This paper proposes a force-free large-active-area light-blocking based switch structure. Our key approach is based on a combination of three basic optical concepts: optical beam shaping (S) that converts an incident optical beam into a diverging sheet of light beam across a desired active area, optical beam floating (F) that allows the sheet of light beam float above the desired active area and optical beam blocking (B) for checking the appearance of an obstruction in space over the touching zone. By adding electronic filters for post processing, unwanted optical noises can also be suppressed. Our SFB-based optical sensor prototype using two light emitting diodes, two plastic cylindrical lenses and eight photoreceivers shows an active area of 27 mm × 29 mm with a measured 23.5-dB electrical on/off ratio. A 100% active area is achieved for a ≥9-mm diameter obstruction. In addition, our SFB-based optical sensor can be operated under a typical 300 lx and a high illumination of 2000 lx without sensor malfunction, verifying that it is suitable for indoor use. So far, 10 sets of our SFB-based optical sensors have been deployed for disabled students to comfortably turn on/off computers for more than 9 months without showing sensor failure.