An Android device is a miniature computer with a touch screen and an array of sensors. There are many used and inexpensive entry-level Android phones and tablets available. In this paper, we describe an innovative method to transform an Android device into a versatile "programmable" I/O sensor module. It uses a serial Bluetooth adaptor and a custom app to obtain the desired sensor measurements and user inputs. The Bluetooth adaptor creates a wireless serial port and the Android app retrieves the desired sensor information, formats the data into a byte stream, and transmits it via the Bluetooth link. The app development is done in the App Inventor platform, which is based on a visual programming environment and can be quickly learned. Students can easily develop basic, functional apps and create a customized I/O module that can be incorporated into microcontroller, digital systems, or embedded systems projects.