user45230: Hey guys I have a project that will require 20 I/O pins with serial communication from a computer to control each of the pins. I would like something as small as possible would a arduino micro or uno work for this project or will i need to go bigger like a mega. I have looked at all the pinouts but am confused to know of which pins I can use as digital I/O or not. 
Thanks

user41951: If you require 20 I/O pins then use Arduino Mega. If it’s too big then use Nano, Micro or Pro Mini (you need a serial thingy to program the Mini) with I/O expanders like PCF8574P with I2C communication or MCP23S17 with SPI communication. 

user11548: There are a couple of options

Easiest = Use an Arduino with a greater number of pins (e.g. MEGA2560)
Easy = Use an add-on board (e.g. Teensy) 
Harder = Use shift registers (see Is there a way to have more than 14 Output pins on arduino?)

Further discussion on these options here.

45230: <p>Hey guys I have a project that will require 20 I/O pins with serial communication from a computer to control each of the pins. I would like something as small as possible would a arduino micro or uno work for this project or will i need to go bigger like a mega. I have looked at all the pinouts but am confused to know of which pins I can use as digital I/O or not. 
Thanks</p>
 41951: <p>If you require 20 I/O pins then use Arduino Mega. If it’s too big then use Nano, Micro or Pro Mini (you need a serial thingy to program the Mini) with I/O expanders like PCF8574P with I2C communication or MCP23S17 with SPI communication. </p>
 11548: <p>There are a couple of options</p>

<ol>
<li>Easiest = Use an Arduino with a greater number of pins (e.g. MEGA2560)</li>
<li>Easy = Use an add-on board (e.g. Teensy) </li>
<li>Harder = Use shift registers (see <a href="https://arduino.stackexchange.com/questions/117/is-there-a-way-to-have-more-than-14-output-pins-on-arduino">Is there a way to have more than 14 Output pins on arduino?</a>)</li>
</ol>

<p>Further discussion on these options <a href="http://forum.arduino.cc/index.php?topic=505084.0" rel="nofollow noreferrer">here</a>.</p>
