No more fumbling with your mobile phone’s or iPod’s tiny touchpads — you can now have a keypad on your forearm, just by combining acoustic sensors and a mini-projector, say scientists.

An international team has come up with such a system, called ‘Skinput’, which has the ability to detect the ultralow-frequency sound produced by tapping the skin with a finger, and microchip-sized projectors now found in some cellphones, the New Scientist reported.

According to the scientists, the system beams a menu or keyboard onto the user’s forearm and hand from a projector housed in an armband. An acoustic detector, in the armband, then calculates which part of the display to be activated.

But how does the system know which icon, button or finger the user tapped? Well, for this, the scientists have identified various locations on the forearm and hand that produce characteristic acoustic patterns when tapped.

The acoustic detector in the armband contains five piezoelectric cantilevers, each weighted to respond to certain bands of sound frequencies. Different combinations of sensors are activated to differing degrees depending on where the arm is tapped, say the scientists.

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