DIY Flying Saucer
This project was inspired by one of those solar powered lawn lights. I had taken the bottom off of one and hung it in a tree. A neighbor commented that it resembled a small flying saucer. Hmmmm.
This unit was constructed from a pair of plastic 'party trays'. The two halves are bolted together thru the middle using a long 1/4 inch bolt along with 4 each nuts, metal washers and rubber washers. Weather stripping was used along the outside edges. The entire assembly is hung from a tree branch using 50 lb mono filament fish line. At night, you cannot see the line, just the saucer flashing away like mad.
Two of the solar cell units were stripped from the existing lawn lights an put in series to produce 4 volts at 70 milliamps. A TIP42 (pnp) transistor is switched on when the light levels are sufficient to charge the lithium-ion cell. A schottky diode could have been used, but it had about 1/4 volt drop. The transistor drops less than 1/10 volt.
Overcharging is theoretically possible, but with a maximum of 4 volts from the solar cells, I do not expect this unit to self-destruct any time soon.
When light levels drop near zero, the unit begins operating. It runs a number of different sequences.
When it senses that the cell voltage has dropped to about 3.0 volts, the unit goes into dormant mode and waits for more light. The cell voltage is determined by reading the (more or less constant) voltage across the 1N4002 diode. As the reference voltage provided by the Lithium cell decreases, the reading at the diode increases. (Note: Yep, I forgot about the internal reference voltage diode and could have save 2 parts and 2 pic pins!).
4 red, 4 green, and 4 blue LED's are hot-glued around the edge of the bottom platter. A white LED is mounted near the center facing straight down. Connection to the circuit board is made via 2 14-pin dip connectors. Pairs of wires are split off and soldered to the individual LED's.
Current limiting the LED's left up to the PIC protection circuits. The two resistors (mounted at the LED's) on RA0 and RA1 are to allow in-circuit programming. The resistor on RC5 is needed for unknown reasons.