Hardware 1-1: Traffic Light Controller

Girls Code Lincoln, Hardware track 1 (Embedded Systems), Lesson 1.

You are building a working traffic signal on a Raspberry Pi: LEDs on GPIO pins, a button that a "car" presses, and logic that must never let two directions go at once. Slides for this lesson are Hardware 1-1_ Intro to Microcontrollers.pdf.

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Parts

Part	Quantity	Notes
Raspberry Pi (3, 4 or 5) with Raspberry Pi OS	1	Set up and booted to the desktop
Breadboard	1	Half-size is plenty
Red / amber / green LED	2 of each	6 total for the full intersection
330 Ω resistor	6	One per LED, never skip it
Momentary push button	1	Any 4-pin tactile switch
Jumper wires (male-to-female)	~14	Pi header to breadboard
Multimeter	1 per table	For the troubleshooting section

Wiring

Each LED: GPIO pin → 330 Ω resistor → LED long leg (anode). LED short leg (cathode) → ground. The button goes from GPIO 26 to ground; the Pi's internal pull-up resistor handles the rest, so no extra resistor is needed there.

Signal	GPIO (BCM)	Physical pin
Main red	17	11
Main amber	27	13
Main green	22	15
Cross red	5	29
Cross amber	6	31
Cross green	13	33
Button	26	37
Ground	GND	6, 9, 39

All seven numbers live in pins.py and nowhere else. If you wire a pin differently, change it there and every script follows.

Run it

cd hw-led
python3 step1_blink.py

gpiozero ships with Raspberry Pi OS. If yours is missing it, pip3 install -r requirements.txt. Press Ctrl+C to stop any script; they all turn the LEDs off on the way out.

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Today's objectives

From the slides, in order:

1. Cycle a light. Green, amber, red in the correct order. Timing is your call.
2. Add a button. A car pulls up and asks for the light to change. Handle button bounce.
3. Add cross traffic. A second trio of LEDs that switches with the first and never creates an unsafe condition.

The ladder

Work down this list. Each file runs on its own and builds on the one above it.

File	Teaches	Objective
step1_blink.py	One pin, one LED, on and off	Wiring check
step2_all_three.py	All three LEDs, one state at a time	Wiring check
step3_traffic_cycle.py	Timed phases in a bounded loop	1
step4_button_test.py	Reading an input, and seeing bounce	2
step5_button_request.py	Button changes the light	2
main.py	Two lights plus a safety interlock	3
step6_pwm_fade.py	PWM brightness and night mode	Bonus
pins.py, lights.py	Shared pin map and light helpers	Imported by the rest

main.py is the reference solution. Try objective 3 yourself from step5_button_request.py before you open it.

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Lesson pieces

1 — A pin is a voltage, not a light

An output pin sets voltage; an input pin reads it. Above 2.3 V is True, below 1 V is False, and in between is where bugs live. led.on() is just "put 3.3 V on this pin", and the LED is what makes that visible.

Mini-task: change MAIN_RED in pins.py to another pin, move the jumper, run step1_blink.py.

Real-world tie: the same read-a-voltage step sits under thermostats, smoke detectors, fuel gauges, and every limit switch on a factory line.

2 — State, not commands

lights.py names four states: RED, AMBER, GREEN, OFF. apply_state() sets all three LEDs from one state, so it is impossible to leave two colours lit by forgetting a line.

Mini-task: add a FLASHING_RED state for a four-way stop.

Real-world tie: state machines run elevators, vending machines, ATM screens, and the checkout flow on any website you have used.

3 — Timing is the program

step3_traffic_cycle.py is phases and delays, nothing more. Change GREEN_SECONDS and the whole intersection feels different. Note MAX_CYCLES: no loop in this repo runs forever, so a script someone forgets on the bench stops on its own.

Mini-task: make the green twice as long as the red, then swap it.

Real-world tie: control loops with a fixed tick are how cruise control, PLCs on an assembly line, and heart-rate monitors work.

