Introduction: Solar Dark Detecting LED Light Bulb

About: I used to teach middle school science, but now I run my own online educational science website. I spend my days designing new projects for students and Makers to put together.

Like so many projects in life, I got this idea while randomly walking around Hobby Lobby. I came across these cute little glass "light bulbs." They have screw off tops, a flat bottom, and only cost $1.50. Who couldn't pass that up?

My original idea was to try and cram a LED, battery, and switch in there. Yeah, that wasn't ever going to happen. My second idea was to make it solar power because as we all know, everything is better with solar. (Yes, even puppies!)

In a nutshell what I ended up making was a little side box that had a solar panel, joule thief circuit, and a switch on it connected to the glass bulb. Oh yeah.... I threw in a simple dark detecting circuit as well. Just cuz.

A fun beginners project for anyone wanting to do some soldering.

I'm putting together some "Light Detecting Solar Circuit" kits on my gadget webpage, Brown Dog Gadgets. All the electronics you'd need to build one of these. Or, if you don't want to do the work I have some of these for sale on my site... with upgrades...

Step 1: What You Need


Hot Glue Gun
Soldering Iron
Helping Hand (Optional really)
Breadboard (Optional, but super handy.)

Wire - I used 22 Gage Insulated as well as some thick magnet wire. Anything will work.
5K ohm Resistor
1K ohm Resistor
1 Zenger Diode
3 White LEDs
PNP Transistor
NPN Transistor
A switch (I used a toggle because toggles are fun to play with...)

4.5V Solar Panel
AAA Battery Holder
AAA NiMh Battery (You could use a AA as well. I just used these for their size.)

Glass Lightbulb from Hobby Lobby
Little Plastic Bead Box that I picked up at Hobby Lobby as well. 6 Pack for $3.

A lot of these supplies can be mixed and matched. AA instead of AAA. Smaller Solar Panel. Different box. Ditch the dark detecting. Blah blah blah. You're all adult, make up your own mind.

Total Cost for parts: $5 or so. I had all these parts on hand to begin with.

Total Time: Less than one hour if you know what you're doing, two hours otherwise.

Step 2: Circuit

This is the circuit we're making, except I'm adding a switch between the LED and the battery. This way the battery can charge even if the switch is off.

Some of you may recognize this circuit from somewhere else. Yes, it's true. I did steal this circuit from who wrote up some awesome solar circuits. (Look, I'm giving credit!)

The first transistor, the PNP 2N3906, acts as a dark detecting. When power (aka sunlight) is hitting the solar panel the transistor closes. When there is power (aka darkness) coming from the solar panel the pathway opens and we get power. If you want, you can ditch this step and control the LEDs via the switch.

After the first transistor we get the famous Joule Thief Circuit. Google this if you've never seen it before. It'll help you with this instructable.

Step 3: Wire Up the Solar

I bought 30 of these panels off ebay for $30. They're amazing. You just have to solder on wires.

Which you can see me doing below....

Step 4: Wrap Your Torrid

This is where you wrap your torrid up with wire.

Again, if you've never done a joule thief circuit before watch a video. Go right now. I'll even give you a link to a video and guide.

Quick reminder for everyone else.

Two wires, coated, equal length.

Wrap around torrid at least 7 times.

Take a wire from one "end" and hook it up with the opposite color from the other end.

Step 5: Breadboard

This is where I highly recommend you use your bread board to test everything out.

I'm not going to give you every detail of the circuit, it's kind of simple.

Needless to say I screwed up a transistor the first time I put it on the board. Go figure.

In the first picture the light is off because the solar panel is getting light.

The second picture I cover the panel and the light goes on. Darkness detecting!

Step 6: Drill Baby Drill

Just drill three holes in the box. I put one in front for the solar panel wires, one on top for the switch, and one out the back for the LED wires.

Size is up to you, and depends on how big of a switch you're using.

Step 7: Soldering the Dark Detecting

If you're not doing the dark detecting part, you can (mostly) skip over this page.

First, don't forget to thread your solar panel wires through the hold first.

Then, solder the zener diode and the 5,000 ohm resistor to the positive wire.

Solder the resistor to the center leg of the PNP transistor.

Solder the zener diode, as well as the positive side of the battery case, to the left leg of the transistor. (Did you breadboard to test this out first?)

Solder the double twisted end of the torrid to the right leg of the transistor.

Step 8: Test Again!

It never hurts to test things again. Use your breadboard. It's your friend.

Step 9: Defuse Your LEDs

If you're smart you'd be using some diffused LEDs.

If you're like me, dumb, and only have clear LEDs you'll have to defuse them.

To do this you can either use a big ball of hot glue OR sandpaper.

I used sandpaper, which reminded me I should order some defused LEDs off ebay.

Step 10: Joule Thief

Off your torrid are two wires.

Take one (doesn't matter) and solder the 1,000 ohm resistor. Then solder the other end of the resistor to the middle leg of your NPN transistor.

The other torrid wire you then solder onto the right leg of your NPN.

Joule Thief is nearly finished.

Step 11: LEDs

I used thick magnet wire because of the look and made it long because I wanted to hide the box away from the bulb, but anything will do provided it fits through the hole in the bulb cap.

First, put the wires through the bulb cap. You can always make it bigger by using your drill.

Next, hook up all positive legs of your LED to one wire, and all negative to another.

***If you're using 8mm or 10mm LED you might want to start with two and see if they'll fit into the bulb. Then, maybe add a third while the other two are inside the bulb. Kind of like a ship in a bottle. Helping hand to the rescue.

Step 12: Hook Up Your Wires in the Box

Take the other end of your LED wires and put them through the hole in the box.

Solder the positive one to the far right NPN leg (the one you already soldered the torrid wire onto).

I soldered the negative wire to the switch. Then run another wire off the switch.

Lastly, solder the new wire off the switch, the negative battery wire, and the negative solar wire all to the left leg of the NPN transistor.

Do a little dance because we're done soldering.

Step 13: Warm Liquid Goo Phase

So now we have a big mess of wires and whatnot that need to go in the box. But what about random exposed wire touching random exposed wire?

Hot Glue Powers Activate!!!!!!

I just cover all the random exposed bits with hot glue. It makes things way easier.

(Word on the street is that the technical term for this is "potting." I prefer calling it the "Warm Liquid Goo Phase.")

Step 14: Shove It All in the Box

This really shouldn't require it's own page, but I am because it can be a real pain.

Be careful and shove it all in the box. Breaking something now would be a big pain to fix.

The goal is to get it all in the box, not look fancy. Maybe you can be fancy the next time you build this?

Step 15: Enjoy!

You should now be finished.

We've now created what amounts to a very fancy looking outdoor solar light. The kind you can buy for less than $5. Only this one looks very impressive.

This is a great beginners circuitry project and cheap to make. Plus it will impress the heck out of everyone you know because of how it looks.

To really impress them use two colors of lights inside that you control using a second toggle. (Go from the NPN to a toggle switch with two positive lines running out. Use a common negative line back. You'll have three wires instead of two.)

If you'd like a prebuilt version of one of these, I have them for sale on my gadget site, Brown Dog Gadgets, or my etsy store. I've tweaked my production version a bit, added a few upgrades.

Or if you'd like to build one on your own I also have kits available that provide you with all the wires and gizmos you need to build a light detecting circuit of your own. (My personal favorite is putting them into Mason Jars for outside lights.)