Introduction: Solar Powered SKULL Blinky LED Pendant Jewelry

This instructable is for solar powered Skull with sinister red LED eyes. It measures about 2 1/2" by 1 3/8", including the USB tab. It has one hole through the top of the board, making hanging easy. Wear it as a necklace, earrings, keychain, bond on a pin, or hang it in a window. The possibilities are up to you, and we'd love to see what you do with them.

The schematic and parts list are included. You are welcome to wire up your own, copying is the sincerest form of flattery. We also have the kit available in our Store.

If you have soldered projects before, this will be easy. There are only 19 pieces to solder onto the board, and all are through hole. Depending on your soldering savvy it may take anywhere from 30 to 45 min. Watch it in action here!

Skull kit01.mp4 from Robin Lawson on Vimeo.

So lets begin!

Step 1: Tools

Here's what you need:

A soldering iron
Diagonal cutters

Helping hands and needle nose pliers are optional, but helpful.

Step 2: Schematic and Parts

Schematic is above.

Parts list is below.

C1 - 47uF ceramic capacitor (medium blue one)
C3 -10uF ceramic capacitor (little blue one)
C5 - 0.033uF Timing capacitor (TINY tan one)
U1 - MCP6542 dual micro-amp comparitor
SC1 through SC6 - BPW34 photo-diode
R6 - 330ohm resistor
R1, R3, R4, R5 - 10M resistor
D1 and D2 - amber LED to blink
Switch - SPDT switch 3-pin 0.1 spaced
C2 - 0.03F coin super cap

Step 3: Capacitors

Start with the three small caps. You may need to bend the leads to make them fit. They may all look similar, but it is important that the correct one go in the correct slot. BTW, they don't have a polarity, so once you figure out which is which you can't get it backwards. Below is a short description of how to read the numbers on them, but for more detail check out this cool electronics wiki.

C1 is the medium one. The tiny numbers on it read "476". The first two digits are the value, the third is the multiplier. Capacitors are measured in pico-Farads. So 476 is 47*10^6 pico-farads.

C3 is little, and has 106 written on the side. Solder C1 and C3 on the top side as shown.

C5 is the TINY tan one and has 333 written on the side. C5 should be soldered on the back side, as it sits really close to the board. If its sitting too high up, simply melt both pads with the soldering iron while pushing down on the capacitor with a needle nosed pliers.

Step 4: Chip

Next the chip. Be very careful of stray electric shocks, they may fry the chip. We know this from experience, ouch. Ground yourself before handling the chip by touching a large metal object first. We find the bolts on outlets are usually grounded, and often nearby.

Line up the chip on the board, U1. The cutout on the chip should match the square on top of the board chip footprint. You can see it in the picture. You may have to squeeze the leads on each side to get it to snap in.

Solder on top as usual.

Step 5: Resistors

Next the resistors. They don't care which direction. Bend the leads so they fit, and pull them through the board so they are nice and tight.

We recommend starting with with R6, its the orange odd one out. Check out this handy dandy link for how to read resistors. Unfortunately, they are tough for color blind people to decipher. But, I hear there is an App for that. ;)

Again, solder from the top.

The remaining 4 resistors are all the same, brown black. Pull them through and solder them from the top.

We recommend flipping it over and trimming the leads at this point.

Step 6: Photo-diodes (AKA Mini Solar Cells)

Next the photo-diodes, aka the mini solar cells. It really REALLY matters that these are put in correctly. The positive end is marked on the board with a +. The positive on the photo-diode has a TINY dot, as well as a metal end which turns into the pin. Match that up with the + on SC1 through SC6. Like with the chip, you may have to gently squeeze the leads together to make it slide into the holes. Solder them on from the top one at a time.

Step 7: Super Cap

And next, the SUPER CAP! This puppy will keep your skull going for several hours after dark. The negative lead is labeled on the body of the cap by a '-' sign on the metal band. The positive lead is labeled on the board. Correct orientation is shown in the picture. Solder on the back side.

Step 8: LED! BLINKY!

Now for the fun part, the blinky bits! D1 and D2 are the red blinky LEDs. It is also important that they be soldered in correctly. But luckily it is easy to tell the polarity. The circle around their holes has a flat side. Line that up with the flat on the LED and you have the polarity correct. Another way to tell which lead is which, the positive lead is the longer one, and should be on the outside as shown.

You can also install the switch at the same time as the LED's. It doesn't matter which way you put it in. One warning, the switch is made of low temperature plastic, so it will easily melt if the soldering iron touches it. By this time you should be a pro at soldering so I wouldn't worry too much about overheating the pins and melting the switch.