4 — Bouncing is real

A push button's metal contacts physically bounce, so one press can register as five. Run step4_button_test.py, press exactly five times per round, and compare the counts. Round 1 counts every raw edge; round 2 ignores anything within 50 ms of the last press.

step5_button_request.py hands the same job to the library with Button(BUTTON, bounce_time=0.05). Now you know what that argument is actually doing.

Mini-task: lower DEBOUNCE_SECONDS to 0.001 and see the bouncing come back.

Real-world tie: debouncing applies to keyboards, elevator call buttons, coin acceptors, and the retry logic in network code.

5 — Minimum green is a safety rule

hold_green() waits the minimum green before it will look at the button. A car already moving through the intersection needs that time. Cutting a green short on demand is what real actuated signals do, and the minimum is why they don't cause crashes.

Mini-task: set MINIMUM_GREEN_SECONDS = 0.1 and press the button repeatedly. Watch the light become useless.

Real-world tie: minimum dwell times guard elevator doors, garage doors, press brakes, and railway crossing gates.

6 — The interlock

is_safe() returns False whenever both directions are green or amber. set_intersection() calls it and raises rather than lighting an unsafe pair. The program crashes instead of causing a wreck.

Amber counts as "go" on purpose: a car already in the box is still crossing. The all-red gap between phases is there for the same reason.

Mini-task: in main.py, call set_intersection(main_lights, cross_lights, GREEN, GREEN) and watch it refuse.

Real-world tie: interlocks stop a microwave running with its door open, a lathe spinning with the guard up, and two trains entering one section of track.

7 — PWM (bonus)

step6_pwm_fade.py fades an LED. The pin is still only ever fully on or fully off; it just switches fast enough that your eye averages it. That ratio is the duty cycle. The slides' motor example is the same trick at a different scale.

Mini-task: dim every LED to 20 % for a "night mode" version of main.py.

Real-world tie: PWM drives motor speed, servo position, LED dimming, and laptop fan control.

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Troubleshooting with a multimeter

Symptom	Check	What to expect
LED never lights	Continuity mode across the jumper	Beep. No beep means a bad wire
LED never lights	LED direction	Long leg toward the resistor and the GPIO pin
LED never lights	DC volts, GPIO pin to ground, while the script says "on"	About 3.3 V
LED is very dim	Resistor value	330 Ω, not 3.3 kΩ. Check the colour bands
Button does nothing	Continuity across the button, pressed and released	Beep only when pressed
Button acts pressed constantly	Which pins of the button you used	Tactile switches connect in pairs; use diagonal corners

ImportError or a pin-factory error on a Pi 5 usually means lgpio is missing: sudo apt install python3-lgpio.

Pick an extension

• Pedestrian crossing. A second button and a walk/don't-walk LED that only runs during all-red.
• Latched request. Remember a press that happens during the cross-street phase instead of dropping it.
• Left turn arrow. A fourth LED and a protected-turn phase, without breaking is_safe().
• Rush hour mode. Longer main-street greens between 4 and 6 pm using datetime.
• Emergency vehicle preempt. Hold everything red, flash the main red, then recover safely.
• Fault detection. Refuse to start if any state would leave all three LEDs dark.

Learning goals checklist

By the end you should be able to:

• Explain the difference between a microcontroller and a single-board computer
• Wire an LED with the correct resistor and polarity, and a button with a pull-up
• Set and read a GPIO pin from Python
• Describe a system as named states with timed transitions
• Explain what switch bounce is and two ways to handle it
• Write a guard that refuses an unsafe combination instead of trusting the caller
• Explain how PWM fakes an analog output
• Find a broken connection with a multimeter

Suggested three-hour block

Time	What
0:00 – 0:35	Slides: embedded systems, microcontroller vs computer, GPIO, PWM
0:35 – 0:55	Breadboard the main light, multimeter check, step1 and step2
0:55 – 1:25	Objective 1: step3
1:25 – 1:35	Break
1:35 – 2:05	Objective 2: step4 and step5
2:05 – 2:50	Objective 3: build it, main.py as reference
2:50 – 3:00	Demo each table's intersection