Pressed state of android selector not working

I made android and arduino apps that control my standing table. I'm uploading this project to github ( https://github.com/neosarchizo/MyStandingDesk )

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/down_button_pressed" android:state_pressed="true" />
    <item android:drawable="@drawable/down_button" />
</selector>

      

        
        
        
      

    

I wanted to use a selector for buttons in MainActivity. If I click the button, then the button image should be changed. But it didn't work. So I change the button image programmatically by checking the MotionEvent.

btnDown.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View view, MotionEvent motionEvent) {

                switch (motionEvent.getAction()) {
                    case MotionEvent.ACTION_DOWN:
                        btnDown.setImageResource(R.drawable.down_button_pressed);
                        sendCommand("a");
                        break;
                    case MotionEvent.ACTION_UP:
                        btnDown.setImageResource(R.drawable.down_button);
                        sendCommand("s");
                        break;
                }
                return true;
            }
        });

      

        
        
        
      

    

Also I wrote the following codes for selector in layout XML file.

<ImageButton
    android:id="@+id/btnDown"
    android:layout_width="fill_parent"
    android:layout_height="0px"
    android:layout_margin="30dip"
    android:layout_weight="1"
    android:scaleType="fitCenter"
    android:background="@android:color/transparent"
    android:src="@drawable/selector_down_button" />