − 2.0) / 0.010 = 100 Ω

Example 2: 9 V battery, red LED, target 15 mA.

R = (9 − 2.0) / 0.015 = 467 Ω  → use the next standard value up, 470 Ω
Resistor power = I² × R = 0.015² × 470 = 0.106 W  (a 0.25 W resistor is fine)

Choosing the next value up keeps the current a little lower, which is the safe direction.

Building the circuit

  1. Battery holder + to the red rail, − to the blue rail (connect the battery last).
  2. Resistor from the red rail to an empty row, say row 10.
  3. LED anode (long leg) in row 10; cathode in row 12.
  4. A jumper wire from row 12 to the blue rail.
  5. Connect the battery. The LED should light.

Electrically this is a simple series loop: battery → resistor → LED → back to battery.

Measuring it

  • Voltage across the LED: red probe on the anode row, black on the cathode row. Expect about 2 V for red.
  • Voltage across the resistor: expect the remainder, for example 3.0 − 2.0 = 1.0 V.
  • KVL check: the two drops should add up to the battery voltage measured under load.
  • Current: either measure in series (break the loop at the jumper and insert the meter in mA mode) or calculate it from the resistor's voltage: I = V_R / R. The second way is safer for the meter and usually accurate enough.

Troubleshooting a dark LED

  • LED backwards? Swap its legs (a reversed LED at these voltages just stays dark).
  • Both legs in the same row? Then the LED is shorted out.
  • Resistor and LED not sharing a row? Use your meter's continuity mode to check the path.
  • Battery flat or rail split? Measure the voltage at the rails.

Common mistakes

  • No resistor. The LED flashes brightly and dies.
  • Placing a part across the centre channel the wrong way. Components that must connect need a shared row.
  • Assuming the rails run the whole length. Check for a split.
  • Leaving the battery connected while rebuilding. Disconnect it before moving parts.

Safety

This lesson uses 3 V or 9 V batteries only. Never use a wall adapter you have opened, a mains supply or a lithium-ion pack for these builds. Lab bench power supplies are used only in a school or workplace lab with your instructor.

Recap

  • Rows of five are connected; rails run along the edges; the centre channel separates the halves.
  • LEDs have polarity and a forward voltage; they always need a series resistor: R = (Vs − Vf) / I.
  • Measure the drops and check that they add up to the supply.

Try it

Open the Do tab and build and measure your LED circuit, or build it in a simulator if you have no parts yet.

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