Solar LED Lamp

Brighten Your Darkest Night

By Michelle Meyer

With a simple circuit, this lamp stores daytime solar energy to power a nighttime LED. This decorative Solar LED Lamp will enhance any yard or patio.

Assembly Time: 2-3 hours
Skill Level: Beginner

Solar Lamp Kit Includes:
PNP Transistor
High Power LED
Heat Sink
3 Rechargeable Batteries
Battery Holder
Solar Panel
Solderless Breadboard

Additional Tools:
Soldering iron
Wire Strippers/Cutters
Electrical Tape
Drill and Drill Bits

Additional Supplies:
6" Diameter Duct Cap
Decorative Perforated Metal Sheet (12" x 18" or 12" x 24")
Rust-Stopping Spray Paint
Parchment Paper (optional)

Step 1: Prepare Materials

The first step is to drill holes on a duct cap so the photoresistor and solar panel can go on the top of the lamp. I drilled a 3/16 inch hole near the edge of the circle for the photoresistor and a 3/8 inch hole in the middle for the solar panel leads. For both of these holes, I started with a 1/8 inch drill bit and worked my way up.

Solar Lamp
Solar Lamp

The next step is to shape the metal sheet. Since the sheet I used was 12" x 24", I cut it down to about 12" x 18". I used the spray paint can to roll the sheet loosely and then shaped it until it was the right diameter to fit in the duct caps. After shaping it to the correct size, I tied the two ends together using wire that I wove through the holes of the sheet.

After the duct cap and metal sheet are prepared, they should be spray painted. This gives the lamp a fancy look with the benefit of protection against rust.

Step 2: Test the Circuit on a Solderless Breadboard

Test the Circuit on a Solderless BreadboardClick to enlarge

While not mandatory, it's a good idea to test the circuit on a solderless breadboard first. Before creating the circuit, I prepared the high power LED. I began the project using one standard LED, but I still wasn't satisfied with the brightness even after adding more, so I switched to a single high power LED. I soldered short wires onto the positive and negative ends and then attached it to the adhesive on the heat sink.

The first thing I placed on the breadboard was a photoresistor, which has low resistance in the light and high resistance in the dark. Next, I added the PNP transistor, which acts as a switch by allowing current through only when the voltage on the base pin (middle pin) is low enough. When it is dark, the voltage on the base pin is low because of the large voltage drop across the photoresistor. This allows current to flow through the transistor to the LED, which I connected to the transistor.

I connected the base pin to ground with a potentiometer, which is used as a variable resistor to change the sensitivity to darkness. The higher the resistance, the darker it must be for the LED to turn on.

Finally, the rechargeable batteries, solar panel, and diode were added. I placed 3 batteries in the battery holder for a combined voltage of 3.6 volts. The diode connects the positive ends of the solar panel and batteries so current can only flow from the solar panel to the batteries. The diode should be oriented so the stripe is closer to the batteries.

Solar LampThe circuit on a breadboard
Solar LampTesting the LED

After assembling the circuit, I began testing. The batteries came charged, so I was able to test the LED and the dark-detecting part of the circuit. I blocked the photoresistor from light by covering it with my finger and sure enough, the LED lit up. This was a good sign, but I still needed to make sure that the solar panel could charge the batteries. To do this, I drained the batteries by putting the circuit in a dark place and then moved the circuit into the sun so the solar panel could charge the batteries. When the LED passed the darkness test for the second time using the energy from the solar panel, I knew my circuit was fully working.

Step 3: Solder and Test

I began by soldering the potentiometer, transistor, and LED wires onto the protoboard.

Next, I soldered one end of the diode (the end with the stripe). After that, I soldered one end of a short jumper wire to the position where the negative end of the solar panel would go.

To attach the solar panel, I passed its cable and alligator clips through the 3/8 inch hole from the top and connected the negative clip to the jumper wire and the positive clip to the other end of the diode.

For the photoresistor, I placed the leads through the hole, twisted a piece of 16 inch wire around each photoresistor lead and applied solder, and then soldered the other end of the wire onto the protoboard.

The BoardThe assembled circuit

After the soldering was complete, I performed the same tests as I did earlier. Unfortunately, this round of testing did not go as smoothly: the LED didn’t light up unless it was in the sun and it didn't respond to the photoresistor.

The first thing I checked was the soldering on the back of the board, but I found that all the connections were correct.

My next thought was that the issue was either the photoresistor or the transistor because the LED wouldn’t turn on and off. The transistor had been exposed to a lot of heat when I was soldering, so I guessed that the transistor might have burned out. I took out the old transistor and soldered on a new one (using a lower temperature and being much quicker this time).

After this replacement, I was pretty confident that the circuit would be working, but the circuit only worked in the sun. I figured it must be a problem with the batteries. I used a multimeter and found that the voltage from the battery pack was just over 2 volts instead of the expected 3.6 volts. I checked the voltage on each individual battery and replaced the battery that read at 0 volts.

Step 4: Assemble Lamp

As the first step of the assembly, I used electrical tape to organize the solar panel wires and tape the photoresistor leads in opposite directions so they wouldn't accidentally touch.

Next, I added a piece of parchment on the inside of the metal sheet to hide the wires and diffuse the light. I cut the parchment paper to about 11.5" x 19", rolled it up to a diameter that fit inside of the metal sheet, and taped it together and to the metal sheet.

Before placing the circuit on the duct cap, I cut a piece of paper and placed it on the bottom duct cap. I did this to prevent any short circuiting that direct contact with the aluminum duct cap might cause.

After I adjusted the potentiometer to the sensitivity of my choice, the lamp was finished!

Finished LookThe finished project

Though I encountered a few issues, I was very pleased with the finished look, especially the brightness. It was even brighter than some of the store-bought lights in my yard!

Warning! Working with electronics can be dangerous. Always use caution and follow all safety procedures. If you are uncertain of the dangers involved with a particular project be sure to seek assistance. Failure to follow safety procedures may result in injury or death.
Michelle Meyer is a student at Menlo School in Atherton, California, and a summer intern at Jameco. She plans to study engineering in college. Her interests include math, physics, sports, and hiking.