Evil Dark Detecting Solar Piggy Night Light

Introduction: Evil Dark Detecting Solar Piggy Night Light

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.

Adding lights to random things in your house is great fun for kids of all ages. Adding solar lights to random things in your house is even more fun. Adding red eyes to a piggy bank to make it evil is probably the most fun you'll have all afternoon.

I found this piggy bank at my local Dollar Store and instantly knew he'd look great with evil red eyes. I could have gone the simple route and wired the eyes to a power adaptor, but where would the fun be in that?

In the end I made a simple light detecting solar circuit that fits inside the pig. I'll stick him in my classroom next week and see if the kids notice him.

This is cheap to put together, easy to build, and long lasting. (Also, you don't have to stick this circuit into a pig. It'll work with anything.)

If you'd like to build this circuit I happen to have a kit available on my gadget site browndoggadgets.com, as well as a variety of solar cells for projects like this.

Step 1: What You'll Need

Container (In my case a $1 piggy bank)
Two Red LEDs (For this project they need to be red)
Two 100 ohm resistors (or 50 ohm works as well)
Two AAA or AA rechargeable batteries
One AAA or AA battery holder
One solar cell with a power greater than 4 volt
One blocking diode (1N914)
One PNP transistor
One 5,000 ohm resistor (or close to it)

I also used two 5mm LED mounts, but you might not need them.

Soldering Iron
Wire Cutter
Hot Glue Gun

Cost: $5-10

Time: 30-60 minutes.

Step 2: The Circuit

I did not come up with this circuit. It's an awesome design by the people over at evilmadscientist.com. Quite simple to make and very effective for a lot of situations.

The limitation to this circuit is that because of the low voltage of the two batteries, we can only power red LEDs. Red LEDs only need 2.4 volts to run, and two rechargeable batteries supply a total of 2.4 volts.

You'll also need a solar cell greater than 4 volts, otherwise you're not going to be able to recharge the batteries.

Other color LEDs....

If you want to use three rechargeable batteries, for a total of 3.6 volts, you can run any color LED you want. You'll just need to use a 5-6 volt solar panel in order to recharge the batteries.

Alternately you could use a voltage booster circuit (aka joule thief) to boost the power of your rechargeable batteries. Meaning you'd only need one or two batteries in order to power higher voltage LEDs. It's not all that difficult to do and I've even wrote an instructable on how to do that.

If you're interested in such a circuit I have a kits available. A fun step up from this project.

Step 3: Wire Up the Solar Panel

Due to the fact that my piggy bank is very awkward to work with, I'm going to be assembling the entire circuit outside and then sticking it inside. The only hitch is that I want the solar panel to stick out through the coin slot. In order for this to work, I soldered on some long wires onto my solar panel.

I think I used about a foot so of wire for positive and negative.

Any solar panel will do as long as it provides a voltage greater than 4 volts. Mine throws out 4.5 volts at 80 mah. You can get your panels from a variety of sources. Online sellers, eBay, old solar garden lights, or even from me!

Step 4: Breadboard Is Fun Board

Before I start to solder everything together I'm going to breadboard it. Just on the off chance I've got a bad part, or I did something wrong. In fact, I actually had the wrong type of transistor the first time I tried this...

The light detecting circuit is very handy. It uses the solar panel to detect light, which directs the flow of power! When power is flowing from the panel the LEDs turn off and the battery charges. When there is no power from the panel the battery kicks in powering the light. Darned useful.

Step 5: Drill Piggy Drill

Before I start all my soldering I drilled eye holes. As the piggy is made of a soft ceramic I need to be careful not to crack the pig nor cut too much out.

I assume a regular wood drill bit would be fine, but to be on the safe side I used a glass/ ceramic drill bit I had. I probably could have drilled through the pig with a pencil...

Be sure to get the right size the first time. Better to drill a hole too small than too big.

Step 6: Turn Up the Heat

First thread the solar panel's long wires down through the coin slot and out the bottom of the pig. Give yourself enough wire to work comfortably with and cut off the rest. (We can always stuff excess wire into the pig at the end.

To the positive wire of the solar panel solder the 5,000 ohm resistor and the blocking diode. make sure the black bar on your blocking diode is on the transistor side. The black bar tells you which leg is negative and in which direction power will flow. (Wired in wrong and it'll block power from ever leaving the solar panel!)

Next, solder the blocking diode and the positive wire from the battery holder to the left leg of the transistor. (You did breadboard this out so you know which leg of the transistor goes where.... right?)

Lastly, wire the other end of the resistor to the middle leg of the transistor.

Annnnnndddddd we're done with the dark detecting circuit.

Step 7: Wire Up the LEDs

First solder the 100 ohm resistors onto the positive legs of the LEDs. (The positive is the long leg.)

Now you have two options. You can either hook those resistors directly up to the transistor OR hook up a wire to each resistor and then hook the wires to the transistor. The first option being slightly easier, the second option will make positioning everything easier.

(I chose the first option and wished I had chosen the second.)

Then solder some wires to the negative legs of the LEDs.

Now solder the two (or one) negative LED wire(s) to the negative solar wire and the negative battery wire. Twist them all together first. It makes things easier.

You've done it! Test out your circuit to make sure it's working.

Most importantly be sure to give yourself ample amounts of wire. It'll make positioning the LEDs inside the pig much much easier.

Step 8: Warm Liquid Goo Phase

Using a hot (melt) glue gun cover all the exposed metal wire bits with glue. This prevents shorts and also is quite fun.

