# A03. 外接 LED 呼吸燈
### 專案說明
外接 LED 發光呈現呼吸效果， 忽明忽暗
### 電路圖
* [LinkIt 7697](https://robotkingdom.com.tw/product/linkit-7697/)
* 麵包板
* LED
* 1KΩ 電阻
* 杜邦線
(圖片由 [Fritzing](http://fritzing.org/home/) 產生, LinkIt 7697 fritzing [下載](https://www.dropbox.com/s/44rgxihq1nri2qn/linkit_7697_fritzing_and_diagrams.zip))
### 積木畫布
使用類比輸出方式來控制 LED 明暗。 讓類比輸出的值(變數 led)， 在 0\~255 隨著時間往返， 呈現一個自然燈光明暗的效果
產生出的 Arduino 程式如下
```
int led;
int fade;
void setup()
{
pinMode(2, OUTPUT);
led = 0;
fade = 5;
digitalWrite(2, LOW);
Serial.begin(9600);
}
void loop()
{
analogWrite(2, led);
led = led + fade;
if (led <= 0) {
fade = 5;
} else if (led >= 255) {
fade = -5;
}
Serial.println(led);
delay(30);
}
```
### **衍伸學習**
**數位 I/O** 和 **類比 I/O** 的積木，都會有兩個類似，但參數帶入方式不一樣的變化型。
* **常數版** : 帶入參數是邊編輯積木時就決定了
* **變數版** : 帶入的參數是執行時動態產生帶入
變數版的好處是，可以透過程式來在**執行時才帶入參數**。下面這範例是我們要把 Pin 0 到 Pin 9 共九根針腳都數位寫入到高電位，就可以看出使用變數版的方便性了。
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