Hi geeks and geekets, this is Marty, the other half of Lumen Jewelry. While developing new circuits for Lumen Jewelery I strive for simplicity. During development of the circuit for the Heart I stumbled onto the simplest circuit that is solar powered, glows, and keeps glowing after it gets dark. It’s only three parts. 3!!! I call it Elemental.
The first part on the left is an amorphous silicon solar cell. Amorphous silicon solar cells have a different band gap than crystalline silicon and work best with visible wavelengths of light. I normally use an AM-1417CA solar cell from Digikey, but any four or more element cell from a cheap calculator should work just as well. NERD SPEAK ON: The solar cell needs an open circuit voltage of at least 2.7 volts in direct sunlight to work in this design. NERD SPEAK OFF.
The middle part is the high-brightness blue LED. I used this LED but any high brightness LED should work. If blue isn’t your thing, other color high-brightness LEDs will work just dandy.
And the third part is a one Farad super capacitor. You read that right, no micro-Farad here, but a FULL FARAD. With this beauty a fully charged Elemental will stay lit for several hours in pitch black. Bigger capacitors will extend running time but also extend charging time, whereas smaller capacitors will do the reverse.
Finally, the leads of the super capacitor are used to join all three pieces and complete the wiring. If you want a larger completed circuit, a through hole LED can be substituted for the surface mount LED I used.
The operation of the circuit is super duper simple. Straight out of the box the supercap needs conditioning, so charge it all day in a window. After that Elemental will fully charge in about an hour under most bright lights. The glow visibly dims in the first hour after a charge, but takes many hours to completely run out of juice. If the piece is exposed to a constant light source, even florescent office lighting (the least favorite light source for most solar cells) is enough to keep the LED happily glowing.