The cable is positioned using a computer-driven battery-powered screwdriver, by winding it up and letting it out.

A battery-powered screwdriver makes it easy to wind the cable up, and the counterweight ensures it’s paid back out when turned the other way.

Photo essay: Power cable drive

Photo 1: Disassembled battery powered screwdriver on the bench

Photo 2: Wiring up a controller circuit for the battery powered screwdriver

Photo 3: Detail: soldering leads to the controller board

Photo 4: Calibrating signals and voltages between controller and screwdriver

Photo 5: Leads attached to the screwdriver motor

Photo 6: The motor returns to its housing, leads attached

Photo 7: Top view of controller board, with leads attached

Photo 8: A cable reel replaces more conventional bits on the screwdriver

Photo 9: Second view of takeup reel and controller attachment device

Photo 10: Screwdriver shell elements prior to reassembly

Photo 11: Remounting the drive motor inside the screwdriver shell

Photo 12: Attaching the screwdriver to the railing pulley-mount plate

Photo 13: Screwdriver motor with positioning cables

Photo 14: Half of screwdriver shell, affixed to railing mount plate

Photo 15: Screwdriver shell with motor in place

Photo 16: Cable wound up on cable spool attached to screwdriver

Photo 17: rear view of screwdriver attached to railing mount plate

Photo 18: Initial test set up, with screwdriver lashed onto railing (note use of polypropelene twine for testing).

Photo 19: detail of screwdriver lashed to railing for test

Photo 20: View from railing during test of screwdriver cable positioning

The motor pulls a cable through a pulley to change the angle of the solar cells in one direction as it takes cable up. The counterweight provides motion in the other direction when the motor turns in reverse and pays cable out. This is a simple, but effective solution that adds only a few watts per day to our overall energy budget.

Controller card for the battery-powered screwdriver motor

To enable remote computer control over our battery powered screwdriver we used an 8-channel controller card that could connect up through the parallel port to our PC. The desirable angles and timings are pre-programmed and run every day as a background application. At night, when the solar cells generate no power, the cells are set into the starting position for the next day.

The controller at work