Operating Voltage: 3.7V DC Solar Current: 25ma max. LED Lamp Operating Current: 10-25ma.
If the solar panel that you use produces more than 20ma, it may be necessary to insert a series resistor between the LED and the solar panel to prevent the LED from burning out. A 50 ohm 1/4 watt resistor is probably about right for the job, the exact value may need to be optimized according to the solar panel that you use.
This concept could easily be expanded to systems with larger arrays of solar cells and more LEDs. The capacitor is not required, but it will keep the LED from flickering if the panel is briefly blocked, such as when a bird flies by. With 7 solar cells, the LED will only light in fairly bright light, if you use up to 10 solar cells, the circuit will work nicely in overcast skies.
For an interesting modification to this circuit, replace the 1000uF capacitor with a 1 Farad/5.5V "Memory Backup Capacitor". An Elna DB-545D105 device was tested on the circuit, after charging up in the sun for a few minutes, the capacitor was able to light the LED for several minutes.
Mount the solar cells on the perf board and solder them into a series string. An easy way to do this is to connect short segments of bare wire-wrap wire to each cell, route the wires through the perf board and solder the ends on the bottom. Connect two wires to the ends of the series string of cells and secure the wires to the circuit board. For outdoor applications, seal the edge of the panel with silicone caulk or other water proof material. Also, seal the mounting screws where they pass through the plexiglass.
Connect the LED and capacitor in parallel, wire them across the two power leads. Be sure to get the polarity correct, otherwise the LED won't light up. Solder the parts together. Be sure to heat-sink the LED leads while soldering, LEDs can be easily destroyed with too much heat.
7-10x photovoltaic cells, rated at 15-25ma each. 1x white LED, high efficiency types work best. 1x 1000uF 15V (or greater) electrolytic capacitor. 1 piece of perforated or printed circuit board. 2 pieces of clear plexiglass. 28 gauge bare wire-wrap wire. 24 gauge speaker wire. miscellaneous screws, nuts, and washers. silicone caulk.
Jameco 1-800-831-4242 http://www.jameco.com/ Digi-Key 1-800-DIGIKEY http://www.digikey.com/ Electronix Express 1-800-972-2225 http://www.elexp.com/