Introduction: Build Your Lie Detector

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The following information is a single lesson in a larger project. Find more great projects here.

Return to Previous Lesson: Create a Finger Bed for Your Lie Detector

Lesson Overview:

Now we'll build the detector!

Step 1: Introduction

Picture of Introduction

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.

Step 2: Place the Components

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 3: Prepare the Probes

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 4: Create Finger Pads

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 5: Connect Components

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.
  4. Using a long black jumper wire, connect the left and right negative (-) rails together.
  5. Connect a medium orange jumper wire from breadboard position J25 to the A2 pin of the Arduino.
  6. Connect a medium blue jumper wire from breadboard position J18 to the A1 pin of the Arduino.
  7. Connect a medium green jumper wire from breadboard position J17 to the A0 pin of the Arduino.
  8. Connect a medium red jumper wire from the top positive (+) rail to the 5V pin of the Arduino.

Step 6: Connect the Finger Pads

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 7: Connect the Battery Clip

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 8: Upload Code

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 9: Test It Out!

Picture of Test It Out!

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.

Congratulations, you have completed this project!

Check out other great projects here.

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