user42187: I am working on a project which uses a set of sensors (HC-SR04). These sensors use pins 2 to 13 of the Arduino Uno. Here you can find the board:
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I also want to add an MP3 shield (VS1053B) to it and it fills all the pins on Arduino. So I decided to add an Arduino Mega and add the sensors to it and use the Arduino Uno and MP3 shield. The problem is with connecting these two Arduinos. As I said the MP3 shield fills A4 and A5 pins too, so I can not use them to communicate with the Mega. What other options do I have?

user42418: Salam, you could somewhat multiplex the HC-SR04 Ultrasonic Sensor by using I2C(IIC) instruction on how to convert those into I2C and by using OctoSonar library can be found here so that only the SDA(pin16) and SCL(pin17) pins on your Arduino UNO will be used. 
https://hackaday.io/project/19950-hc-sr04-i2c-octopus-octosonar

user359: You can multiplex the trigger line. With 3 pins you can address up to 8 sensors. If you multiplex the reply line as well (so you can abstract out the latency) you only need 2 additional pins for the actual trigger and reply.

42187: <p>I am working on a project which uses a set of sensors (HC-SR04). These sensors use pins 2 to 13 of the Arduino Uno. Here you can find the board:</p>

<p><a href="https://i.stack.imgur.com/Q6ynm.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Q6ynm.jpg" alt="enter image description here"></a>.</p>

<p>I also want to add an MP3 shield (<a href="https://www.waveshare.com/music-shield.htm" rel="nofollow noreferrer">VS1053B</a>) to it and it fills all the pins on Arduino. So I decided to add an Arduino Mega and add the sensors to it and use the Arduino Uno and MP3 shield. The problem is with connecting these two Arduinos. As I said the MP3 shield fills A4 and A5 pins too, so I can not use them to communicate with the Mega. What other options do I have?</p>
 42418: <p>Salam, you could somewhat multiplex the HC-SR04 Ultrasonic Sensor by using I2C(IIC) instruction on how to convert those into I2C and by using OctoSonar library can be <a href="https://hackaday.io/project/19950-hc-sr04-i2c-octopus-octosonar" rel="nofollow noreferrer">found here</a> so that only the SDA(pin16) and SCL(pin17) pins on your Arduino UNO will be used. 
<a href="https://hackaday.io/project/19950-hc-sr04-i2c-octopus-octosonar" rel="nofollow noreferrer">https://hackaday.io/project/19950-hc-sr04-i2c-octopus-octosonar</a></p>
 359: <p>You can multiplex the trigger line. With 3 pins you can address up to 8 sensors. If you multiplex the reply line as well (so you can abstract out the latency) you only need 2 additional pins for the actual trigger and reply.</p>
