Do you want to unlock your device after some interval then you can do that by using following procedure -

You have to create a alarm manager that will trigger after sometime of interval and then you can use KayguardManager class of android that is use to lock and unlock android device.

KeyguardManager km = (KeyguardManager) context
 .getSystemService(Context.KEYGUARD_SERVICE);
   final KeyguardManager.KeyguardLock kl = km
     .newKeyguardLock("MyKeyguardLock");
   kl.disableKeyguard();

   PowerManager pm = (PowerManager) context
     .getSystemService(Context.POWER_SERVICE);
   WakeLock wakeLock = pm.newWakeLock(PowerManager.FULL_WAKE_LOCK
     | PowerManager.ACQUIRE_CAUSES_WAKEUP
     | PowerManager.ON_AFTER_RELEASE, "MyWakeLock");
   wakeLock.acquire(); 

here PowerManager class used to indicate that our application needs to have the device stay on.

for this you have to add a permission in your manifest file like this :

android.permission.WAKE_LOCK