PCF8574 LED Control with ESP32 (I²C GPIO Expander)

This example shows how to control 8 LEDs using the PCF8574 I²C GPIO expander with an ESP32. The sketch cycles through each LED by sinking current through the expander pins, demonstrating how the PCF8574 works and why wiring matters.

The PCF8574 is one of the simplest and most useful chips you can add to an ESP32. It gives you 8 extra GPIO pins over I²C, which is perfect when you run out of pins or want to simplify wiring.

What this example does

This sketch creates a simple LED chaser using the PCF8574.

• Initializes the PCF8574 at address 0x20
• Configures all 8 pins as outputs
• Turns LEDs ON one by one
• Turns them OFF again before moving to the next

The important detail is how the LEDs are driven. This example uses current sinking, not sourcing.

⚠️ Important concept: PCF8574 sinks current

This is the most important thing to understand:

• The PCF8574 cannot source current reliably
• It is designed to sink current (pull to GND)

That’s why the comment in the code says:

LEDs must be connected with the cathodes to the expander

Correct wiring

Each LED should be connected like this:

• LED anode (+) → VCC (3.3V or 5V via resistor)
• LED cathode (-) → PCF8574 pin

So when the pin goes LOW, it pulls the line to ground → LED turns ON.

When the pin goes HIGH, it stops sinking current → LED turns OFF.

Libraries used

This example uses:

• Adafruit_PCF8574.h

Install it via Arduino Library Manager.

Wiring the PCF8574 to ESP32

Typical I²C wiring:

• VCC → 3.3V
• GND → GND
• SDA → GPIO21
• SCL → GPIO22

Optional:

• Address pins (A0–A2) define the I²C address (default = 0x20)

Code walkthrough

Creating the PCF8574 object

Adafruit_PCF8574 pcf;

This creates the expander instance.

setup()

Serial.begin(115200);
while (!Serial) { delay(10); }

Standard Serial initialization.

Initializing the PCF8574

if (!pcf.begin(0x20, &Wire)) {
  Serial.println("Couldn't find PCF8574");
  while (1);
}

• 0x20 is the default I²C address
• &Wire tells it to use the default I²C bus

If the chip is not found, the code stops.

Configuring all pins as outputs

for (uint8_t p=0; p<8; p++) {
  pcf.pinMode(p, OUTPUT);
}

The PCF8574 has 8 pins (0–7). This sets all of them as outputs.

The main loop

for (uint8_t p=0; p<8; p++) {
  pcf.digitalWrite(p, LOW);
  delay(100);
  pcf.digitalWrite(p, HIGH);
}

What happens here

For each pin:

• LOW → sinks current → LED ON
• wait 100ms
• HIGH → stops sinking → LED OFF

Then it moves to the next LED.

This creates a simple running light effect.

Why LOW = ON?

This confuses a lot of people at first.

Normally:

• HIGH = ON
• LOW = OFF

But here it’s reversed because:

• The LED is powered from VCC
• The PCF8574 pulls it to ground

So:

• LOW → current flows → LED ON
• HIGH → no current → LED OFF

Common mistakes

1. Wiring LEDs the wrong way

If you connect:

• LED anode → PCF8574
• LED cathode → GND

It will not work properly.

2. Forgetting resistors

Each LED still needs a current limiting resistor.

3. Wrong I²C address

If pcf.begin(0x20) fails:

• Check A0–A2 pins
• Scan I²C bus to confirm address

4. Expecting high current drive

The PCF8574 is not meant to drive heavy loads.

• Good for LEDs, buttons, logic signals
• Not good for relays (without transistor)

Expanding this example

Once this works, you can easily extend it:

• Button inputs on unused pins
• LED patterns or animations
• Driving relays via transistors
• Combining multiple PCF8574 chips (up to 8 addresses)

Why use PCF8574?

It’s ideal when:

• You run out of ESP32 GPIOs
• You want cleaner wiring (only 2 I²C lines)
• You need multiple simple outputs or inputs

Final thoughts

This is a simple example, but it teaches one critical concept:

👉 Not all GPIO expanders behave like normal pins

The PCF8574 is designed around current sinking, and once you understand that, everything else makes sense.

From there, you can build much more complex projects with very little extra wiring.