LED Reaction Timer

The finish line

What you will build

The timer records five valid reaction trials in milliseconds, detects early presses, and reports an average and best time.

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

   Build the cue circuit

   Connect the LED through its resistor and verify polarity.

   Point it toward the player without glare.

2. Step 2

   Wire the button

   Connect between pin 2 and ground with internal pull-up enabled.

   Mount it firmly.

3. Step 3

   Test input edges

   Print button state and confirm one press changes HIGH to LOW.

   Add a short debounce interval.

   Builder checkpoint: After test input edges, the first subassembly should stay aligned when handled gently.

4. Step 4

   Create random waiting

   Choose a 2-5 second random delay after a ready message.

   Keep the LED off.

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

5. Step 5

   Detect false starts

   If the button is pressed before the cue, cancel the trial.

   Require full release before reset.

6. Step 6

   Measure valid response

   Store millis when LED turns on and when a new press arrives.

   Subtract and print the result.

   Builder checkpoint: After measure valid response, operate the build slowly and confirm that measured reaction time in milliseconds begins without binding.

7. Step 7

   Collect five trials

   Record each valid result and ignore only clearly labeled false starts.

   Allow a rest between trials.

8. Step 8

   Report summary

   Calculate average and minimum valid time.

   Explain why more trials improve confidence.

   Builder checkpoint: At the final checkpoint, The timer records five valid reaction trials in milliseconds, detects early presses, and reports an average and best time.

See the engineering

Why it works

Input

button press after a light cue

Output

measured reaction time in milliseconds

Motion

human event-to-digital timing result

Energy losses

button bounce, display delay, false starts, timer resolution

const int ledPin=9, buttonPin=2; long total=0; int validTrials=0;
bool pressed(){ return digitalRead(buttonPin)==LOW; }
void waitRelease(){ while(pressed()) delay(5); delay(30); }
void setup(){ pinMode(ledPin,OUTPUT); pinMode(buttonPin,INPUT_PULLUP); Serial.begin(9600); randomSeed(analogRead(A5)); }
void loop(){
  if(validTrials>=5){ Serial.print("Average ms: "); Serial.println(total/5.0); while(true){} }
  Serial.println("Ready"); waitRelease(); unsigned long waitTime=random(2000,5001), start=millis();
  while(millis()-start<waitTime){ if(pressed()){ Serial.println("False start"); waitRelease(); return; } }
  digitalWrite(ledPin,HIGH); unsigned long cue=millis();
  while(!pressed()){} unsigned long reaction=millis()-cue; digitalWrite(ledPin,LOW);
  Serial.println(reaction); total+=reaction; validTrials++; waitRelease(); delay(800);
}

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 cue type or hand used change the measured result?
2. Where did button bounce affect the build most strongly?
3. What evidence shows that elapsed-time measurement explains the motion?
4. Which change would improve measured reaction time in milliseconds without creating a new problem?