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Andruino

Official Description

Now you can use this Arduino USB SERIAL Communicator and connect to your Andruino board! NO ROOT REQUIRED. All you only need is an Android device with USB Host support.

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Example SERIAL:
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void setup()
{
Serial.begin(9600);
Serial.println("Hello from Arduino!");
}

void loop()
{
int incomingByte = 0;
delay(500);
String content = "";
char character;

while(Serial.available())
{
character = Serial.read();
content.concat(character);
}

if (content != "")
{
Serial.println("Received data: " + content);
}
}