An "electric eye" alarm detects a break in the beam of light not with a simple "electric" cell but with this type of cell
.... element for many types of sensors, including pressure sensors, load cells
,... [DOC] [TLE] Video and Optical - Techlib.com [PAR] Video and optical sensors are well-established as consumer products but here ... 
stay open until the chime signal from motion detection turns off the transistor.