Device Note - Traffic Light LED Module

Device Note - Traffic Light LED Module

What It Is

A traffic light module is a small PCB with three LEDs (red, yellow, green) and built-in current-limiting resistors. It replicates the layout of a pedestrian or road traffic signal at a scale that fits a breadboard.

Why It Is Used

Three separately controllable LEDs in a familiar real-world form factor make it easy to introduce sequenced output, state machines, and colour-coded status indicators without wiring individual components.

Pin Overview

This module uses a 4-pin header:

• GND — ground
• R — red LED control (active high)
• Y — yellow LED control (active high)
• G — green LED control (active high)

Connection in this course:

• R -> PB12
• Y -> PB13
• G -> PB14

All three are standard GPIO output pins on the Bluepill.

Control Behavior

Each pin drives one LED independently. Commands in this course treat them as exclusive states:

| Command | Effect | |---|---| | TRAFFIC_RED_ON | Red on, yellow off, green off | | TRAFFIC_YELLOW_ON | Yellow on, red off, green off | | TRAFFIC_GREEN_ON | Green on, red off, yellow off | | TRAFFIC_ALL_OFF | All LEDs off |

The current traffic light state streams in the compact USB telemetry frame every 120 ms as traffic_light (0 = off, 1 = red, 2 = yellow, 3 = green).

Wiring Rules

• Resistors are built into the module — do not add external series resistors or the LEDs will be very dim.
• Active high: set the GPIO pin HIGH to turn the LED on.
• All three LEDs share the same GND connection on the module.

Common Failure Modes

• GND not connected — no LEDs light regardless of pin state.
• Pin and LED colour swapped — wrong LED turns on for a given command; check R/Y/G -> PB12/PB13/PB14 mapping.
• Module connected to 5 V GPIO directly — the STM32 PB pins are 5 V tolerant, but running the module from 5 V logic without 3.3 V supply may over-drive the LEDs; keep at 3.3 V.
• All LEDs glow faintly — floating GPIO pin; set the pin as push-pull output, not input.

Classroom Notes

• The familiar traffic light sequence (red → yellow → green → yellow → red) is an excellent introduction to state machines and time-sequenced automation.
• Challenge students to replicate the UK highway code sequence using the automation tool.
• Ask "why does a real traffic light not just have one wire?" — leads into discussion of independent control lines vs multiplexing.
• The traffic_light telemetry value (0–3) is a good example of encoding state as a number.

Integration With This Course

• Pairs well with Lesson 2 (LEDs & GPIO & Blinking) — first LED control exercise.
• Pairs well with Lesson 3 (Commands, Modes & State) — introducing named commands and state reporting.
• Pairs well with Lesson 10 (Automation Tool & Control Logic) — sequencing traffic light states using automation steps and delays.
• Telemetry key: traffic_light (0 = off, 1 = red, 2 = yellow, 3 = green)
• Automation variable token: {traffic_light}