This hobby circuit has been used in
some museum displays and exhibits when the electronics needed to trigger an
audio/visual presentation is behind a plastic or glass panel. A finger pressed
against the glass, over a designated area triggers the switch.
Infrared light reflected off a
finger is used to activate this switch circuit. Drawing only 30uA from a 3v supply,
this electronic circuit will detect a human finger with a range of about 1 inch.
The sensor uses an inexpensive infrared LED and a matching photo diode. The
circuit is immune to ambient light.