GSR to MIDI
Goal:
Create a computer interface
that based on the biofeedback of multiple users.
Define terms:
-
Galvanic Skin Response(GSR)
o
A change in the electrical properties of the skin in
response to stress or anxiety; can be measured either by recording the
electrical resistance of the skin or by recording the weak currents generated
by the body
o
One of the number of sensors used in lie-detectors
-
Musical Instrument Digital Interface(MIDI)
o
A universal language used by digital music
instruments for communicate
-
Analog to Digital Conversion(ADC)
o
Periodic samples of a analog electrical signal
represented as digital binary code
-
Biofeedback
o
The body’s physiological response to the environment
o
These responses can either be voluntary, such as a
muscular twitch, or involuntary, such as respiration, heart rate, perspiration,
or electrical skin response
Step 1: Measuring GSR
-
Skin makes very small voltage
o
Using a multi-meter we measured a small voltage
generated between two finger tips, ~20millivolts
-
Amplification
o
We attempted to use a simple LM386 op amp to amplify
the 20millivolt signal to a measurable range between 0 and 5 volts
o
We was able to amplify a signal, but found that it
did not fluctuate with environmental changes or self-imposed stress
- Relaxomat
o
We found an obsolete GSR device from “West” Germany
used as a relaxation trainer
Step 2: ADC to MIDI
-
I had previously created a MIDI controller using 8
potentiometers and a PIC 18F452 microcontroller
-
Using the 8 ADC input on the PIC I sampled the levels
of each potentiometer and output it as MIDI controller information
Step 3: Hacking Relaxomat
-
I traced the electrode inputs of the Relaxomat back
to a LM324N quad op amp
- Testing the voltage on each pin of the op amp I discovered that the voltage on pin 1 changed in direct relationship to the sound and display of the Relaxomat
Step 4: Reducing the voltage
-
The fluctuating voltage produced by the Relaxomat’s
op amp was from ~ 0 to 9Volts
-
To reduce the voltage range from 0-9V to a range of
0-5V to read into the PIC’s ADC inputs
-
I added a 10k potentiometer between the Relaxomat
output and the PIC’s ADC input. This allowed me to fine tune the resistance and
precisely drop the voltage from 9Volts to 5Volts
Step 5: Compose GSR symphony
-
I plan to connect a number of Relaxomat devices to
the GSR to MIDI converter
-
Routing the GSR MIDI to synthesized sounds and video
signals in Max each user of the device will control sonic and visual elements
of the environment through biofeedback
