Solar Powered Light

In this guide, we will show you how to create a light which charges during the day and turns on at night. A solar panel is used to detect the time of day and to charge the batteries. Best of all, this can be accomplished through a simple circuit with cheap components. The device can be placed anywhere that receives light during the day that you would want to see at night, such as a pathway in your backyard. Plus, seeing the light turn on and off as you wave your hand over the solar panel is awesome, especially if you built the light yourself.

We used the following materials

• 5V Solar Panel
• Rechargeable AA Lithium Ion Batteries
• Battery Protection Plate (TP4056)
• PNP Transistor (SS8550)
• 3V LEDs
• 100 ohm resistor
• Perfboard
• Battery Case
• Heatshrink tubing
• Wires for soldering
• Superglue
• ABS Plastic sheets
• Plastic corner guards
• Soldering iron and solder

We recommend buying more of the electronic components than you need in case one of them is defective.

The above circuit is the "brains" of the device; it contains the logic which turns on and off the LEDs and charges the battery. When light is hitting the solar panel, its positive end should have roughly 5V more than its negative end. The triode (transistor) acts like a switch which only connects the battery to the LEDs if the solar panel has lower voltage than the battery. Our batteries were about 1.2V so we used three of them in series. This makes sure that they have enough voltage to power the 3V LEDs while having less voltage than the 5V solar panel. The battery protection plate is there simply to make sure that the battery is not undercharged or overcharged, which can reduce battery life. It cuts off ourput current if the charge on the battery is too low and cuts off input current if the charge is too high.

Before soldering everything, it is a good idea to test the electronic components first on a breadboard or perfboard. Wire up the circuit as shown above on the breadboard with a test LED and make sure that it turns on when you cover the solar panel. You may need to solder pins onto the battery protection plate.

If the light doesn't turn on:

• Use a voltmeter to check the voltage of the battery pack and make sure it is over 3V. Follow this to the ends of the LED to find where the faulty connection/component might be.
• One issue you may encounter is that the pins for the battery voltage output on the battery protection plate are not right next to the B+ and B- text on some protection plates.

If the light doesn't turn off:

• Make sure the battery output is less than 5V.
• Many modern solar panels only accept light from a single wavelength. Check the output voltage of the solar panel and make sure it is greater than that of the battery. If it is not, try shining a flashlight on the solar panel to see if the panel is defective.
• Make sure the solar panel is connected to the circuit properly using a voltmeter.

After dealing with any issues, solder the circuit to the perfboard and make sure to leave plenty of room for the LEDs to be soldered on later.

The next step is to make the box/casing that will house the circuit, batteries, and LEDs. We planned our casing to have a top cross section that would fit the solar panel's exact dimensions because devices like this need as much solar charging as they can get.

Cut out pieces of plastic to your desired sizes. One way to do this which worked for us is to measure out the size on a piece of paper and tape that paper to the sheet of plastic, then use a retractable safety knife to slice through the plastic. The plastic corner guards also need to be cut to size. This is tricky to do with a safety knife and we ended up using a hand saw, then sanding down the edges.

Mark out where the LEDs will be inserted and drill a hole the size of the LED head. The drill bit size will vary based on the size of the LED that you buy. This page may be helpful in choosing the right drill bit size. Make sure to test that your LEDs can fit in all of the holes before gluing, because otherwise it will be a pain to fix later.

Join the side panels at the corners using super glue. The plastic will need to be held at a 90 degree angle while the glue is drying. To prevent you from gluing the plastic to the table, we recommend taping the edge with duct tape since it can be easily removed after letting the superglue dry for about a minute. Some pieces of the duct tape may stick to the superglue if it is left there overnight.

After gluing the side panels together, glue on the corner pieces to hide the ugly corners. As before, we recommend using duct tape to secure the corner pieces while the glue is drying.

The bottom panel will be attached after the circuit is inserted, while the top will be the solar panel, so this is all that needs to be done with regards to the casing for now.

First, prepare the wires to be soldered. It is very useful to color code the wires so that you know which end is positive and which end is negative. We used 16 LEDs with four on each side, but feel free to use a different amount. Cut off one length of wire of each color for all the LEDs you plan to use and strip both ends. Make sure the lengths are long enough to where your circuit will not be pulling the wires taut inside the casing.

Solder the wires to the ends of the LEDs. Then put on the heat-shrink tubing and heat it with the soldering iron.

Test the LEDs to make sure that the wires are color coded correctly and that the soldered connection is sound.

Solder the LEDs into the perfboard and make sure that the circuit functions, switching on and off all of the LEDs.

First, glue the solar panel in as the top of the box. As before, use duct tape to secure the edges and ensure no glue leaks out. Also, remove the plastic coating from the top of the solar panel before gluing because glue which leaks through the cracks can stick this plastic coating to the panel, which makes it look worse.

Tape the circuit board to the side of the casing or bottom of the solar panel, and do the same with the battery.

Plug the LEDs into the sides of the box and tape or glue them in place.

Finally, glue on the bottom panel, using duct tape as usual.

The device can now be placed in any location that you want an automatically charging light to brighten the way at night. Have fun!