I am interested in learning how to wire a speaker and load cell, and use them to transfer weight to sound.

Process and reflection: 

I really enjoyed this week’s domain-specific building session. Figuring out how the load cell works and designing a base for it was definitely the highlight for me—it was super satisfying to see it come together.

One thing I learned the hard way: I need to be more mindful of the hardware when I’m debugging. I ended up spending almost two hours trying to fix the code, only to realize the real issue was a loose solder connection. Lesson learned—next time, I’ll check the physical connections first before getting lost in the code.

/*

    60-223 Intro to Physical Computing, fall 2025

    Domain-specific Skill Building exercise: Smart Cup

    It’s a “smart coaster” that weighs what’s on it (via an HX711 load cell) and plays an MP3 sound (via a DFPlayer Mini) when the weight crosses two milestones: first at 18 g, then again at 30 g. After you remove most of the weight, it resets so it can trigger again next time.

    Pin mapping:

    Arduino pin | role  | details

    ------------------------------

    10+11        output   DFPlayer mini 

    5+4          input    XFW_HX711

    A0    output   PAM8302A

*/

code is entirely wr

#include <SoftwareSerial.h>

#include <DFRobotDFPlayerMini.h>

#include "HX711.h"

// ---- DFPlayer on D10/D11 ----

SoftwareSerial mp3Serial(10, 11);     // D10=RX, D11=TX

DFRobotDFPlayerMini mp3;

bool dfp_ok = false;

// ---- HX711 on D4/D5 ----

#define HX_DT 4

#define HX_SCK 5

HX711 scale;

// ---- Calibration ----

float CALIBRATION_FACTOR = 7050.0f;   // adjust for your cell/wiring

// ---- Trigger config ----

const float WATER_TARGET_G   = 18.0f;   // first trigger

const float SECOND_TARGET_G  = 30.0f;   // second trigger

const float HYST_G           = 8.0f;    // re-arm below (18 - 8) = 10 g

const int   TRACK            = 1;       // 0001.mp3

const int   START_VOLUME     = 14;      // 0..30

bool armed18 = true;   // play once when >= 18 g

bool armed30 = true;   // play again when >= 30 g

void setup() {

 Serial.begin(115200);

 while (!Serial) { }

 Serial.println(F("Boot: HX711 + DFPlayer (18g & 30g triggers)"));

 mp3Serial.begin(9600);

 dfp_ok = mp3.begin(mp3Serial);

 if (dfp_ok) {

   mp3.volume(START_VOLUME);

   mp3.EQ(0);

   Serial.println(F("DFPlayer online."));

 } else {

   Serial.println(F("DFPlayer init FAILED (check RX/TX/GND/SD /MP3/0001.mp3)."));

 }

 scale.begin(HX_DT, HX_SCK);

 scale.set_scale(CALIBRATION_FACTOR);

 delay(300);

 Serial.println(F("Place EMPTY CUP on the coaster; taring in 2s..."));

 delay(2000);

 scale.tare(15);

 Serial.println(F("Tare complete."));

}

void loop() {

 float grams = scale.get_units(5);

 Serial.print(F("mass[g]=")); Serial.print(grams, 1);

 Serial.print(F("  18g=")); Serial.print(armed18 ? "armed" : "done");

 Serial.print(F("  30g=")); Serial.println(armed30 ? "armed" : "done");

 // First trigger: >= 18 g

 if (armed18 && grams >= WATER_TARGET_G) {

   Serial.println(F(">= 18 g -> PLAY ONCE"));

   if (dfp_ok) mp3.play(TRACK);

   armed18 = false;

 }

 // Second trigger: >= 30 g

 if (armed30 && grams >= SECOND_TARGET_G) {

   Serial.println(F(">= 30 g -> PLAY AGAIN"));

   if (dfp_ok) mp3.play(TRACK);

   armed30 = false;

 }

 // Re-arm both after weight drops well below 18 g

 if (!armed18 || !armed30) {

   if (grams <= (WATER_TARGET_G - HYST_G)) {

     armed18 = true;

     armed30 = true;

     Serial.println(F("Below 10 g -> re-armed both triggers"));

   }

 }

 if (dfp_ok && mp3.available()) { mp3.readType(); mp3.read(); }

 delay(120);

}