We recommend soldering while upside down, as the LED and switch cover the pads on the top. Flip it over and solder them on.

What is the best part about this step? You're done! AND if you are in a bright naturally lit room, and soldered it all correctly, IT WILL NOW BLINK! The LED's will run directly off the ambient light collected by the photo-diodes. Or for even more power, stick the USB tab into any available socket and they'll REALLY shine like below. WINNING!

Skull kit02.mp4 from Robin Lawson on Vimeo.

Step 9: FUNction!

The first time you charge your skull we recommend conditioning the super caps. What does this mean? Charge it above and beyond full capacity and store this for a few hours. It keeps the caps from having a memory and undercharging. We also recommend cleaning off the flux with orange clean. The flux may corrode the leads over time. See the next step for details.

But you are probably impatient to PLAY WITH THE BLINKY!! We understand. Skull operation is super simple. It has two modes of behaving controlled by the switch. We call them "Store" and "Direct:

How to play:
1. Switch into “Direct” mode (unlabeled). The piece now gets its energy directly off the surrounding light. If it’s dark, NO blinky. If it’s bright, blinky awesomeness for all!
2. Not all light sources are the same, have fun figuring out which ones make the piece blink.

How to charge:
1. Switch into “Store” mode.
a. USB – plug it in. Wait 2 minutes till its really, really bright. If the LED's are constant, its full.
b. Solar – find a light source it likes and leave it under for 10-15 minutes. See step 2 of “How to play” to figure out what light it likes.
2. Enjoy the blinky awesomeness now.
3. Store the charge for later by flipping the switch to “Direct”.

Endurance: In “Store” mode it'll run about 3 hours on a full charge in complete darkness. In “Direct” mode it'll blink as long as the room is bright. It's powered by SCIENCE!

Cleaning: Wash it with a warm damp soapy cloth. It may stop blinking when wet, don't panic. Let it dry out and it will start blinking. Do NOT put it in the dishwasher or washing machine (if you can help it). Both are too hot and may damage components.

Step 10: Extra Credit

You are welcome to string up your blinky skull as a necklace, pair of earrings, key chain, or simply window dressing. You can also bond a pin onto the back and wear it on a backpack or shirt. However, make sure there is an insulating layer of epoxy or glue between the metal pin and the back of the circuit board. The skull is very low power, and can easily be shorted out by touching the back (or sweating on it, ew.) A metal pin will short out the skull and it won't work properly.

If you want to go the extra mile you can seal it in epoxy, which is easy to do. Here's how:

1. Clean it. Its best to remove the solder flux before sealing, so it doesn't discolor over time or corrode the metal.
a. The project must be completely dead before cleaning. Let it run down overnight or in a dark place, and keep the switch in "Store". Then when cleaning flip the switch to "Direct" so it doesn't re-charge.
b. We recommend full strength orange clean and a toothbrush to remove all the flux. Scrub it thoroughly.
c. Rinse with cold water
d. Allow it to dry overnight.

2. After cleaning, do your best not to touch it with your fingers. Rubber gloves are recommended. The oil on your skin may discolor the metal, and interfere with the epoxy adhering to the parts and board.

3. Clear 5 or 10 min epoxy will work great for sealing. Just work fast, it gets gummy quick and hard to work with. Mix up a small batch, not more than an ounce. You can always mix up more if you need it.

4. Using a disposable brush seal one side. Avoid sealing the switch and USB tab. Everything else can be covered with epoxy and continue working, even the photo-diodes.

5 Allow the epoxy to completely cure. You'll know its done when it stops being sticky. This usually takes 24 hours, however don't be too impatient as un-cured epoxy will retain your finger prints. You want your blinky to be shiny don't you?

6. Repeat steps 4 and 5 for the other side. You're done!


hulse (author)2017-04-20

This was a rather challenging project for me, though a great learning experience. Some of the solder points are tiny and close together, especially where the photo-diodes go. When I finished, the skull blinks but only in Store mode (after charging with USB). Nothing happens in Direct mode, even trying different kinds of light. Can you suggest where the problem may be?

scott-42 (author)2014-12-17

Why did you use a dual comparator if the second output (pin 7) is not connected? Doesn't that waste power (~0.6µA) to have the second one? Otherwise there is a DIP8 version of the 6541 that would work.

lumenjewelry (author)scott-422014-12-21

I mostly use a dual comparator instead of a single because the extra 0.6uA draw doesn't make the pieces visibly dimmer, while only stocking dual comparators reduces the number of parts we stock. It also lets us add more LED's and blinking phases to a comparator design with minimal disruption.

