pcf8591

Overview

The pcf8591 node reads analog values from the PCF8591 8-bit ADC/DAC converter via I2C. This versatile chip provides 4 analog inputs and 1 analog output in a single package, making it an affordable solution for basic analog sensing. It supports single-ended and differential input modes, with configurable I2C addresses allowing up to 8 devices on the same bus. The built-in DAC can output a variable voltage for controlling analog circuits.

Properties

Output

{
  "raw": 178,
  "voltage": 2.30,
  "channel": 0,
  "mode": "single",
  "vref": 3.3,
  "timestamp": 1640000000000
}

Wiring (I2C)

  PCF8591           Raspberry Pi
  -------           ------------
  VCC  ──────────── 3.3V (Pin 1)
  GND  ──────────── GND  (Pin 6)
  SDA  ──────────── SDA  (Pin 3 / GPIO 2)
  SCL  ──────────── SCL  (Pin 5 / GPIO 3)
  A0   ──────────── GND  (address bit 0)
  A1   ──────────── GND  (address bit 1)
  A2   ──────────── GND  (address bit 2)

  AIN0 ── sensor/pot  (analog in ch0)
  AIN1 ── sensor/pot  (analog in ch1)
  AIN2 ── sensor/pot  (analog in ch2)
  AIN3 ── sensor/pot  (analog in ch3)
  AOUT ── load        (analog output)

Example: Multi-Channel Sensor Dashboard

Read all 4 analog channels to monitor different sensors and convert raw readings to meaningful values.

// Function node: Parse 4-channel analog data
// CH0: LDR (light sensor)
// CH1: NTC thermistor
// CH2: Potentiometer (user dial)
// CH3: Soil moisture sensor

var voltage = msg.voltage;
var channel = msg.channel;
var vref = msg.vref;
var result = flow.get('sensorData') || {};

switch(channel) {
    case 0: // LDR: lower voltage = brighter
        result.light = ((vref - voltage) / vref * 100).toFixed(1);
        break;
    case 1: // NTC: Steinhart-Hart approximation
        var resistance = voltage / (vref - voltage) * 10000;
        var tempK = 1 / (1/298.15 + Math.log(resistance/10000)/3950);
        result.temperature = (tempK - 273.15).toFixed(1);
        break;
    case 2: // Potentiometer: 0-100%
        result.dial = (voltage / vref * 100).toFixed(0);
        break;
    case 3: // Moisture: higher voltage = drier
        result.moisture = (100 - (voltage / vref * 100)).toFixed(0);
        break;
}

flow.set('sensorData', result);
msg.payload = result;
return msg;

Example: LED Brightness with DAC Output

Use the DAC output to control LED brightness based on ambient light readings from the ADC input.

// Function node: Auto-brightness LED control
var lightVoltage = msg.voltage;
var vref = msg.vref;

// Light level: 0 (dark) to 100 (bright)
var lightLevel = ((vref - lightVoltage) / vref * 100);

// Invert: darker ambient = brighter LED
var ledBrightness = 100 - lightLevel;
ledBrightness = Math.max(5, Math.min(100, ledBrightness));

// Convert to DAC value (0-255 for 8-bit)
var dacValue = Math.round(ledBrightness / 100 * 255);
var dacVoltage = (dacValue / 255 * vref);

msg.payload = {
    ambientLight: lightLevel.toFixed(1),
    ledBrightness: ledBrightness.toFixed(0),
    dacRaw: dacValue,
    dacVoltage: dacVoltage.toFixed(2)
};

// Send dacValue to pcf8591 DAC output
msg.dacOutput = dacValue;
return msg;

Example: Differential Voltage Measurement

Use differential mode to measure the voltage difference between two points, useful for bridge sensors and current sensing.

// Function node: Differential pressure sensor
// Using differential mode: AIN0 - AIN3
var raw = msg.raw;      // 0-255, centered at 128
var vref = msg.vref;

// Convert differential reading
// 128 = 0V difference, 255 = +Vref, 0 = -Vref
var diffVoltage = ((raw - 128) / 128) * vref;

// Convert to pressure (example: MPXV7002 sensor)
// Sensitivity: 1V per kPa, offset at 2.5V (= 0 kPa)
var pressure_kPa = diffVoltage * 1.0;

// Convert to other units
var pressure_psi = pressure_kPa * 0.14504;
var pressure_mbar = pressure_kPa * 10;

msg.payload = {
    raw: raw,
    diffVoltage: diffVoltage.toFixed(3),
    pressure_kPa: pressure_kPa.toFixed(3),
    pressure_psi: pressure_psi.toFixed(4),
    pressure_mbar: pressure_mbar.toFixed(1)
};

return msg;

Common Use Cases