user19559: I have an Arduino Uno and I want to connect;

RFID Reader (RC522)
WiFi Module (ESP8266 ESP-01)
SD Card Module
RTC (Real Time Clock - DS1307)

Same time but pinouts are not enough. Is there a way to doing this with Uno or I have to buy an Arduino Mega ? I saw "Expansion Sensor Shield"s but I do not know what is the logic of using this ? 
Regards 

user19513: Let's see which device uses which communication protocol:

WiFi Module (ESP8266 ESP-01) uses Serial communication.
RTC (Real Time Clock - DS1307) uses I²C communication.
RFID Reader (RC522) uses SPI communication.
SD Card Module also uses SPI communication.

Now, the Arduino UNO supports One I²C, One SPI and One Serial (by default) communication port. 
So, you can easily connect your Wifi and RTC modules.
Now, actual problem is: "How to access multiple SPI interfaces on Arduino" (and that is a solved problem).
Hope it helps and you don't have to buy an Arduino MEGA.

19559: <p>I have an Arduino Uno and I want to connect;</p>

<ul>
<li>RFID Reader (<em>RC522</em>)</li>
<li>WiFi Module (<em>ESP8266 ESP-01</em>)</li>
<li>SD Card Module</li>
<li>RTC (<em>Real Time Clock - DS1307</em>)</li>
</ul>

<p>Same time but pinouts are not enough. Is there a way to doing this with Uno or I have to buy an Arduino Mega ? I saw "<em>Expansion Sensor Shield</em>"s but I do not know what is the logic of using this ? </p>

<p>Regards </p>
 19513: <p>Let's see which device uses which communication protocol:</p>

<ol>
<li><strong>WiFi Module (ESP8266 ESP-01)</strong> uses Serial communication.</li>
<li><strong>RTC (Real Time Clock - DS1307)</strong> uses I²C communication.</li>
<li><strong>RFID Reader (RC522)</strong> uses SPI communication.</li>
<li><strong>SD Card Module</strong> also uses SPI communication.</li>
</ol>

<p>Now, the Arduino UNO supports <strong>One I²C</strong>, <strong>One SPI</strong> and <strong>One Serial</strong> (by default) communication port. </p>

<p>So, you can easily connect your <strong>Wifi</strong> and <strong>RTC</strong> modules.</p>

<p>Now, actual problem is: "<a href="https://arduino.stackexchange.com/questions/8425/how-to-access-multiple-spi-interfaces-on-arduino">How to access multiple SPI interfaces on Arduino</a>" (and that is a solved problem).</p>

<p>Hope it helps and you don't have to buy an <strong>Arduino MEGA</strong>.</p>
