I am using an arduino uno with a wireless sd shield to measure galvanic skin response.
The setup uses two tens machine style electrodes, and measures the voltage coming into an analog input. The whole thing is portable and powered by a 9v duracel battery.
It is designed to take a voltage reading every 2 seconds, and print that to a micro sd card, so that it can take continuous readings over many hours.
I finally got the whole thing working and measured the voltage with the two electrodes placed about 2 inches apart on my chest over several hours. The graph of the readings shows a steady increase in the readings from about 40 to about 160 over about one and a half hours then a flatter range of between around 140 to around 180 over the next two hours. Basically a linear increase for two hours followed by sideways movement for two hours. I suspected this might be due to sweat build up under the electrodes slowly increasing conductivity, until they became saturated after 2 hours. (Attached image 1).
I decided to test the setup on a potato for several hours. If the graph shape was solely due to sweat, then since potatoes don't sweat the graph shape should be basically flat. Unfortunately the potato gave an even more extreme graph. It first shows a very rapid increase in measurement from about 270 to 350 over 15 minutes, followed by a more gentle linear increase from around 350 to around 410 over the next 2-3 hours. (Attached image 2).
The potato readings are clearly not due to sweat. This was done on a wooden table with no other interference. Inside a dry room in a warm house.
The wires used to take readings are fully insulated so that no bare copper is exposed.
The smaller fluctuations between readings are to be expected, but the general graph shape seems to suggest an underlying problem with the sensor setup.
Does anyone have any idea what might be causing these readings? Could it be something to do with the ground?
(I have no experience with electronics before this project!)
(The setup has:
9v battery going into the power jack, with proper 9v connector soldered to jack having positive on the inside.
3.3v going to first electrode via resistor.
Second electrode going back to arduino with the wire split so that one end goes into an analog input and the other end goes to a ground pin via a resistor.
An RTC with its own battery powered from the 5v output, with a ground wire going into another spare ground on the arduino, and info wires going into two more analog pins.
The arduino is fully sealed inside a plastic tub. Only the battery and electrodes are outside the tub.
I am using foam to cushion the arduino inside the tub, and the foam is touching many parts of the electronics, but I don't think it is conductive.)