Construction

Sensor

A rough prototype was constructed by etching copper clad boards at home using Kapton tape mask and 2:1 ratio H2O2 / muriatic acid bath.

Each half of the sensor is comprised of two electrodes etched onto 6 x 6 x 0.062 inch PCB. SMB edge-launch connectors connect to the plates. Spacing and sealing is provided by two 31 mil thick closed-cell adhesive foam sheets, cut to shape with approximately 0.75 inch wide perimeter. To form the ports, barbed 1/4 inch dripper-irrigation tubing connectors were trimmed to fit flush into 0.173 inch holes.

Capacitor plates are 1.0 x 2.25 inch each.

The barbed connectors were trimmed on one end near the flange, so they would extend slightly less than the PCB thickness when inserted into the hole. They were epoxied into place.

The mylar film was cut from emergency survival blanket, 1 mil or less in thickness. It was stretched out and pulled taut with tape. There were some slight residual creases left. The adhesive backer was removed and the foam was applied to the PCBs first, then the PCB was pressed down onto the mylar. The mylar was trimmed closely to shape with a knife.

Electronics

As a first implementation, the EVAL_AD7746EB evaluation board is used for the hardware. This board consists of an AD7746 connected a Cypress FX2 microcontroller, and from there to a host over USB. Short 6 inch coaxial cables connect the sensor to the board.

Software

The AD7746 eval board ships with a Windows GUI. This is fine for initial testing, but isn't useful for data collection and integration with other tools. With a little searching, I found the source code for the LabView application that forms the GUI. In this zip file was an excerpt from the Cypress EZ-USB firmware header file that documented the USB vendor-specific commands supported by the firmware. This provided enough information to write a simple Linux app to communicate with the board.

The app relies on fxload to load the Analog-provided FX2 firmware, and libusb to issue control packets. The firmware receives these vendor specific commands and issues I2C transactions to the AD7745 as commanded. The RDY# pin and LED can also be read / controlled.

This app provides the ability to configure the AD7746 and it streams capacitance and temperature measurements to stdout. From here they are logged to a file and sent to a plotting script.

The next step will be to stream them to a tool such as WinSDR or jAmaseis.