Picture of Evil Dark Detecting Solar Piggy Night Light

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, as well as a variety of solar cells for projects like this.

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Step 1: What You'll Need

Picture of 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.

Cats Science Club made it!1 year ago

Had a great time building this. The directions Joshua gives are easy to follow. He does an amazing job with all of his instructables.


Thats one evil looking piggie!

Thanks! Still works great! We have two. One for Cool Cat and one for Ally Cat.

hkortus1 year ago
What does it do????
crudites2 years ago
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.
shanakoenig3 years ago
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.

Nevermind, late night user error, works great now :)
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!!
JoshuaZimmerman (author)  shanakoenig3 years ago
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.
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 +.