In this instructable I will show you how to make a solar powered bike stereo box that fits in a milk crate, which you may already have on your bike! This instructable assumes you have some proficiency in woodworking and soldering, and is really about the solar to battery circuitry and the options it provides. This system could be used for any variety of applications that run off 12v DC.
In designing this project the box went through many phases, first it was battery powered, then the solar was added, and more extra bits kept finding their way into the box. Because of this the photos will appear somewhat inconsistent, so I have provided some easy to read drawings for clarification.
Cut holes in the center of two of your sides to match your speakers. You can use a hole saw if your speakers are small enough, or drill a small hole and use a scroll or jig saw. Solder about a foot of wire onto the speaker terminals and screw them onto the inside sides of your speaker sides so that they fit on the holes you just made. You can round over the outside edge of your holes with a 1/4" round-over router bit.
Glue the sides and speaker sides together to make a box. Glue the diver in the middle flush with the bottom. Finally glue the bottom on the box. (You might want to make sure the box fits the crate before you glue it up, you may need to shave a 1/16" off all the pieces depending on the accuracy of your crate.) Should look like the picture when viewed from the top.
Step 5: Fit the Top
For the top, put four of the threaded inserts into the top corners of the box and drill through holes that line up on the top. The top should over hang about an inch past the back. The top should also be the same size as the solar panel. Mark the mounting holes from the soler panel on the top and install the other four threded insterts on those spots. You should now be able to screw the top onto the box, and the solar panel onto the top. I used 1" 10-32 square drive machine bolts for all of these. You can optionally add a 1/8 pice of plexy glass with the same mounting holes as the solar panel on top of the solar panel to protect it.
Layout the holes for your power input jack, audio input, and switch on the front side of your crate. Make sure they align with holes in your milk crate, I used one of the handle holes for everything. Drill the holes and install the parts.
Solder everything to the switch based on this schematic. What this all does is let you switch between running the amp of the battery versus running the amp off the included AC adaptor while the solar cpanel charges the battery. This is important becasue you don't want the solar panel charging the battery while the battery is in use. If the box is not connected to AC putting it into AC mode serves as a generic off mode, but the battery will still charge. The solar panel comes with alligator clips; I just cut those off to use the wires. All of the grounds (black wires) can be soldered together, just the positive (red) wires have to go through the switch.
At this time you might want to consider any other possible uses for 12v DC. For instance, you could add a 12v car cigarette lighter outlet to power any accessories you hay have for one of those. I took apart a 12v car to 5v usb adaptor and added that to my box so I could charge my usb capable accessories. (You could even charge your ipod while it plays over the speakers.) Just connect any 12v additions to the power cable that goes to the T-amp.
If you can, use speaker wire or some other shielded wire as it will cut down on the likelyhood of all your cables becoming antennae for background noise.
The battery should sit in one speaker well and the amp in the other. Fill the remaining space loosely with poly fill. The divider prevents the speakers back noise from canceling each other out, and the polyfill keeps the back noise from bouncing back out the front of the box.
Step 10: Assembly
Put on the lid, panel, etc. And put the box in your crate. Finish with Thompson's Water Seal. Won't be really waterproof, but will help if you get caught in the rain.