subject: Solar Panel Lights Can Light Up Your Landscape [print this page] Solar Panel Lights Can Light Up Your Landscape
This was a project undertaken for Burning Man 2002. The goal was to build solar path lights for the Alternative Energy Zone.
Anybody might ask, "Why *build* a solar path light when anybody can go to a home supply store and buy one for under $10?"
The answer is that learning and creating is fun! It's way cool to fiddle with bitty little electronic parts until suddenly There is Light! Anybody derives pleasure from seeing a homemade light, in all its funky glory, lighting the paths of the AEZ. Plus, building a solar path light is a great introduction to circuits and solar power!
This project was made possible by the collective efforts of several members of the AEZ mailing list. A special thanks goes to the stunningly talented AEZ members who built the Solar CDs, Jeff Barlow of the AEZ, and Head Hardware Wonk, Mike Tarrant of Blinky Red Things in Hushville.
To the right is one of the super cool Solar CDs. They play the music of the sun to the tune of about 4.4 volts.
"Hey, Kidz! Please be aware that this project involves using sharp tools, potentially toxic fumes, and hot molten metal!
There are also wires that might poke your eye out! This isn't a project for just anybody! Be smart and be safe.
Now, go have some fun!"
The Flow of Electricity
The Two LEDs Circuit: Day and Night
During the day when the sun is shining on the Solar CD, the electricity provided by the solar cells flows into the batteries, recharging them for their night's work.
Anybody might ask, "What keeps the LEDs from turning on during the day?"
The answer is that the current provided by the Solar CD seeks out any open path. Most of the current flows directly from the Solar CD and into the batteries. But a small amount of current flows through 2.2k resistor R1, where it meets the higher resistance of 10k resistor R2. An even smaller amount of current flows through R2; the rest of this current flows into the "base" lead of transistor Q1, which "turns on" Q1, creating an open path for current to flow through 2.2k resistor R3 and then through Q1 from collector to emitter. This prevents current from flowing into the base of transistor Q2. If current flowed into the base of Q2, the LEDs would turn on.
The width of the red arrows attempts to denote the amount of current flowing at each location in the circuit. Please note that all voltage and mA numbers are approximate. The actual voltage and mA varies depending on conditions.
Anybody asks, "What turns on the LEDs at night?"
When the sun sets, the Solar CD stops producing electricity, and the batteries take over all the work and start pumping their stored current into the circuit. The current runs into one-way valve, diode D1, and can go no further. So a small amount of current flows across 2.2k resistor R3, and since it can't flow across transistor Q1 (which is turned off), the current flows into the base lead of transistor Q2: this "turns on" Q2. Now, this creates an open path so that most of the current can flow across the LEDs and through Q2, which, of course, makes the LEDs light up!
Anybody says, "I have a vague understanding of current and electricity. I know it has something to do with 'bitty particles' moving around, but it still all seems like magic to me!"
Jeff writes, "Actually, the 'bitty particles' are all electrons and they all have the same charge: -1. It only seems like magic because the part that moves is too small to see. The traditional beginner crutch is to think of water flow in pipes. You can't see that either but you know it's there because you can see the water when it comes out the end. Saying that you have a certain current flow in Amps is just a shorthand for saying that some (very large) number of electrons have passed a certain point in one second. It's the same idea as measuring water flow in gallons per minute. Keep in mind that the various currents differ only quantitatively. One might be 10mA and another 50A (50,000mA) but they're both just electrons passing a point, and electrons are all the same."
