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LED Anatomy

Run the LED Anatomy MicroSim fullscreen

About This MicroSim

An LED only works one way round. Put it in backwards and nothing happens — no smoke, no error, no clue. That silent failure is why this diagram exists.

There are two independent markings for the same thing, which is deliberate on the manufacturer's part:

Leg length. The longer leg is the anode (positive, toward +). The shorter is the cathode (negative, toward ground). Reliable until somebody trims the legs to fit a breadboard neatly, at which point it is gone forever.

The flat edge. One side of the round plastic rim is flattened, and that flat is always on the cathode side. It survives trimming, and it is the one to teach as the primary check — the leg length is the convenient shortcut, the flat is the fallback that always works.

A useful mnemonic that holds for both: the cathode is the cut-short leg on the chamfered flat side.

The dome is worth a mention too. It is not just a coloured cover — it is a lens, shaped to focus the light the semiconductor die emits into a beam. That is why an LED looks much brighter head-on than from the side, and why a diffused LED and a clear one of the same colour behave differently in a project.

How to Use

  • Hover any numbered marker on the illustration, or any label in the panel, to read what that part is and what it does.
  • Compare markers 3 and 4. The right-hand leg is visibly longer — that is the anode.
  • Find the flat on the rim at marker 2. It is on the same side as the short leg, and it is the marking that survives having the legs trimmed.
  • Switch to Quiz to be asked instead of told.

Lesson Plan

Bloom level: Remember (L1) -- identify

Learning objective: Given a zoomed illustration of an LED, the learner identifies the anode, cathode, and flat-edge marking used to orient it correctly.

Before (2 min). Hand out real LEDs and ask which way round they go. Most groups find the leg-length difference on their own, which is the right starting point.

With the diagram (5 min). Name all four parts, then ask the question that matters: what do you do once the legs are trimmed? That is what sends people looking for the flat.

After (10 min). Build the circuit with the LED deliberately backwards first. Nothing happens, nothing breaks, and the group has now met the failure mode in a controlled way rather than in the middle of debugging something else.

Check for understanding. Ask: "Your LED is in the breadboard with both legs cut to the same length and it will not light. What do you check?" The flat on the rim, and then whether the resistor and the loop are right. A student who says "the longer leg" has not noticed the legs are now identical.

Embedding This MicroSim

Paste this into any page of the book, adjusting the relative path to docs/sims/ for the page's depth:

<iframe src="../../sims/led-anatomy/main.html" width="100%" height="650" scrolling="no"></iframe>

Specification

The full specification below is extracted from Chapter 16: Physical Computing and Electrical Safety Basics.

Type: infographic
**sim-id:** led-anatomy<br/>
**Library:** Interactive Infographic Overlay (diagram.js, callout engine)<br/>
**Status:** Specified

Purpose: Let a learner identify anode, cathode, and the flat-edge marking on a zoomed illustration of a real LED before handling one.

Bloom Taxonomy: Remember (L1)
Bloom Taxonomy Verb: identify

Learning objective: Given a zoomed illustration of an LED, the learner identifies the anode, cathode, and flat-edge marking used to orient it correctly.

Image style: Close-up flat illustration of a single 5mm LED, legs extended, with a visible length difference between the two legs and a flat edge on the dome

Image dimensions: 800x600 (landscape)

Callouts (4 total): 1. "Anode (longer leg)" color #4A90D9; 2. "Cathode (shorter leg)" color #E67E22; 3. "Flat edge" on the dome, color #9B59B6; 4. "Light-emitting dome" color #F5A623

Interactive features: Hover or click each callout to reveal a one-sentence explanation; a "Flip It" button shows the same LED reversed so learners practice recognizing orientation from either side

Implementation: Interactive Infographic Overlay Guide (callout engine) — `diagram.js` + `style.css` render numbered markers over the generated LED illustration

References