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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?

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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 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.

  • You can also multiplex buttons with LEDs, this way you just need 3 pins for commons + 3 pins for LEDs + 3 pins for inputs. – Cano64 Aug 21 '15 at 19:06
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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.

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  • What I am planning to do is that use rgb leds, So if user 1 presses some button then red color and then next one green. So I guess that this method would not be able to work that way, because they require different voltages. And all this process have to be dynamic. I don't know which button will the user be pressing. – Clarskon Aug 22 '15 at 5:03

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