Stand-Alone Volume Control

(apologies for the cross-posting. I was not sure about the most appropriate forum)

I am an artist and have reach a point in my research where I need to call for help. I am interested in an interface which regulates the volume of a radio based on a person's proximity. The range I am looking for is roughly within a foot of the interface and I am looking to have the volume increase as the person gets closer.

I have a simple circuit which works with optics/lights but would prefer the interface to be robust to lighting. I believe a capacitance-based regulator would be great. Any help would be great.

Thanks.

>[i]...I have a simple circuit which works with optics/lights but would prefer the interface to be robust to lighting...[/i]

Any circuit based on the light modulation is robust to lighting (TV remote control for example).