Buy arduino, buy sound shield, buy distance sensor shield, plug into each other, burn example code, stuff components inside box. Not only is this not a hack but he might as well have just bought a ...

The microcontroller-based concept was chosen because I wanted to scale the sensor system later to suit other applications. Here is the diagram of my Arduino infrared close proximity sensor. As you can ...

The sensor shown here is essentially two plates mounted side-by-side, attached to an Arduino Nano using the Capacitor library which uses just two pins, one digital and one analog.