If the unused comparator still bugs you, it's easy to recruit. Just lift pin 6 of U1 (-inB) into the air and connect it to pin 1, 5 and/or the pin 6 pad with a resistor and/or capacitor. Pin 7 will now oscillate with some pattern which can then be feed into a copy of the circuit formed by C3, D1, and D2.

MoonDocker (author)2013-12-23

Thanks for the instructable. This was a wonderful christmas gift for my wife. Works and looks great! I will be ordering some of the other items you have in your store. Simple and easy to put together. I did hot glue a piece of fabric to the back of the skull. Maybe you could include a piece of felt?

lumenjewelry (author)MoonDocker2013-12-23

Thanks, I'm glad you enjoyed it. The felt or cloth on the back is a really good suggestion. We will add that to our instructables and look into adding it to the kits.

masood1234 (author)2013-10-07

Sorry correction to my last question, I meant replacing C3 not C2 with a 10uF tantalum.

lumenjewelry (author)masood12342013-10-07

I wouldn't recommend using a Tantalum cap for C3. The polarity of C3 changes with each flash and Tantalum caps tend to explode if reverse charged. Aluminum electrolytic caps are less volatile, so I'd be willing to try a 10-15 volt 10uF aluminum cap as C3. I'd expect an aluminum cap to work for a few months at least. Lastly eyes have a logarithmic response to brightness, so using a 1uF to 4.7uF ceramic cap for C3 only dims the flashes to about 1/3 as bright. (using a smaller C3 would also let you shrink C5 for faster flashing without hurting run time)

masood1234 (author)2013-10-07

Further question about the hard to find capacitors. If I were to replace C1 with a 47uF tantalum I take it the polarity would be same orientation as the supercap, C2. What about replacing C2, also with a 10uF tantalum, how would its polarity go?

masood1234 (author)2013-10-05

I'm Interested in making this project but I cant source the 47uf fixed ceramic capacitor. Where are these available from. Would a tantalum substitution work

lumenjewelry (author)masood12342013-10-05

Digikey stocks some TDK capacitors in this size.  445-8540-ND (or at Mouser 810-FK26X5R0J476M )  A Tantalum cap or two 22uF ceramic caps in parallel would also work fine.  The circuit isn't too picky about what is used for C1 as long as it's a much higher value than C3. 

pandyaketan (author)2013-07-20

Firstly, a very fine & compact project!
5***** !
This seems more of a usb powered led project than a solar one.
What is the "solar only" circuit?? (No USB port or USB CHARGING of any kind!)
Would this "solar only" circuit just "blink" or throw some light also??!
Would like to make a "solar only" circuit - using photodiodes/ldrs, leds, and capacitors (hopefully, no resistors since they are a waste!!)


lumenjewelry (author)pandyaketan2013-07-26

For "solar only" just leave off R6 and the usb-connector.

With the above mods, the circuit would still just "blink". To "throw some light" you either have to accept a much shorter run time, or use a much larger solar array and storage system. (and re-design the circuit to use more power)

If you want the simplest "solar only" project that glows, see our 3 piece solar powered LED instructable.  (which will scale up just fine with different parts)  For anything fancier, you're going to need to use a few resistors in your solar project as part of a timing circuit (R1, R3, R4, R5) or as current limiters (R6). 

out-of-the-box (author)2013-07-16

I made a 555 timer led bracelet, I wanted to use a super cap but I cant find where to buy them, is there any online shops you would recommend?


I usually buy stuff from or  They ship fast and let you easily search by component electrical parameters. 

I'd suggest using this super-cap or this super-cap for your bracelet.  They should run it for 5-10 minutes on a charge.  For better run time I'd also suggest switching to a low power 555 timer like the TS555IN.  With a TS555IN, high brightness LEDs, and some power optimization you should be able to get your bracelet to run for 1/2 to 1 hour on a 1F super-cap.  You could also pick up a usb-b connector as a convenient charge port. 


im using the tlc 555 timer, runs on a minimal of 2 volts (might be 4) the timmer is perfect i just use one small battery. Thanks for the links! Ill look into it

out-of-the-box (author)2013-07-16

i love this! If i had extra cash I would buy your kit. I love the way the componets are laid out, adds detait to the skull. This is a future purchase!

About This Instructable




Bio: We’re life-long tinkerers, siblings, and fourth generation engineers. We’re not quite sure which of us had the idea of putting LED’s on ... More »
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