Mission 15 · Stage 2

Reaction Timer

millis() with if conditions

Measure button reaction time with millis() after a random GO signal.

Pin connections

Part 1		Part 2
Arduino pin 2	→	Button pin 1
Button pin 2	→	Arduino GND
Arduino pin 13	→	Resistor pin 1
Resistor pin 2	→	GO LED anode (+)
GO LED cathode (-)	→	Arduino GND

See it

Time reactions with millis()!

Wait… wait… GO! Press as fast as you can — millis() measures your speed.

Games and sports timers use millis() so other code can still run.

The story

The problem

delay() would freeze everything — you need a clock that keeps ticking.

Think of it like

millis() is a stopwatch that never stops counting milliseconds.

Meet the parts

Runs your sketch

Arduino

Brain

Loading part…

Press when GO lights up

Button

Input

Loading part…

Protects the GO LED

Resistor

Safety

Loading part…

Tells you when to press

GO LED

Signal

Loading part…

How it works

Wait random time

No delay() — we compare millis() to a target time stored in waitUntil.

if (state == WAIT && millis() >= waitUntil)

Show GO

LED turns on and we record the exact moment with goTime.

digitalWrite(GO_LED, HIGH);
goTime = millis();

Measure reaction

Subtract goTime from now to get milliseconds since GO.

reaction = millis() - goTime;

Then loop back to step 1

Build the circuit

Follow these steps in order. Match the wires to the colors shown.

1
Place Arduino
Place the Arduino (uno) on the breadboard.
Arduino placed!
Loading part…
2
Place Button
Place the Button (btn1) on the breadboard.
Loading part…
3
Place Resistor
Place the Resistor (r1) on the breadboard.
Loading part…
4
Place GO LED
Place the GO LED (led1) on the breadboard.
Loading part…
5
Connect Arduino pin 2 to Button (btn1) 1.l
Connect Arduino pin 2 to Button (btn1) 1.l.
6
Connect Button (btn1) 2.l to Arduino GND
Connect Button (btn1) 2.l to Arduino GND.
7
Connect Arduino pin 13 to Resistor (r1) 1
Connect Arduino pin 13 to Resistor (r1) 1.
8
Connect Resistor (r1) 2 to GO LED (led1) anode (+)
Connect Resistor (r1) 2 to GO LED (led1) anode (+).
9
Connect GO LED (led1) cathode (-) to Arduino GND
Connect GO LED (led1) cathode (-) to Arduino GND.

Try it

Open Serial Monitor — wait for GO!, then press fast.
Read your reaction time in milliseconds.

Peek at code

State machine

enum State { WAIT, GO, SCORED };
State state = WAIT;
unsigned long goTime = 0;
unsigned long waitUntil = 0;

State enum tracks WAIT, GO, and SCORED — the program mode changes with if.

scheduleWait()

void scheduleWait() {
  digitalWrite(GO_LED, LOW);
  waitUntil = millis() + random(2000, 5000);
  state = WAIT;
}

Sets a future millis() target for the random wait — non-blocking timing.

Reaction math

void loop() {
  if (state == WAIT && millis() >= waitUntil) {
    digitalWrite(GO_LED, HIGH);
    goTime = millis();
    state = GO;
    Serial.println("GO! Press the button!");
  }
  if (state == GO && digitalRead(BUTTON_PIN) == LOW) {
    unsigned long reaction = millis() - goTime;
    Serial.print("Reaction: ");
    Serial.print(reaction);
    Serial.println(" ms");
    state = SCORED;
    delay(2000);
    scheduleWait();
  }
  delay(10);

loop() uses if on state and millis() to start GO and compute reaction time.

Show full sketch (reaction-timer.ino)

const int BUTTON_PIN = 2;
const int GO_LED = 13;
enum State { WAIT, GO, SCORED };
State state = WAIT;
unsigned long goTime = 0;
unsigned long waitUntil = 0;
void scheduleWait() {
  digitalWrite(GO_LED, LOW);
  waitUntil = millis() + random(2000, 5000);
  state = WAIT;
}
void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(GO_LED, OUTPUT);
  Serial.begin(9600);
  randomSeed(analogRead(0));
  scheduleWait();
  Serial.println("Reaction Timer — wait for GO!");
}
void loop() {
  if (state == WAIT && millis() >= waitUntil) {
    digitalWrite(GO_LED, HIGH);
    goTime = millis();
    state = GO;
    Serial.println("GO! Press the button!");
  }
  if (state == GO && digitalRead(BUTTON_PIN) == LOW) {
    unsigned long reaction = millis() - goTime;
    Serial.print("Reaction: ");
    Serial.print(reaction);
    Serial.println(" ms");
    state = SCORED;
    delay(2000);
    scheduleWait();
  }
  delay(10);
}

Quick quiz

Q1. Where does repeating work belong?

