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