Exemplary embodiments are described wherein an auxiliary sensor is removably attached to a touchscreen computing device and is communicably coupled to the device to provide additional user input to the computing device. In accordance with some embodiments, the additional user input may be used to affect how the computing device interprets and processes gestural user input from the touchscreen, or to simulate such input. In accordance with an illustrative embodiment, the auxiliary sensor establishes an optical path near the surface of the touchscreen and sensing user contact by observing blockage of the optical path by a user's pointing member.