As a kid, I loved Christmas and Halloween themed necklaces that were a string of small LED lights, with a small coin cell battery. They had some problems though, their wires were thin and broke easily, and the batteries did not last very long. They quickly became trash.

More than a few years later, in late fall 2024, I was planning a trip to Disney World with my girlfriend, who was in a Mandalorian-driven Star Wars phase. Little did she know I was planning to propose to her there. To make that trip a bit more special, I came up with the idea of making glowing lightsaber-inspired necklaces for us to wear - one green, one red. Given my past experiences, I decided to run this with solar and supercapacitors, so that it would never need batteries replaced. And it worked!

However, these necklaces were not perfect. They looked great from a distance, but up close they were clearly handmade. The lightsaber blades (case epoxy) were tricky to get perfect, and the solar panel "box" that formed the hilt was hard to get quite right (hot glue plus cast epoxy worked but wasn’t as elegant as I had hoped). The necklace was also just rather large. While small enough to be a necklace, it is rather too large for daily wear. Currently they are hanging in the window as a window decoration, they glow at sunset every evening (hundreds of cycles now and no obvious degradation, about 30 minutes for the red and 3 hours for the green one). It actually worked rather well at Disney World, where we would alternate between bright sun and dark ride line, charge and discharge (the solar panel, when lit by sun, shuts off the light circuit).

I had planned to dedicate time to making a “perfect” version of the lightsaber necklace, but as a year passed and it became apparent these were not going to be daily wear items, I decided to come up with a version that would be even smaller and simpler which might be worn more often. Thus, the scintillation solar necklace idea was developed. 

The second version of the necklace is a ‘diamond’ (moissanite) with LEDs set behind it. They sparkle, controlled by an attiny mcu (unlike the fully analog lightsaber version). It reuses the same solar charging and super capacitor circuit as the original, but uses an LED boost driver instead of carefully selected LEDs to set the LED current. The idea is to create the ‘sparkle’ of the diamonds in sun artificially by switching between four different LEDs behind the stone.

Both of these build off my earlier solar business card project, where I had been testing solar panels, and was surprised to find that the tiniest solar panels actually work - you just have to be paranoid about managing choosing components with tiny amounts of leakage current. Neither use an 'energy harvester' IC, and they seem to work just fine without it (perhaps those would be worthwhile if you wanted this to charge only on indoor light).

Lightsaber Necklace Build Details

Components (the schematics in GitHub also have JLCPCB part numbers for some options as well)

CXHP2R7125R-TWX 6.3*11 2.7V 1.2F EDLC (main limit here is size to fit inside the hilt. Don’t use hybrid capacitors, they don’t like being fully discharged and this will be), CXHP2R7255R-TW another option
TSM260P02CX RFG -20V, -6.5A, SINGLE P-CHANNEL PO
BAT43WS RRG DIODE SCHOTTKY 30V 200MA SOD323F (TSMC, very low voltage drop)
ZEPTOSMDC0015F PTC RESET FUSE 15MA 13VDC 0201
AM-1456CA-DGK-E AMORPHOUS SOLAR CELL 7.95UW 2.4V 25MM_X_10MM
KXOB25-03X4F-TR  SOLAR CELL GEN3 2.76V 30MW
UCLAMP2501T.TCT TVS DIODE 2.5VWM 7.5V SLP1006P2T
DF2B5PCT,L3F TVS DIODE 3.6VWM 22VC CST2
HLMP-CM1A-560DD LED GREEN CLEAR RADIAL LEAD T/H Broadcomm (this circuit depends on the voltage range of the solar panel, the super capacitor, and the LED all aligning, so you can’t just use any LED. I use the PTC fuse here as a way of allowing a “flare” of brighter light briefly if fully charged with some LEDs. Although my main plan was to use high power...