By Sean M. Montgomery and Ira M. Laefsky

Excerpted from a Primer on Biosensing in MAKE Volume 26.

When you experience an arousing stimulus, like an evocative question, a startling noise, or even a disturbing thought, your body generates a variety of psychophysical responses.

One of these is micro-pulses of sweat released after a 1- to 2-second delay from apocrine sweat glands that are tied to the arousal systems in your body via adrenaline and other hormones. The reason your palms might get sweaty during public speaking or a job interview, for example, is because your mind is on high alert and every small stimulus generates one of these pulses.

Each pulse of sweat increases the electrical conductance of your skin, and when this conductance is measured and tied to arousing stimuli, it’s referred to as galvanic skin response (GSR). The Truth Meter measures GSR for display on an LED or for input to a microcontroller. It’s called a Truth Meter because GSR is an important component of lie detector (polygraph) tests used to assess how nervous subjects are while answering questions during interrogation.

So how do we measure GSR? The first step is to understand the signal. With each pulse of sweat, skin resistance decreases suddenly and creeps slowly back up as the sweat evaporates. The Truth Meter circuit transforms this pattern of drops and slow recoveries into sharp spikes deviating from a steady baseline, to light an output LED or trigger some other action.

The resistance sensor itself is simply 2 metal cuffs attached around your fingers with a piece of velcro. Hypoallergenic metals used in jewelry and those less reactive with skin, such as stainless steel, are the best, but any solderable metal will work. We recommend copper or brass foil. Fingers are a good place for measuring GSR because apocrine sweat glands occur in very high concentrations on fingers and palms.