Traffic Light Controller

The finish line

What you will build

The model cycles through valid light states, accepts one pedestrian request, and never illuminates red and green together.

Before you build

Materials, tools, and safety

Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult guidance recommended for wiring and cutting.

Build it

Step-by-step instructions

1. Step 1

   Plan valid states

   Draw red, red-yellow transition if desired, green, yellow, and pedestrian red states.

   Choose safe durations.

2. Step 2

   Build the light housing

   Place LEDs vertically with labels and opaque dividers.

   Keep leads from touching.

3. Step 3

   Add current resistors

   Connect each anode to its own 220-330 Ω resistor and digital pin.

   Connect cathodes to ground.

   Builder checkpoint: After add current resistors, the first subassembly should stay aligned when handled gently.

4. Step 4

   Wire the button

   Connect it between pin 2 and ground using the internal pull-up.

   Label pressed as LOW.

   Watch for: If this stage binds or drifts, inspect blocking delays before adding more parts.

5. Step 5

   Test one LED at a time

   Upload a short output test and verify color-to-pin mapping.

   Correct wiring with power disconnected.

6. Step 6

   Program the states

   Use millis timing and one function that sets all three LEDs.

   Prevent conflicting outputs.

   Builder checkpoint: After program the states, operate the build slowly and confirm that safe red-yellow-green LED sequence begins without binding.

7. Step 7

   Add the request flag

   Record button presses and serve the request at the next safe transition.

   Clear the flag afterward.

8. Step 8

   Audit ten cycles

   Log every state and press the button at different moments.

   Confirm no unsafe color combination.

   Builder checkpoint: At the final checkpoint, The model cycles through valid light states, accepts one pedestrian request, and never illuminates red and green together.

See the engineering

Why it works

Input

elapsed time and pedestrian button event

Output

safe red-yellow-green LED sequence

Motion

digital events-to-visible state changes

Energy losses

button bounce, timing drift, wiring errors, blocking delays

enum State { GREEN, YELLOW, RED };
const int redLed=8, yellowLed=9, greenLed=10, buttonPin=2;
State state=GREEN; unsigned long stateStarted=0; bool request=false, lastButton=false;
void setLights(bool r,bool y,bool g){ digitalWrite(redLed,r); digitalWrite(yellowLed,y); digitalWrite(greenLed,g); }
void enter(State next){ state=next; stateStarted=millis(); if(state==GREEN)setLights(0,0,1); if(state==YELLOW)setLights(0,1,0); if(state==RED)setLights(1,0,0); }
void setup(){ pinMode(redLed,OUTPUT); pinMode(yellowLed,OUTPUT); pinMode(greenLed,OUTPUT); pinMode(buttonPin,INPUT_PULLUP); enter(GREEN); }
void loop(){
  bool pressed=!digitalRead(buttonPin); if(pressed && !lastButton) request=true; lastButton=pressed;
  unsigned long elapsed=millis()-stateStarted;
  if(state==GREEN && (elapsed>=8000 || (request && elapsed>=3000))) enter(YELLOW);
  else if(state==YELLOW && elapsed>=2000) enter(RED);
  else if(state==RED && elapsed>=(request?8000UL:6000UL)){ request=false; enter(GREEN); }
}

Make it behave

Test, troubleshoot, and tune

Keep experimenting

Try another version

Build together

Classroom and access options

Reflect on the design

1. How did button press timing change the measured result?
2. Where did button bounce affect the build most strongly?
3. What evidence shows that finite-state control explains the motion?
4. Which change would improve safe red-yellow-green LED sequence without creating a new problem?