Copy/Paste the following code into a blank Arduino sketch. Then, compile and upload it to your Metro. If you see the LED running through the script, continue to the next page. If you're not seeing it work properly, check the FAQ below for help.

 * (PROJ04) Metro (and Metro Express!) Fidget Spinner Tachometer
 * Desc: Count fidget spinner RPMs (and beat your high scores)
 * Original code by Tony Dicola for Adafruit Industries
 * by Brent Rubell and Asher Lieber for the Metro Explorers Guide

// include the LCD library code:
#include <LiquidCrystal.h>
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(7, 8, 9, 10, 11, 12);

// How many arms does the spinner have?
#define SPINNER_ARMS         3
// 1kB sample size
#define SAMPLE_DEPTH        256
// delay between light samples
#define SAMPLE_PERIOD_US   150
// min. speed, depends on reflective-ness of spinner, noise thresh.
//#define THRESHOLD           127
// wait 2s between measurements
#define MEASURE_PERIOD_MS  2000

// rpm high score
float rpmHighScore = 0.00;
// threshold value
int threshold;
// photo light sensor pin
int photoSensor = A0;
// led pin
int led = 2;

void setup() {
  // Init. serial monitor @ 115200 baud
  // set up the LCD's number of columns and rows:
  lcd.begin(16, 2);
  lcd.print("Metro Tachometer");
  // set up LED
  pinMode(led, OUTPUT);

void loop() {
  int sensorCalibrate = 0; 
  // Set depending on light balance
  threshold = 40;

  // PAUSE between measurements 
  lcd.print("GET READY...");
  analogWrite(led, 255);

  // pause between sampling sensor
  // shown as a countdown on the screen! 
  for (int i = 3500; i > 0; i--) {
  // init. empty sample array
  uint16_t samples[SAMPLE_DEPTH] = {0};
  // start time
  uint32_t start = micros();

  // lcd during spin
  lcd.print("SPIN IT");
  lcd.print("score: ");
  for (int i = 0; i < SAMPLE_DEPTH; i++) {
    // sample the photo light sensor
    samples[i] = analogRead(photoSensor);
    // serial output
    Serial.print("\nSample: ");
    // keep the player occupied while sampling
    if (i == int(SAMPLE_DEPTH/4)) {
      lcd.print("keep going!");
    else if (i == int(SAMPLE_DEPTH/3)) {
     lcd.print("almost there!");

  // time elapsed (uS)
  uint32_t elapsed_uS = micros() - start;
  // time elapsed (sec)
  float elapsed = elapsed_uS / 1000000.0; 

  // Find the min and max values in the collected samples.
  uint16_t minval = samples[0];
  uint16_t maxval = samples[0];
  for (int i=1; i<SAMPLE_DEPTH; ++i) {
    minval = min(minval, samples[i]);
    maxval = max(maxval, samples[i]);

  // Serial Monitor Values 
  Serial.print("\n Samples taken, : ");
  Serial.print(elapsed, 3);
  Serial.print(" seconds");
  Serial.print("\n Max Sample Val: ");
  Serial.print("\n Min Sample Val: ");

  // Check the amplitude of the signal (difference between min and max)
  // is greater than the threshold to continue detecting speed.
  uint16_t amplitude = maxval - minval;
  if (amplitude < threshold) {
    // Didn't make it past the threshold so start over with another measurement attempt.
    lcd.println("didnt spin fast enough, re-spin!");

  // Compute midpoint of the signal (halfway between min and max values).
  uint16_t midpoint = minval + (amplitude/2);

  // Count how many midpoint crossings were found in the signal.
  // These are instances where two readings either straddle or land on
  // the midpoint.  The midpoint crossings will happen twice for every
  // complete sine wave cycle (once going up and again coming down).
  int crossings = 0;
  for (int i=1; i<SAMPLE_DEPTH; ++i) {
    uint16_t p0 = samples[i-1];
    uint16_t p1 = samples[i];
    if ((p1 == midpoint) || 
        ((p0 < midpoint) && (p1 > midpoint)) ||
        ((p0 > midpoint) && (p1 < midpoint))) {
      crossings += 1;
  // Compute signal frequency, RPM, and period.
  // The period is the amount of time it takes for a complete
  // sine wave cycle to occur.  You can calculate this by dividing the
  // amount of time that elapsed during the measurement period by the
  // number of midpoint crossings cut in half (because each complete
  // sine wave cycle will have 2 midpoint crossings).  However since
  // fidget spinners have multiple arms you also divide by the number
  // of arms to normalize the period into a value that represents the
  // time taken for a complete revolution of the entire spinner, not
  // just the time between one arm and the next.

  Serial.print("\n MP Crossings: ");
  Serial.print("\n Elapsed: ");
  float period = elapsed / (crossings / 2.0 / SPINNER_ARMS);
  Serial.print("\n Period: ");

  // Once the period is calculated it can be converted into a frequency
  // value (i.e revolutions per second, how many times the spinner spins
  // around per second) and more common RPM value (revolutions per minute,
  // just multiply frequency by 60 since there are 60 seconds in a minute).
  float frequency = 1.0 / period;
  float rpm = frequency * 60.0;

  // Print out the measured values!
  Serial.print("Frequency: ");
  Serial.print(frequency, 3);
  Serial.print(" (hz)\t\tRPM: ");
  Serial.print(rpm, 3);
  Serial.print("\t\tPeriod: ");
  Serial.print(period, 3);
  Serial.println(" (seconds)");

  // high score checker 
  if(rpm > rpmHighScore) {
    rpmHighScore = rpm;
    lcd.print("you beat the");
    lcd.print("high score!");


Not Working?

LCD is blank/garbled/glitchy

Make sure all your wires are both plugged in correctly and not loose. During the assembly step where you tied everything together, make sure everything is snugly plugged into both the Metro and the breadboard. 

LED not turning on?

Check the LED wiring. Possibly you have a wrong resistor value.

Still not working?

Post up on our community support forums!

This guide was first published on Aug 18, 2017. It was last updated on Aug 18, 2017. This page (Code) was last updated on Mar 17, 2020.