Kitchen scale on an Arduino Uno

// Kitchen scale: a bar load cell on an HX711, weighed in grams.
//
//   pin 3  the HX711's DT      pin 2  its SCK
//   VCC -> 5 V, GND -> GND     the cell's four wires are on the far side
//
// A load cell's whole signal is about a millivolt at full load, which no
// Arduino ADC can see. The HX711 is a 24-bit converter made for it, with its
// own two-wire output: wait for DT to go low, clock 24 bits out on SCK, then
// one more pulse to ask for channel A at gain 128 again. HX711.h does that,
// and this sketch is the tutorial every kit prints.
//
// # Three numbers, and the order they come in
//
// * The raw reading with nothing on it is not zero. No real cell is
//   balanced, and this one's zero is about 43 000 counts. So the first thing
//   the sketch does is tare(): read ten times, and call the average zero.
// * set_scale() is counts per gram. This cell is 1 mV/V at 5 kg, which at
//   gain 128 is 2 x 128 x 2^23 x 1 mV/V / 5000 g = 429.5 counts a gram. On a
//   real cell you find it with a known weight: read_average(10) with and
//   without it, and divide the difference by its grams.
// * get_units(5) is the average of five readings, less the tare, over the
//   scale. At ten readings a second that is half a second a line.
//
// # The last digit wanders
//
// The HX711's input noise is about 50 nV, some twenty counts at gain 128:
// a twentieth of a gram. Drag the load and watch the tenths settle, then
// change get_units(5) to get_units(1) and watch them wander.
//
// # Floats you can print, and nothing more
//
// This board has no floating point. get_units() is a float built bit by bit
// from whole numbers, so it prints; doing sums on it would not link. The
// scale is taken to a hundredth, which is far finer than the cell.

#include "HX711.h"

const int DT_PIN = 3;
const int SCK_PIN = 2;

HX711 scale;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: kitchen scale");

  // begin() waits for the chip's first settled reading, four conversions.
  scale.begin(DT_PIN, SCK_PIN);
  Serial.print("raw with nothing on: ");
  Serial.println(scale.read_average(5));

  scale.set_scale(429.5);
  scale.tare(10);
  Serial.println("tared: drag the slider to put a weight on the cell");
}

void loop() {
  Serial.print(scale.get_units(5), 1);
  Serial.println(" g");
}