Solar panels are cool, right? All techniques of extracting energy from environment have always fascinated me, with solar panels standing out because of their inexpensiveness and ubiquity. I've been wanting to make something incorporating solar panels for ages, and finally got around to it.
What we'll be making today is a simple night light powered off solar energy. Yes, the same thing you see in gardening department of every hypermarket and gardening supply stores around the world.
The difference is, we'll put a new spin on the old idea of solar lights by assembling the whole fixture inside a mason jar! Wait, why did you say "old news"?
Well, it didn't matter to me if it was new stuff or not, I still wanted to make a solar-powered night lamp. After about a month of attempts and toil smeared thin across 2 or 3 years (yes, I did stow this project away at the end of every summer, and I really had better things to do almost all of the time) I finally produced a viable result.
My build even had one distinction from the mason jar lamp mentioned above - instead of scavenging store-bought solar lamp we'll solder together our own Frankenstein's monster! That's more fun than just fitting Shenzhen-assembled PCB into another enclosure, right?
Step 1: Ingredients
- One mason jar. Possible improvement for your build: make it frosted inside for better light diffusion.
White LEDs and 56Ω or 68Ω resistors x3. Possible variation for your design: get any color and any number of
LEDs, then recalculate the current-limiting resistor value e.g. here. For example of calculation see my blog post mirroring this instructable.
4 solar panels rated at 4V@100mA, some reasonably sized capacitors and two Schottky diodes.
One lithium battery, one charger,one 47kΩ resistor, one 2N3904 PNP transistor and one switch.
One solderless breadboard for prototyping and testing - strongly recommended! Even if you decide to just assemble a carbon copy of my device, there's still a solid chance of at least one of your components malfunctioning/being counterfeit. Breadboard your design before the final assembly.
Two-component epoxy to plaster solar panel assembly to the lid of mason jar. Get one in your local hardware store.
Finally, when every item on the list is checked off, we can get started!
Step 2: Schematics
The diagram is pretty straightforward - two panels are connected in series to produce a maximum of 8V, then in parallel to double (hopefully, in perfect conditions) output current.
The voltage is then fed to big capacitors to accumulate energy. Solar panels are a woeful current source - even 100mA of drain will collapse the voltage to near zero.
The energy extracted and stored in capacitors goes to battery charger (set to really low current to further guarantee absence of huge currents).
The rest of our circuit is just a PNP transistor switching LEDs ON when the current through base is LOW, and OFF when it's HIGH. Low current = low lighting, so that suits our purposes.
Step 3: Testing
First of all, we're going to have to modify our charger. According to this other guy who tested TP4056-based charger boards (datasheet), the factory-provided current setting resistor sets the charging current to 1A. That's good for wall warts (AC power supplies) but my solar panels wouldn't be able to put out more than 200mA even under perfect conditions!
This is why we needed an extra 47k resistor - it will set the charging current to 25mA.
This one modification should be enough to get to testing. Assemble everything on the breadboard according to schematics. Now move the whole testing rig to the great outdoors!
The purpose of this venture is to find out if your battery is adequately sized for your solar panel assembly. If it is:
a) The battery will be close to full charge (or even fully charged) every evening (near the end of solar day) - that's 4.2V.
b) It will be depleted (around 3 - 3.2V) at the end of night.
Picking oversized battery will result in under-utilizing it's capacity.
I.e. it's going to discharge during the night and only charge to 3.5V during the day - that's a sign that you have to find a battery with lower capacity.
After making sure your setup works we can stuff it into more permanent housing and solder it together.
Step 4: Assemble the Top
Compose 4 panels into one and stick it on top of the lid. What kind of glue is up to this task? 2-component epoxy that I've mentioned in ingredients section above.
Step 5: Assemble the Rest of the Circuit
See my post for detailed assembly instructions with lots of pictures, involving some intricacies with keeping moisture out of the jar.
Step 6: Enjoy!
Well, it was an interesting project for someone starting to dabble in energy harvesting and solar power particularly.
As a side effect, it culminated in something useful that you can hang in your garden, which is a plus!