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Aside from sensing biometrics, Virtual Meditation's sensors needed to be minimally invasive. The final script called for two types of biofeedback: heart rate and touch (physical contact between the two volunteers).


Sensing Heart Rate--

EKG style (senses electric signals)
electrode versions are too invasive
finger versions would get in the way of natrual hand-holding
either version might be washed out by our GSR sensor (negating the higher accuracy)

Optical style (usually IR light beamed through an earlobe or finger)
less accurate than EKG (motion can give a false reading)
might be washed out by our stage lights


Virtual Meditation uses ear clip style optical heart rate sensors from Pasco Scientific. The overriding design philosophy for our sensors is minimal-invasiveness, and on that front, ear lobe sensors are clearly the winner.



Sensing Touch--

Pressure Sensor (holding some kind of air bladder between the two volunteer's hands)
simple mapping -- the harder you press, the higher the signal
how do you stop/start holding hands (does the balloon fall if you let go or is the balloon attached to a glove?)
you're not holding someone's hand anymore -- you're holding this weird 'thing' with a tube coming out
fatigue issues (both for the volunteers and the air bladder)

Galvonic Skin Response (GSR)
complex mapping (actually measuring skin resistance: surface area touching, sweat, sensor placement, etc)
great for seeing when they touch (on/off), hard to control gradiations
touching doesn't have to be hand to hand (any skin to skin will work)

We decided to build our own GSR sensor, and weld the ends to a pair of rings. Each volunteer puts a ring on their finger at the beginning of the performance, and we pipe a very low current across the two rings-- we know when the volunteers are touching because they complete a circuit. The rings are adjustable, and we can place the rings on the volunteer's outside hands so they're not encumbered when they hold each other's inside hand.


DAC --

Finally, we need a way to get the sensor reading into the computer. We're using a LabJack U12 analog->digital converter to read our sensors. The labjack runs off the computer's USB, so we have the ability to migrate to laptops for running the show in the future.