Hacked Gadgets Forum

November 19, 2007

DIY Polygraph

at 5:07 am. Filed under Complex Hacks, Computer Hacks, DIY Hacks, Electronic Hacks

Jordan Crittenden and Edwin Lai built this DIY Polygraph, their goal was to construct it for $50.00.

“The basic design of the polygraph is shown in the diagram to the right. The four analog circuits: plethysmograph (pulse rate meter), GSR meter, breathing rate meter, and audio preamplifier, are attached to the first four pins of the Mega32 analog to digital converter (ADC). The polygraph can then be run in one of two modes: pulse, GSR, and breath sampling or audio sampling. This is simply because the microcontroller is the limiting factor in processing power. In the first case, the microcontroller samples the three signals in a polling loop. The samples are taken at a fast enough rate (100Hz) that it appeared to be sampling each signal in real time. In the second case, the microcontroller exclusively samples audio at 8kHz.

In PGB (pulse, GSR, breath) mode, the microcontroller waits for a signal from a concurrent MATLAB program (running on a laptop) indicating a data request. The data is sent to the laptop over an RS-232 link via a serial to USB adapter and it is plotted in real time. The MATLAB program performs the signal processing, such as computing pulse and breathing rates as well as skin resistance.

In audio sampling mode, once a set of 32 sequential samples is acquired, the microcontoller performs a 32 point DCT on the audio signal to extract the frequency decomposition of the subject’s voice. At 8khz, 32 samples cover 4ms of speech and are enough to capture the primary features of human speech. The signal power is computed and the average DCT is updated with the new data. A concurrent MATLAB program periodically polls the microcontroller for the current DCT coefficients. These values are then plotted in a time-based heatmap, along with the signal power and average DCT. “


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12 Responses to “DIY Polygraph”

  1. Chetan Says:

    Hello guys I’ve designed Plethysmograph using LM358 but i’d like to know more abt your circuit so that i can learn more from you guys. so plz help me out. cheers.

  2. Tutorial mikrokontroler dan teknologi elektronika « Darie nge-blog Says:

    […] Polygraph (hacked-gadgets) […]

  3. Surya Says:

    Hi friends………
    i had been trying to execute ur MATLAB as well as C pgms for testing….buts its showing error….would u tell me which compiler u used for c pgming & which matlab version……

  4. Alan Parekh Says:

    Hi Surya,

    You may have to click on the link to get more details since I am not sure that the creator of the device is monitoring this comment thread.

  5. where can i buy this Says:

    i want to buy this one pleases contact me phone 00971509898353

  6. Alan Parekh Says:

    This is a DIY project so you would need to make your own.


    […] Polygraph (hacked-gadgets) […]

  8. ravikiran Says:

    i want to know that which simulator you used in your project

  9. Milarepa Says:

    I built a GSR meter with interface in a ‘gaming’ sort of framing, as a project in a beginners programming course. It’s way less sophisticated but alright as a proof of concept and for learning!

  10. GSR-Monkey - Skin Conductance Measurement System - Hacked Gadgets - DIY Tech Blog Says:

    […] Via: Hacked Gadgets Comments […]

  11. wasim Says:

    Hi Alan
    Can you send how to connect the circuits plograph
    (or Simulation all circuits) with each other, and this controller to my mail(w.basheer@yahoo.com).

  12. Alan Parekh Says:

    Hi Wasim,

    It was just featured here. Please click on the link in the article for additional information.

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