Cati Vaucelle
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HARDWARE - by Cati Vaucelle
Visualizing the unseen always leads to fascinating and playful exploration. All devices emit background signals (electrostatically, magnetically, acoustically, and optically) that are characteristic of particular devices and also sometimes indicate that device’s mode of operation.
CIRCUIT DESIGN
RESULTS
The bracelet contains an electrode antenna made from copper fabric and simple electronics that capacitively pick up the frequencies emitted from a laptop LCD screen.
The user wears the bracelet and is coupled to the circuit’s ground via the bracelet’s inner electrode. The present design uses a high-impedance pickup electrode (actively shielded from the body) on the outside of the bracelet.
The circuit’s front-end high-pass filter is designed to accept frequencies ranging from 50 kHz up to the 3Mhz-level rolloff of the amplifiers (lower frequencies can be accommodated by dropping the high-pass cutoff frequency).
A peak detector extracts the resulting envelope, and the detected signal goes through two stages of amplification, providing a net gain of 55 dB.
This envelope drives an LED that provides qualitative visual feedback on the local electric field intensity, changing brightness with field strength. The circuit board is sewed and connected to the bracelet.
My first results were measured with an oscilloscope. The bracelet picks up frequencies above 50 kHz, such as radiated from a typical LCD laptop screen.
I noticed that, as expected, the further away the bracelet is from the LCD laptop screen, the smaller the signals on the oscilloscope became.
With my current antenna I can detect induced LCD signals up to 14 inches away from the laptop display (see figure a and b) – more range is easily possible with more amplification and a more sensitive front end.
Figure a. The bracelet is located at 14 inches away from the display – the oscilloscope shows the front-end output.
Figure b. The bracelet is located at 1 inch away from the display.
Data from the bracelet can be retrieved and analyzed by a computer for more detailed visualization (through my custom designed software application) in addition to the immediate analog LED feedback on the device.
My original circuit sketch. Next step, Eagle layout editor for PCB design!
In the second test, I embedded all electronics within the bracelet itself, exclusively using floating battery power, grounding only to the body, and only displaying a progressively illuminated LED to give us feedback on the level of signals in my passband.
The circuit is still quite responsive - the LED progressively turns on starting at 12 inches from the LCD screen, achieving full brightness at a 1-inch proximity.
In this picture, the battery-powered bracelet is at 1 inch away from the screen, where the feedback LED is fully illuminated.