Summary: I plan to design a wireless sensor using an old solar sidewalk lamp for the Project14 Solar Powered Yard Gadget contest sponsored by Element14.
The Idea: I will be up-cycling an old sidewalk light. Five years ago, we bought several of these items for lighting the sidewalk and entry to our house. However, since then several of the lights have stopped working. On the ones that do still work, the clear plastic part of the light has yellowed and cracked due to UV exposure. The original cost of these items was minimal and normally they would be trashed after a few years and new lights purchased. However, for this contest, I want to give new life to these lights and add some functionality. I will be rebuilding one or more of these lights into a new light with a built-in wireless sensor.
Starting Point: For starters, each light has a small solar panel on top with a LED light in a clear plastic section below the solar panel. The rest of the product is a metal enclosure with plastic end pieces and a single plastic stake to anchor it in the ground. During the day, the product charges an internal battery and after the sun goes down the light comes on.
To research the feasibility of this project, I broke open one of the old lights and looked at what was inside them. After removing several screws, I could see the internals. At the heart of the product is a single AA Nickle Cadmium (NiCd) battery which fits between two battery terminals with wires attached. The wires go to a small PCB with three components. The main component is what appears to be a single piece solar battery charger/power management IC marked ANA658 in a 4 pin TO-92 package. Also on the board is a leaded inductor and the LED. The wires from the solar cell also attach to this board.
After taking out the NiCD battery, I measured it on my digital multi-meter and found that it was non-working (measures zero volts). So, I disconnected the solar cell from the PCB, stripped the wires and measured the output on the multi-meter. Putting the solar cell in the sunlight from a nearby window shows the open circuit voltage is approximately 2.4V. So, that part still works.
Next Step: I plan to characterize the solar cell to determine exactly how much power I can get from it.
To be continued...