More Pins for an ESP8266 with an MCP23017

An ESP8266 does not have many pins that you can use freely. This test setup adds 16 more with an MCP23017, an I/O expander that the ESP8266 controls over I2C. A led on one of the new pins is the example: it is switched on and off through the expander.

Why an expander

Every project on this site that needs more inputs or outputs than the module has runs into the same limit. The MCP23017 gives 16 pins over two wires, so a small ESP8266 can drive many leds, relays or buttons. The MCP23017 I/O expander guide describes the chip in detail: its pins, its I2C address and its registers.

The circuit

The I2C connection uses two wires, one for the clock (SCL) and one for the data (SDA). In the Arduino core for the ESP8266, the default pins for I2C are GPIO4 for SDA and GPIO5 for SCL. Follow the wires in the diagram for your own build.

Use it

The expander has two ports of eight pins each. The led shows the basic use: set a pin of the expander as an output, and switch it on or off from the ESP8266, exactly like a pin of the ESP8266 itself. This is a test setup, made to prove that the expander works with the ESP8266.

Good to know

• The repository holds the diagram and is under the MIT licence. It has no code.

The project is in the ESP8266-MCP23017 repository on GitHub.