Wireless GSR Progress

by Carson Reynolds

In order to develop a wireless galvanic skin response (GSR) sensor several design issues need to be overcome. Firstly, is the design of a GSR amplifier that doesn’t require tuning and is precise. Secondly, this amplifier needs to be interfaced with a wireless transmitter. Lastly, the entire device needs to be packaged in a manner that makes it comfortable to use. Recent work with the Mindgames Group at Media Lab Europe, has produced several designs that satisfy these requirements.

Galvanic skin response is a term that covers a variety of electrical phenomena on the surface of the skin. The psychophysiology community recommends measuring skin conductivity response (SCR), because it is a variety of electrodermal activity that maps accurately to the activity of sweat glands [1,2]. This means that a sensor should treat the skin as a conductor and keep voltage constant across the surface of the skin.

In recent months we have developed 15 designs for GSR amplifiers. After informal testing, we’ve settled on a single design, which measures SCR. From the schematic for this design, we see that it consists of signal conditioning circuitry and a PIC microcontroller. The microcontroller interfaces with wireless transmitters via a UART port. A printed circuit board prototype for this sensor has been constructed and inital tests performed.

The skin conductivity response consists of two components: the tonic and phasic [3]. The tonic is slow moving, oscillating over the course of days. The phasic is fast moving, and spikes sharply when a person is startled, and generally increases when a person is psychologically aroused. Many GSR and SCR amplifiers include some adjustment so that the tonic portion can be removed and the phasic measured more accurately. The design presented above, includes a potentiometer to trim the tonic portion off and adjust the amplifer’s output.

A new design removes the need for this adjustment by incorporating a 16-bit analog to digital converter (ADC) with enough resolution to ignore the tonic offset. The electrical schematic for this approach is considerably less complex, due mainly to the removal of the PIC microcontroller. The ADC chosen provides a I2C interface which can be interfaced with wireless transmitters like the BlueCore 2. The resulting board is considerably smaller. So much smaller that it could conceivably be packaged into a design the size of a ring.

What remains is to fabricate an interface board which physically connects to the BlueCore 2 Bluetooth chipset. A physical housing for this also needs to be constructed. One design being considered is an ambient display [4] in the shape of an orb that would house the electronics. A second, more ambitious design, would be a ring sensor that would consist of 3 layers: amplifier board, bluetooth board, and watch-style battery.

One interesting application for this sensor could be a tug-of-war version of Relax To Win, in which a crowd of participants play the game and simultaneously form a Bluetooth network that forwards sensed information.

[1] Lykken, D. T., & Venables, P. (1971). Direct measurement of skin conductance: A proposal for standardization. Psychophysiology, 8. 656-672.

[2] Fowles, D. C., Christie, M.J., Edelberg, R., Grings, W.W., Lykken, D.T., Venables, P.H. (1981): Committee report: Publication recommendations for electrodermal measurements. Psychophysiology Research. 18. 232-9.

[3] Boucsein, W. (1992). Electrodermal Activity, Plenum Series in Behavioral Psychophysiology and Medicine, Plenum Press.

[4] Wisneski, C., Ishii, H., Bahley, A., Gorbet, M., Braver, S., Ullmer, B., Yarin, P. (1998). Ambient Displays: Turning Architectural Space into an Interface between People and Digital Information: Cooperative Buildings. Springer Verlag, February 25-26, 1998. http://citeseer.nj.nec.com/wisneski98ambient.html