Increasing the numbers of I/O pins on Arduino Uno

Question

I want to make a game of Tic-tac-toe using an Arduino, LEDs and bouncy buttons. Hence I need nine inputs and nine outputs. But you see that an Arduino only has 13 I/O pins. So is there a way I can use the analog pins or a way to reduce the number of I/O pins using an ingenious way?

Answer 1

Yes, just use the analog pins. The analog pins are both analog inputs and digital I/O pins. The functions happen to be shared on the pins, in the same way that the interrupt pins are shared with digital I/O, the TX and RX pins are shared with digital I/O and the SPI pins are shared with digital I/O.

Every I/O pin (analog or digital, etc.) can be used as digital I/O.

You can also reduce your required number of I/O pins by using a technique known as multiplexing.

For instance, to read nine buttons you could have them arranged (as in Tic-tac-toe) in three rows of three buttons. You then have three I/O pins for the columns and three I/O pins for the rows - that's just six I/O pins for nine buttons.

The technique is widely used for keypads, but they are just buttons, so the same technique can be applied to any buttons.

• The Matrix Keypad how-to

The same can go for LEDs. There are various specialist LED multiplexing techniques that allow for many LEDs to be driven from just a handful of I/O pins.

• How to Multiplex an LED Grid... (YouTube)
• Charlieplex Library for Arduino

Answer 2

If multiplexing is too complicated for you, you can wire all buttons to one analog pin using resistors. This way you only need 9 pins for LEDs and 1 analog pin for buttons.