Arduino Polygraph

I’ve invited some fellow writers for dinner and this book went missing.

Paraphrasing John Goodman in The Big Lebowski: “This is what happens, Larry.”

Instead of sending who-stole-the autobiography-of-Klaus-Kinski email. I’ve decided to make a Nano Lie Detector (AKA Polygraph) for next dinner.

Parts

• Arduino Nano
• Analog VU-meter x2
• Heart Rate sensor
• Jumper cables
• Aluminum foil
• Switch
• Buzzer
• 5V power source

Theory

A polygraph is a device that measures physiological indicators while a person answers a series of questions. Deceptive answers will produce certain physiological responses that can be differentiated, for example: increased heart rate and changes in sweat gland activity (galvanic response).

There is a cheap sensor to measure heart rate and with some easy to find parts, a galvanic response sensor can be made. Arduino will read both sensors and display the information using a vintage analog VU-meter.

Obstacles

I’ve found 2 small issues during this project. One related to the VU-meter and the other one related to GSR. To move analog VU-meter needle you have to send voltage through a digital output pin using PWM and I’ve forgot that not all digital pins are capable of producing PWM. For Arduino Nano: pins 3, 5 and 6 can be used but not 4.

The other issue was related to galvanic response sensor. I’ve tested several ideas found on the web - no circuit at all, amplifications, filters, etc – but readings were not reliable until I’ve connected one jumper to a 300k ohm resistor and Ground besides A1 input.

Code

Code just reads heart rate sensor and sends coordinated tics to left needle, then reads analog input 1 (higher values means more finger sweat and a possible lie) to move right needle. Finally a comparison triggers the buzzer. There is a small setting named Threshold for the Heart Rate that has to be manually configured. It is the limit to start considering new heart beats. A potentiometer could be added to manually adjust this setting (instead of hard-coding)

Custom case

Two 3D-printed rectangles, 4 screws and one handle from a drawer. Cardboard will work as well.

Download 3d printed parts for the Arduino Polygraph lie detector

Demo

Where to go from here

A button to differentiate answers, local storage and reports could be good improvements.

Customizations, Arduino and Raspberry projects

https://www.instagram.com/ronibandini/
https://twitter.com/RoniBandini