Digital pot LED dimmer on an Arduino Uno

// An MCP4131 digital potentiometer: a knob the Uno turns over SPI.
//
// The MCP4131 is a 10 kohm track with a wiper that stops at 129 places, 0
// to 128, set by a two-byte SPI command. Here its ends are on 5V and GND,
// so the wiper is a voltage from 0 to 5 V, and that voltage lights an LED
// through 220 ohms. A0 reads the wiper too.
//
//   MCP4131  CS -> pin 10, SCK -> pin 13, SDI -> pin 11
//            P0A -> 5V, P0B -> GND, VDD -> 5V, VSS -> GND
//            P0W -> 220 ohm -> LED -> GND, and P0W -> A0
//
// Every half second the sketch moves the wiper to the next step of a fade
// up and down, and prints the step and what A0 reads. The LED stays dark
// until the wiper passes the LED's forward voltage, about 1.8 V.
//
// THINGS TO TRY. Change the value property to 100k: the wiper's volts are
// the same but the LED is dimmer, because the track feeds it less current.

#include <SPI.h>

const int CS = 10;
const uint8_t STEPS[] = {0, 32, 64, 96, 128, 96, 64, 32};

uint8_t i = 0;

void setWiper(uint8_t value) {
  digitalWrite(CS, LOW);
  SPI.transfer(0x00);           // write, wiper 0
  SPI.transfer(value);
  digitalWrite(CS, HIGH);
}

void setup() {
  pinMode(CS, OUTPUT);
  digitalWrite(CS, HIGH);
  SPI.begin();
  Serial.begin(9600);
  Serial.println("MCP4131 on SPI: CS 10, SCK 13, SDI 11");
}

void loop() {
  uint8_t step = STEPS[i];
  setWiper(step);
  delay(5);
  int a0 = analogRead(A0);
  Serial.print("wiper ");
  Serial.print(step);
  Serial.print(" of 128, A0 reads ");
  Serial.print(a0 * 5.0 / 1023, 2);
  Serial.println(" V");
  i = (i + 1) % sizeof(STEPS);
  delay(500);
}