Introduction
Materials
Safety
Procedure
Collect Data
Analysis
Further Investigations
How does our skin respond to stress?
Build a “lie detector” based on skin resistance.
You have probably heard that a polygaph test can tell if you are lying or telling the truth. Explain how you think it works.
- Experiment board
- GoLink
- GoLink header
- 2 clip leads (Note: test them for continuity.)
- aluminimum foil
- tape
- circuit parts:
- resistor – 30K to 100K (100K = brown-black-yellow)
- short bits of wire
Important: Unplug your circuit from the computer each time you work on it. This is standard practice and you should make it a standard habit.
The Experiment Board
If you are not familiar with how the experiment board works, go to this activity for an introduction.
Plug the header into the experiment board so that each pin has its own row to connect wires to. If it helps, use colored markers to make it clear which is which. It is customary to use black for ground, red for +5V, and some other color for the signal.
The Sensor
Make a way to detect the conductivity of your skin. This can be done by making two “sleeves” of aluminum foil, held tightly by tape.
Optional: measure the resistance between these two sleeves with your multimeter. This value is the ideal resistor for the circuit.
The Circuit
Put the sleeves in series with a resistor that is between 30K and 100K — that is, about the same resistance as between your fingers. (Note: 100K = brown-black-yellow.) Here is the circuit as a schematic. Explain why the sensor output is related to the finger conductivity in this circuit. Conductivity is the inverse of resistance. Why does the voltage reading go up as the conductivity goes down?
- Hint: You know that the signal voltage is somewhere between 0 and +5 Volts. What determines how close it is to one or the other extreme? What would the signal voltage be if the resistance of the fingers were 0?
Here is a picture of the circuit.
- Adjust your setup until the reading is reasonably steady.
- Enlarge the graph so that you can note small changes.
- Try to alarm or stress the person using the lie detector. Nothing dangerous or harmful, please! What do you observe?
- Change roles — let the experimentor become the subject. Try it again. What do you observe?
A standard reaction to alarm or stress is to sweat. This salt increases the conductivity of the surface of the skin and changes its conductivity. This is called the Galvanic Skin Response. Can you observe this phenomenon with this experiment?
Do some research on what survival value the galvanic skin response would have.