DIY Polygraph

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. “