Unity Writes Arduino via Serial Port

Tap 1 0 keys in Unity will make Arduino LED light on/off.

Arduino

const int ledPin = 13;
int ledState = 0;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  ledState = recvSerial();
  delay(100);
  if (ledState == 1)
    digitalWrite(ledPin, HIGH);
  else
    digitalWrite(ledPin, LOW);
}

int recvSerial() {
  if (Serial.available()) {
    int serialData = Serial.read();
    switch (serialData) {
      case '1':
        return 1;
        break;
      case '0':
        return 0;
        break;
      default:
        return -1;
    }
  }
}

Unity 3D

1. Enable Serial Port: Menu bar File > Build Settings... > Button Player Settings... > In the pane select tab Other Settings and find Api Compatibility Level switch the default .Net 2.0 Subs to .Net 2.0

2. Create an asset, for example an Empty GameObject. Add Component and enter ArduinoControl to create a C# file:

   using UnityEngine;
   using System.Collections;
   using System.IO.Ports;
   
   public class ArduinoControl : MonoBehaviour {
     public string portName;
     SerialPort arduino;
   
     void Start () {
       arduino = new SerialPort (portName, 9600);
       arduino.Open ();
     }
     
     void Update () {
       if (arduino.IsOpen) {
         if (Input.GetKey ("1")) {
           arduino.Write ("1");
           Debug.Log (1);
         } else if (Input.GetKey ("0")) {
           arduino.Write ("0");
           Debug.Log (0);
         }
       }
     }
   }

3. Enter Serial Port Name in Inspector, for example /dev/cu.usbmodem1411, and run the scene.