Ultrasonic switch

Circuit of a new type of remote control switch is described here. This circuit functions with inaudible (ultrasonic) sound. Sound of frequency up to 20 kHz is audible to human beings.

http://www.electronic-circuits-diagrams.com/sensorsimages/sensorsckt7.shtml

Infrared Head Phones

Using this low-cost project one can reproduce audio from TV without disturbing others. It does not use any wire connection between TV and headphones. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from TV to headphones.

http://www.electronic-circuits-diagrams.com/audioimages/audiockt3.shtml

Infrared Head Phones

Using this low-cost project one can reproduce audio from TV without disturbing others. It does not use any wire connection between TV and headphones. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from TV to headphones. Without using any lens, a range of up to 6 metres is possible. Range can be extended by using lenses and reflectors with IR sensors comprising transmitters and receivers.

http://electronicsworld.tripod.com/audioimages/audiockt3.html

Visible and Infrared Light Detectors

The sensors used are silicon phototransistors and Cadmium Sulfide (CdS) photocells. Both of these sensors allow less current to flow when they are dark than when lighted. Phototransistors change their conductance while photocells change their resistance depending on the intensity of the light falling on them.

http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html