Use your finger as a cable - and a speaker!

Came across this kind of cool link from a friend.

http://www.electronicproducts.com/Digital_ICs/Video_Graphics_Audio/Disney_technology_transmits_sound_through_body_can_only_be_heard_when_touched_to_ear.aspx

We have looked at some of Disney Research's stuff before. They have sketched on ways and methods to control for example media players by certain touches and gestures on and with the body. Google for their touché system!

Wide-band signaling

In the perspective of human body communication through capacitive coupling, wide band signaling has mixed blessings if it is not properly designed. 

As shown in Fig. 1, the signal strength or power on the transmitter side is an important parameter to enable communication through human body or any object which has good dielectric properties. The signal strength can be altered by changing voltage or current or both on the transmitter side and therefore driver circuit associated with capacitive plates has an important role to play. Similarly, receiver sensitivity is also an important parameter which could help in signal detection at lower transmitting voltages and currents.

As shown in Fig. 2, there is special importance of ground electrode layout compared to signal electrode in the coplanar capacitor configuration. Both of them form two plates of coplanar capacitor and electric field is confined in between them starting from the signal plate and ending at the ground plate. So the question of using single signal electrode is not valid argument.

Polarization and change in capacitance

The concept of electric field polarization is a convenient explanation for describing the phenomenon of change in capacitance between the positive and negative plates of a coplanar capacitor. Any object which has certain dielectric properties when placed under the influence of external electric field, it gets polarized. This concept has also been illustrated in Fig. 1. This polarized object causes the redistribution of charges on positive and negative plates of the co-planar capacitor which can now accomodate more charges, not shown in Fig. 1. This requirement of extra charges which results in change in capacitance of the plates according to C = Q / V relationship is fulfilled by the source which could be a static cell or a dynamic signal generator. 

Similarly human body or wood or paper can also alter the capacitance of the plates which could be detected by the circuit and if the electrodes (positive and negative plates of the coplanar capacitor) are not properly designed then it could also trigger false alarms. 

Can this be safe?

Ricardo is demonstrating the use of his transceiver for body-coupled communication by strapping it to his head. Irfan, not shown in the picture, is standing by with another transceiver in his hand.

When shaking hands, effectively bridging Portugal and Pakistan :), they also demonstrate that a communication link is established.

Ricardo Matias in action (and sorry for the somewhat shaky photo).

The power levels are orders of magnitude less than those of mobile phones and operating frequencies are selected to be well above the "biological frequencies" of the human body. The body has a frequency profile up to some kHz.