Heartbeat-Responsive Flowing Lights

What I wanted to learn, and why it interested me: I want to learn about how to solder an LED strip, and build a live-varying LED display depending on a sensor value. 

Process and reflection: 

The process of learning about the basics of how to manipulate and wire an LED strip is very entertaining, especially because I can see instant, colorful feedback from the lights. Once I understood the fundamentals, I chose a sensor that seemed the most interesting to me, a pulse sensor, and used that as an input to control the LED display.  I experimented with multiple ways of translating the pulse rate to LEDs: flashing to each heartbeat, creating wave/ ripple effects with each beat, creating gradients, and mapping the pulse rate to color and speed of the individual LEDs on the strip. It was interesting to play with different libraries to control the LED strip!

In terms of hardware, both the LED and the pulse sensor were easy and straightforward to wire. The most challenging part was soldering wires onto the LED strip. It was difficult to hold both the wire and the strip in place while soldering on a small area. Because I was also inexperienced in soldering, the wires came apart multiple times, and I spent a tedious amount of time getting them to stay in place. 

/*

    60-223 Intro to Physical Computing, fall 2025

    Domain-specific Skill Building exercise: [Heartbeat Responsive Flowing Lights]

    [This sketch reads a pulse sensor and smooths the analog signal to reduce noise. It detects each heartbeat, calculates the correact BPM, prints both the smoothed sensor value and BPM to the Serial Monito,and sends a flowing pulse of LEDs down a trip, with the LED color changing according to the current BPM (blue for slow heart rate, green for normal, and red for a fast hear rate.]

    Pin mapping:

    Arduino pin | role  | details

    ------------------------------

    A0             input   pulse sensor

    6              output  external LED Strip

    Code implementation assisted by ChatGPT, 9/26/2025

*/

#include <Adafruit_NeoPixel.h>

#define LED_PIN     6

#define NUM_LEDS    18

#define PULSE_PIN   A0

Adafruit_NeoPixel strip(NUM_LEDS, LED_PIN, NEO_GRB + NEO_KHZ800);

// Pulse detection

int threshold = 550;

int pulseState = LOW;

int pulseIndex = -1;

int fadeAmount = 10;

// BPM calculation

unsigned long lastBeatTime = 0;

float bpm = 0;

// Smoothing

const int smoothSize = 5;

int values[smoothSize] = {0};

int index = 0;

// Simple moving average smoothing

int smooth(int reading) {

 values[index] = reading;

 index = (index + 1) % smoothSize;

 long sum = 0;

 for (int i = 0; i < smoothSize; i++) sum += values[i];

 return sum / smoothSize;

}

void setup() {

 strip.begin();

 strip.show();

 Serial.begin(9600);

}

void loop() {

 int rawValue = analogRead(PULSE_PIN);

 int sensorValue = smooth(rawValue); // Smoothed reading

 // Print smoothed sensor value

 Serial.print("Smoothed: ");

 Serial.print(sensorValue);

 // Detect heartbeat

 if (sensorValue > threshold && pulseState == LOW) {

   pulseState = HIGH;

   pulseIndex = 0; // Start new pulse

   unsigned long currentTime = millis();

   if (lastBeatTime > 0) {

     unsigned long interval = currentTime - lastBeatTime; // ms between beats

     bpm = 60000.0 / interval; // Correct BPM calculation

     Serial.print("  BPM: ");

     Serial.print(bpm);

   }

   lastBeatTime = currentTime;

 }

 if (sensorValue < threshold && pulseState == HIGH) {

   pulseState = LOW;

 }

 Serial.println();

 // Update LED strip

 updatePulse();

 fadeStrip();

}

// Move pulse down the strip

void updatePulse() {

 if (pulseIndex >= 0 && pulseIndex < NUM_LEDS) {

   strip.setPixelColor(pulseIndex, bpmColor(bpm));

   pulseIndex++;

 }

}

// Fade LEDs gradually

void fadeStrip() {

 for (int i = 0; i < NUM_LEDS; i++) {

   uint32_t color = strip.getPixelColor(i);

   int r = (color >> 16) & 0xFF;

   int g = (color >> 8) & 0xFF;

   int b = color & 0xFF;

   r = max(0, r - fadeAmount);

   g = max(0, g - fadeAmount);

   b = max(0, b - fadeAmount);

   strip.setPixelColor(i, strip.Color(r, g, b));

 }

 strip.show();

}

// Map BPM to color

uint32_t bpmColor(float bpm) {

 if (bpm < 70) return strip.Color(0, 0, 255);   // Blue

 if (bpm < 100) return strip.Color(0, 255, 0);  // Green

 return strip.Color(255, 0, 0);                 // Red

}