At this point you'll probably want to try putting in your LED holders if you've not already. (Unless you're not using them.) Don't glue them in.

Step 9: Hook Up the Eyes & Stuff

This part is very annoying. You need someone with long and skinny fingers to help you. Or cheap child labor. Either or.

I found that it was easiest to first stick the LEDs in, running them up to the eye holes and out a bit. Then attaching the mounts to the eyes, and then sticking them in. In all honesty just do whatever you can to get them in.

Once they're in place you may or may not want to put some glue down to keep them in place. As my glue gun is way way way to big to go into the pig, and as I don't trust my glue gun aim, I applied melt glue by hand (finger).

Note: This isn't very safe. Hot glue is... HOT.

So I squeezed some glue onto the table, waited a couple of second for it to slightly cool, picked it up with my finger, and applied it inside the pig behind the LED. I was only mildly burned.

Then just stuff everything else inside the pig. Battery pack, extra wires, the transistor circuit. Everything.

Step 10: Enjoy!

Well that's it. You're now set. Put the pig next to a window and you'll always have a night light. An evil looking one.

For extra evil you might want to use a black or red piggy bank. Or paint it evil colors. If I had any artistic skills I would paint wings on my piggy.

This simple solar circuit is easy to make and quite a bit of fun. You can adapt it into most any sort of container or project.

If you're having problems finding solar cells I have quite a few of these cells for sale on my gadget site browndoggadgets.com. I also have all the parts needed to build this available as a kit, as well as a different kit that will let you use different colored lights.

If you make a piggy, post a picture! I'd love to see what people come up with.

1 Person Made This Project!


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9 years ago

What does it do????


9 years ago on Introduction

If I use 3.6 rechargeable battery and 3.6V solar cell, do I need to change the resistor 5k? Since when I tried, there wasn't any light on at all.


10 years ago on Step 2

I'm pretty new to electronics so I apologize in advance for the basic questions.

I have actually tried using 3 batteries and 3 solar cells each in series so I can drive my LEDs brighter, but once I do, the dark detecting part no longer works. It works fine with 2 batteries and 2 solar cells or 1 battery 1 solar cell and a joule thief at the end. Though none of these drive my LEDs as bright as I want and I assume to most efficient way to drive 10 - 15 LEDs in parallel is with 3 batteries and 3 solar cells in parallel. (I will add further solar cells in series to provide enough charge to my batteries to run my LEDs all night).

I assumed that this was because the voltage difference between the solar cells and the batteries is no longer enough to trigger the switch. I tried changing my 5k resistor first to a 10 k and then a 1k hoping one would solve the problem but it didn't. Can you please explain why, and if there is a way to modify this circuit to work with 3 batteries. If not I can also try making a switch with a photoresitor to control the "on/off", but I am not sure of the technical details to do that, from what I have read it would involve a voltage divideer circuit.

Any help would be greatly appreciated.

BTW I just bought a solar panel and one a simple joule thief from you, I think your IC circuits are the same as rcisneros is using in his solar light string instructible and I want to play around with that, I can't get god results form the one in mine.


Reply 10 years ago on Introduction

Nevermind, late night user error, works great now :)


Reply 10 years ago on Introduction

It works but the transistor seems to be limiting the current, the LEDs (in parallel) are only getting 2.4 volt . I accidentally connected my battery positive to the emitter end of the transistor on my breadboard and my LEDs got blinding bright, as they should be receiving 3.4 volts each.

Based on other posts I read about joule thief circuits I figured the 2N306 transistor rated for 200mA was limiting it, so I switched to a 2N4403 resistor rated for 600mA, but it didn't make a difference.

Do you have any suggestions? I want to run 10-15 LEDs (including color changing LEDs, I know they have a problem getting past red with a joule thief, but I plan to add a diode and cap at the end to fix this) at full brightness. Size of enclosure isn't an issue single I will need several solar cells to support this.

If using 1-2 batteries and a joule thief more efficient then directly charging 3 or 4 batteries and then running the LEDs straight off them?

Thanks in advance for any help!!


Reply 10 years ago on Step 2

Red LEDs are only rated for 2.6V and the circuit makes them quite bright when I've done it.

If you're using a Joule Thief and getting dim LEDs then you're doing something wrong. A Joule Thief circuit should create some very bright LEDs.

The Joule Thief is great when doing solar as it means you don't need as strong of a solar panel when charging up a single battery. Doing a 3 or 4 battery setup shouldn't make much of a different.

The biggest difference is that the Joule Thief will such your amps down faster than using regular batteries (as the nature of it sacrifices amps for volts). It all depends on what you're using it for.


Reply 10 years ago on Introduction

Thanks for the prompt reply :)

Yes, I figured I was doing something wrong wrong with the joule thief part of the circuit, probably my toroid wrapping. Tried one with 22 AWG wire I had around but could only fit 6 turns, tried another that I wrapped with 30 gauge wrapping wire with 15 turns. I have seen many joule thief circuits on the web and some seem to use 7-10 turns, others 20 others 40. Somewhere I read the more tuen the more volts you will get, other time that it raised the inductance, or that it doesn't really matter.

I would love an explanation on the relationship of the number of wraps to the circuit.

Also I love to know why when I use the PNP transitor for the dark detecting part of my circuit the voltage provide to my load goes down to 2.4. When I attach the load + to the collector leg of the PNP (basically bypassing the switch), I get 3.4 volts (as expected) to my load +.