Analog board on an Arduino Uno

// Analog board: the YL-40 PCF8591 module's three sensors over I2C, and its
// DAC following the trimmer.
//
//   YL-40   SDA -> A4   SCL -> A5   VCC -> 5 V   GND -> GND   (jumpers fitted)
//
// The PCF8591 is four 8-bit analog inputs and one analog output on two
// wires. On the YL-40 three of the inputs have a sensor on a jumper: the
// light-dependent resistor on AIN0, the thermistor on AIN1 and the blue
// trimmer on AIN3. Each is a divider, so the LDR reads lower in more light
// and the thermistor lower when warmer, which surprises everyone once.
//
// The library reads each channel twice and keeps the second byte, because
// the chip always sends the conversion before the one just asked for. The
// DAC copies the trimmer, so turning it brightens the LED on AOUT.
//
// Drag the three sliders on the module.

#include <Wire.h>
#include <Adafruit_PCF8591.h>

Adafruit_PCF8591 pcf = Adafruit_PCF8591();

void setup() {
  Serial.begin(9600);
  if (!pcf.begin()) {
    Serial.println("Couldn't find PCF8591");
    while (1) delay(10);
  }
  pcf.enableDAC(true);
  Serial.println("Analog board: light, temperature and trimmer");
}

void loop() {
  uint8_t light = pcf.analogRead(0);
  uint8_t temperature = pcf.analogRead(1);
  uint8_t trim = pcf.analogRead(3);
  pcf.analogWrite(trim);

  Serial.print("light ");
  Serial.print(light);
  Serial.print(", temperature ");
  Serial.print(temperature);
  Serial.print(", trim ");
  Serial.print(trim);
  Serial.print(", AOUT ");
  Serial.print(pcf.int_to_volts(trim, 5.0));
  Serial.println(" V");
  delay(250);
}