Galvanic Skin Response, or Electrodermal Activity has been used since the 1920's in police lie detector's, and recently has enjoyed a little bit more of the limelight as an arousal and stress meter. Some awesome folks at MIT's Affectiv Lab have made tools to help autistic children see empathy(http://affect.media.mit.edu/). It's also well known for it's use within scientology as the e-meter.
It's a simple yet pretty amazing technology. When people get excited or stressed they secrete micro sweat which is often imperceivable to the human eye. The greatest concentration of sweat glands is on your finger tips, so this is a good place to measure the change, although the underside of your wrist isn't too bad as well if you need to be moving around while you measure this.
I recommend finger based electrodes, or if you need to wear something for a longer period, the underside of your wrist.
Classic GSR devices are made with Wheatstone bridges, a classic circuit to measure small impedance differences. What makes this implementation interesting to me, is the lack of circuitry. It's different from the classic Wheatstone bridge implementation as it's using AC instead of DC current through the audiojack of the phone. It's putting out a 300Hz sine wave - AC instead of the expected DC signal. 300Hz turns out to be close enough to DC for this technique to still work. Looking at impedance changes with increasing frequency can give you a range of interesting variants on GSR, turning the phone into more of an LCR meter. Many different things can be done with this, but for now, let's stick with measuring excitement!
Ok - let's make it:
Step 1: Making the Electrodes!
Get an old iphone headset that you will never use again or buy one for $5 on Amazon. Buy some TENS electrodes, or clip electrodes on amazon, get a 4.7K resistor and a 0.1uF capacitor, and a hook them up as per the diagram. The stripping and preparation of the iphone headset cable is the most difficult part as the wires inside it are so small.
PUT THEM TOGETHER!
For the IPhone to register the microphone as a voltage input, you need to place an impedance where the microphone input of your(now severed) cable used to be. The circuit diagram below describes a functional set up, and also shows my initial bread boarded attempt. It took me two severed cables to get this right. The first cable was an old standard iphone headset cable which has a lot of shielding that's hard to remove around the mic cable strand. I'd recommend getting a cheap 4 pin cable elsewhere as 4 waxed wires are easier to modify.
Note: You must melt the wax off the ends of each of the 4 tiny wires with the tip of your soldering iron, and tin them, to expose them properly.
I made a revised pair of electrodes after determining the trick to enable the iphone to measure electrical impedances through the iPhone jack. The final photo shows my final electrodes, neatly heatshrinked with velcro wrap around finger pads.
Remember, the 4.7K resister is required as per the circuit diagram for this to work with the IPhone, so other instructions that show a straight connection will not work. The finger electrode designs are applicable, but please remember to add in the resistor as shown in the circuit diagram! That's it really, if you do this right, you are ready to download the app and try it out...
Step 2: Test It!
Download the free GSR app from the app store, plug in your cable, and off you go! Here is a link to the app: App store link!
It's also available and opensource on github: HERE! You'll need to install XCode to compile it onto your phone, and purchase an apple developer's license to update it. Collaborations and updates welcome!
Before you plug in the cable you should hear a tone when you press the start button. After you plug it in, your plot will switch to becoming a GSR meter! Put the electrodes on and get those awkward questions ready!
Sit down with your friend and see if you can excite them(or stress them out)!