A. loop() ✓
B. setup()
C. pinMode only

Why: Correct—loop() runs again and again.

Q2. Why use millis() instead of delay() while waiting?

A. millis() lets the program check other conditions ✓
B. millis() is louder
C. delay() reads buttons faster

Why: Correct — millis() counts time without freezing the whole sketch.

Code lab — try on your own

Make waits shorter — change random(2000, 5000) to random(1000, 3000) in scheduleWait().
Hint: Line 9.
Comment line 28 explaining reaction = time since GO.
Hint: The reaction = millis() - goTime line.

Code walkthrough

A line-by-line tour of the sketch — the same steps as in Robo Gurukul Studio.

Program overview

Technical

Sketches have globals, then setup() once, then loop() forever.

In this project

Measure button reaction time with millis() after a random GO signal.

Why here

Read from top to bottom. Hover words or lines for help!

const int BUTTON_PIN = 2;
const int GO_LED = 13;
enum State { WAIT, GO, SCORED };
State state = WAIT;
unsigned long goTime = 0;
unsigned long waitUntil = 0;
void scheduleWait() {
  digitalWrite(GO_LED, LOW);
  waitUntil = millis() + random(2000, 5000);
  state = WAIT;
}

setup()

Technical

Runs one time when the board turns on.

In this project

Sets up pins and libraries for Reaction Timer.

Why here

One-time setup belongs here—not in loop().

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(GO_LED, OUTPUT);
  Serial.begin(9600);
  randomSeed(analogRead(0));
  scheduleWait();
  Serial.println("Reaction Timer — wait for GO!");
}

loop()

Technical

Runs again and again after setup() is done.

In this project

This is the main action you see in Reaction Timer.

Why here

Repeating work (blink, read sensors) goes here.

void loop() {
  if (state == WAIT && millis() >= waitUntil) {
    digitalWrite(GO_LED, HIGH);
    goTime = millis();
    state = GO;
    Serial.println("GO! Press the button!");
  }
  if (state == GO && digitalRead(BUTTON_PIN) == LOW) {
    unsigned long reaction = millis() - goTime;
    Serial.print("Reaction: ");
    Serial.print(reaction);
    Serial.println(" ms");
    state = SCORED;
    delay(2000);
    scheduleWait();
  }
  delay(10);
}

Try this: Change numbers in loop(), then compile and run the simulator.

pinMode

Technical

Tells a pin if it listens or drives something.

In this project

Gets the Reaction Timer circuit ready in the simulator.

Why here

Goes in setup() because we only set pins once at the start.

  pinMode(BUTTON_PIN, INPUT_PULLUP);

digitalWrite

Technical

Turns a pin ON or OFF.

In this project

Controls lights, motors, or buzzers in Reaction Timer.

Why here

Goes in loop() so it can keep changing while the program runs.

  digitalWrite(GO_LED, LOW);

digitalRead

Technical

Checks if a pin is ON or OFF.

In this project

Reads buttons or sensors in Reaction Timer.

Why here

Goes in loop() so we can react when something changes.

  if (state == GO && digitalRead(BUTTON_PIN) == LOW) {

analogRead

Technical

Reads a sensor number from 0 to 1023.

In this project

Turns a sensor signal into a number for Reaction Timer.

Why here

Goes in loop() to keep checking the sensor.

  randomSeed(analogRead(0));

delay

Technical

Waits for some time. Nothing else runs during the wait.

In this project

Controls speed so you can see Reaction Timer in the simulator.

Why here

Right after an action that should stay the same for a moment.

    delay(2000);

millis

Technical

Counts milliseconds since the board started.

In this project

Tracks time in Reaction Timer without using delay().

Why here

In loop() when you need to know how much time passed.

  waitUntil = millis() + random(2000, 5000);

random

Technical

Picks a random number in a range.

In this project

Adds randomness in Reaction Timer.

Why here

In loop() when you want different values each time.

  waitUntil = millis() + random(2000, 5000);

begin

Technical

Starts talking to the computer screen (serial monitor).

In this project

Lets Reaction Timer print debug messages.

Why here

Goes in setup() once before any Serial.print.

  Serial.begin(9600);

Technical

Sends text to the serial monitor without a new line.

In this project

Shows values from Reaction Timer on the screen.

Why here

In loop() or setup() when you want to see what the board is doing.

    Serial.print("Reaction: ");

println

Technical

Sends text to the serial monitor and starts a new line.

In this project

Prints one line of output for Reaction Timer.

Why here

In loop() when each reading should appear on its own line.

  Serial.println("Reaction Timer — wait for GO!");

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