Download Spy Kit: Lie Detector Instructions copy

Spy Kit: Lie Detector Project
Photo-Based Lesson
Lesson Overview
In this lesson, you will create a real-life version of the Lie Detector circuit prototyped in
123D Circuits.
Skill Prerequisites
● Successful creation of prototype
circuit in 123D Circuits
● Previous experience uploading
code to Arduino IDE
Learning Objectives
● Create a working breadboard
circuit
● Translate concept of variable
resistance from potentiometer
in prototype 123D Circuit to wire
probes
● Introduce concept of Galvanic
Skin Response
Background
This project utilizes an Arduino Uno with a few basic electronic components to measure
changes in skin conductance, or galvanic skin response, that occur when the subject is
telling a lie. The subject’s fingers are connected to electrodes, which completes a
voltage divider circuit as shown below, where R1 is the subject’s skin and R2 is a fixed
resistor.
(source:
http://en.wikipedia.org/wiki/Voltage_divider#Resistive_divider)
As the subject’s fingers get sweaty, their skin’s resistance will decrease which will result
in a larger voltage reading at Vout.
Code Summary:
The goal of the software is to establish the subject’s baseline skin resistance, detect
when it significantly changes, then enable an indicator LED. The maximum allowable
voltage variance is declared first, I used a value of 0.08 (or 8% change), but this value
can be adjusted to control the sensitivity and accommodate for differences in electrode
construction. Because the voltage across the subject’s skin will be relatively noisy, we
will compute the average of multiple consecutive measurements, a sample size of 10
results in stable readings.
There is a button connected from ground to an Arduino data pin with an internal pull-up
resistor enabled. When the button is pressed, the electrode voltage is saved as the
baseline that future values will be compared against. The user should put on the
electrodes and press the button once their readings have stabilized. When the button is
pressed, the LED will be off and the test can begin. The interrogator asks the subject
questions, and if the LED turns on it signifies a substantial change in skin resistance
due to increased moisture (a.k.a. probably a lie!).
Note: be sure to press the calibration button before each question to ensure that you’re
comparing against the subject’s current sweat level.
Materials
Two 3D printed Finger Beds
Half-size Breadboard
10mm LED
130 Ω Resistor
1k Ω Resistor
Breadboard-friendly Pushbutton
Jumper Wires
9V Battery Clip
9V Battery
Tin Foil
Rubber Bands
Construction Tools and Materials
Arduino Uno
USB Cord
Computer with Arduino IDE
Electrical Tape
Glue Gun/Hot Glue
Wire Strippers
Scissors
Step 1: Placing Components
Summary: Begin to assemble the Lie Detector circuit using real-world components. You
will place the components in the same breadboard location as in the 123D Circuits
model. Refer to Steps 1-3 if you want to review the function of each component.
When working with your breadboard, orient it towards you vertically, so that the power
rails are at the left and right sides and the lettered row is at the top.
1. Place the LED so that the anode (negative, shorter lead) fits in breadboard position
F23.
2. Place the anode (positive, longer lead) into breadboard position F25.
3. Orient the pushbutton so that the leads point out left and right.
4. Place the pushbutton so that the upper right lead (Terminal 12) fits in breadboard
position F18. The upper left lead (Terminal 11) should fit in breadboard position E18.
5. Place the 130 Ω resistor (R1) between breadboard positions I20 and I23.
6. Place the 1k resistor (R2) between breadboard positions E17 and F17.
Step 2: Prepare the Probes
Summary: Prepare the long jumper wires for inclusion in the circuit.
1. Cut two 1in. x .5in rectangles out of aluminum foil. This foil will increase the surface
area for your probes.
2. Using wire strippers, strip off one end of the extra long, red jumper wire.
3. Secure the exposed end to one of the pieces of aluminum foil using electrical tape.
4. Using wire strippers, strip off one end of the extra long, white jumper wire.
5. Secure the exposed end to one of the pieces of aluminum foil using electrical tape.
Step 3: Creating Finger Pads
Summary: Create the finger pads using your 3D printed objects as well as some simple
household materials.
1. Lay out the two 3D Printed Finger Beds with the finger side facing up.
2. Using a small amount of hot glue or double-sided tape, connect the aluminum foil and
red jumper wire to one of the 3D Printer Finger Beds.
3. Repeat the same process with the white jumper wire and the other Finger Bed.
4. Place both Finger Beds together with the foil on the inside. The grooved sides should
be facing out.
5. Connect the Finger Beds together by looping the rubber band around the outside
several times. The grooves on the top of the finger beds should help the rubber band
stay in place. Remember, you will need to place a finger inside, so don’t wrap too
tightly!
Step 4: Connect Components
Summary: Connect the components to each other and to the Arduino.
1. Connect a short black jumper wire from breadboard position J20 to the right negative
(-) rail.
2. Connect a short black jumper wire from breadboard position A17 to the left negative
(-) rail.
3. Connect a short black jumper wire from the right negative (-) rail to the GND pin of
the Arduino.
3. Using a long black jumper wire, connect the left and right negative (-) rails together.
4. Connect a medium orange jumper wire from breadboard position J25 to the A2 pin of
the Arduino.
5. Connect a medium blue jumper wire from breadboard position J18 to the A1 pin of
the Arduino.
6. Connect a medium green jumper wire from breadboard position J17 to the A0 pin of
the Arduino.
7. Connect a medium red jumper wire from the top positive (+) rail to the 5V pin of the
Arduino.
Step 5: Connect the Finger Pads
Summary: Connect the finger pads to your circuit.
1. Connect the long red lead wire to the right positive (+) power rail.
2. Connect the long yellow lead wire to breadboard position G17.
Step 6: Connecting the Battery Clip
Summary: In this step, you will connect the battery clip to the circuit.
1. Connect the black lead wire of the 9V battery clip to the other GND pin of the
Arduino.
2. Connect the red lead wire of the 9V battery clip to the bottom Vin (Voltage in) pin of
the Arduino.
Step 7: Uploading Code
Summary: Upload the Lie Detector code on to the Arduino Uno.
If this is your first time using Arduino IDE, please visit
http://arduino.cc/en/Guide/HomePage for information about installing and using the
program.
1. Open Arduino IDE.
2. Copy the code from the 123D Circuits Lesson into Arduino IDE.
3. Select your board (Arduino Uno).
4. Connect the Arduino to the computer via USB.
5. Select the appropriate serial port.
6. Click the Upload button.
7. Once code has successfully uploaded to the Arduino, disconnect the USB.
Step 8: Test it Out
Summary: Time to test out your circuit on a human being!
1. Ask your subject to place one of their fingers inside of the Finger Bed. Make sure
Finger Bed fits snugly around your subject’s finger, you need to make sure the foil is
touching their skin on both sides.
2. Connect the 9V Battery to the Battery Clip. You should see a small green light
illuminate on the Arduino, indicating that the Arduino is on.
3. Press the pushbutton to calibrate the circuit. If already on, pushing the button should
cause the LED to go out.
4. Ask your subject a Yes or No questions and see what happens. If they begin to
sweat, a common response when someone is lying, the LED will illuminate.
5. Push the button again to calibrate the circuit before asking another question of when
testing a different subject. Individuals have varying skin resistance, and skin resistance
will naturally decrease the longer a finger is in the Finger Bed, so it is necessary to reset
each time.
Photos