There are many lock screens available for your Android phone to protect your phone from unauthorized use including several apps and patterns. Hidden Lock is a free app for Android which as the name suggests is a hidden lock for your phone without any password. Hidden Lock adds an additional security for your phone as it locks your device by turning off touch screen and menu shortcuts. There are no password, no special codes, no patterns required to unlock it. When you enable the hidden lock, you specify the location of the unlock button on the screen along with the icon size. Unless somebody presses the real location on the screen, no one will be able to unlock your phone. This is pretty useful when you have a large screen Android phone.
AS you can see from the above images, you can select the unlock button and its size using the slide and then disable the default lock on your Android phone.
Here is a video demo of Hidden Lock.
Hidden Lock works with Android 2.1 and above and there is also a pro version with more features available.