Home made solar boombox
A while ago I got a pair of computer speakers for free from Newegg, when I bought a wireless keyboard and mouse for 21 USD. Now, I already had 3 pairs of computer speakers, and since we only have two computers, two pairs were just sitting there taking space and having no use.
I had some solar panels just lying around, as well as some rechargable batteries. So one day I came up with the idea of modding those speakers and transforming them into a digital boombox.
The first step was to take the speakers apart, and extract the amplifier and the speakers themselves, while discarding their case and converting them from 110 AC to 7-9V DC.
This step was pretty hard, because being cheap and made in China, they didn't have screws. Instead, they were glued together. With some patience and a small round saw blade I was able to take them apart. I needed to cut the wires because they were sealed with some silicone which was impossible to remove.
To make it even harder, the transformer was in one speaker, and it send AC current to the other speaker, which rectified it via 4 diodes and a capacitor.
Since I didn't want to feed it AC, and I didn't want 1.4 volts being dropped because of the diodes, I removed them and the capacitor, and connected my power cable to where the capacitor was.
The next step was to build an enclosure to house the whole thing (amplifier, speakers, batteries, etc.) Initially I wanted to use some sort of cardboard/polystyrene sandwich material, but cutting it properly with the tools I had was impossible. The polystyrene in the middle was being teared, and gluing the boards together was not that easy either.
Eventually, I've decided to use some wood, as it is stronger and easier to glue, and it looks nicer too. The problem with normal wood (or compressed wood boards) is that it takes special tools to cut, tools I don't have in my apartment. And cutting round holes in the middle for the speakers is not the most simple thing either.
I went to a crafting store looking for some suitable wood and tools, and I was surprised to find sheets of balsa wood for a very decent price (about 2-3 bucks/sheet). I needed only 2 sheets, but got 3, just in case.
For those of you who don't know what balsa wood is, here is a simple explanation:
The balsa wood is the nature's alternative to expanded polystyrene.
It's a very lightweight wood, and pretty strong for it's low density. It's main use is RC planes and gliders, but as you can see it can be used for other projects as well. Having such a low density, it can be cut with an Exacto knife, and cutting portions in the middle is easy as well. Just draw a line, and stab that line with the knife. Eventually you will be able to take that piece away, and then just file and sand the edges so they will become smooth and eliminate the excess material.
Making the enclosure was a relatively easy task, it took only about a few hours. The balsa wood is easy to cut and glue, and the only thing required is patience and attention.
After the front panel was done, I glued the speakers to it. Normally I was supposed to use some screws, but I was afraid the wood might crack, so the glue seemed to be a better alternative. They are small speakers, so I don't have to worry too much about vibrations.
One other aspect when designing the enclosure is to keep in mind that it's main purpose is to prevent the sound waves from the back of the speakers from going back in the room. The sound in the back of the speakers can cancel the sound from the front, so, ideally, the enclosure should absorb all the sound. This is pretty much impossible, but nevertheless, an enclosure that absorbs most of the sound is desirable. Because of this, it should be hermetically closed, so that the air can not escape. Given that objective, I wanted to seal it all with silicone, but that had a small disadvantage: sealing it makes it impossible to replace the batteries after a few years, when they can't charge as well as when they are new.
My first idea was to make a hole in the bottom, and have the batteries externally. This had some disadvantages though, such as making it look more ugly than it already was, and the possibility of the batteries falling out and ripping the wires, which would have required opening the box again.
After some tests with the lid panel closed, but not sealed, I determined that it sounds reasonably well even if not totally sealed, so I just placed the batteries inside. Everything but the lid panel was sealed with the silicone, inside and outside.
Now that the amplifier/speaker system was in place, I had to put a sound source. The best idea seemed to be buying a defective mp3 player off eBay, and fixing it for my purpose. After some search, I found a Samsung YP-T8Z 1GB that wouldn't work unless having the external power connected, for about 40 USD.
When I got it, I noticed that it did work while the power was disconnected, but the sound was just mono, and it had something rattling inside. Opening it was easy. It had no screws or glue, and it could be open with a small flat screwdriver or just with your nails. First I had to detach the lateral panels, and then the middle panels.
The problem was that the stereo headphone jack was broken :/ I had no other alternative but to remove the whole jack, and soldering 3 wires. That was pretty hard, because I needed an SMD soldering tool, and I didn't have one so I had to improvise. Placing a piece of copper wire on top of my soldering gun did the trick.
One additional problem with that mp3 player is that it can not be charged with solar panels. For some reason, it always reported that the battery was full, so it didn't want to charge it. This is strange because the solar panels had a 5.5V output, which was lowered by a diode, so in theory it should have worked, but it didn't. Most likely a software problem which requires a very specific voltage and current.
So what I did was open it again, remove the Li-Ion battery and replace it with 3 NiMh batteries, which I am charging independently (directly from the solar panels, not through the mp3 player). The pictures above show the mp3 player being tested from some AAA batteries. It worked just fine.
This system has a few advantages:
1. About 50 hours of playtime, rather than 20 (the current draw is 50 mAh, the batteries are 2500 mAh)
2. Easier to replace.
3. Easier to charge.
As you can see, I am using 6 solar panels. Each is 6V, 90 mA. The amplifier battery is 7.2V 4A, and the mp3 player uses 3 NiMh batteries. So the big battery pack is charged from 4 solar panels (2 in series, to in parallel), while the small battery pack is charged by 2 solar panels in parallel. The diodes are on the back of the solar panels, and their back is glued on the lid.
The final step was connecting all the wires inside, gluing the batteries and performing some final tests.
This project took about 20 hours and cost about 90 USD.
It looks ugly, but there can be some improvements, such as a plastic faceplate, or the wood being painted.
It is actually cheaper, if you just buy a commercial stereo system and attach an mp3 player to it, although it won't charge from the sun. However, doing it yourself is more fun, and you can learn some stuff from it, such as how to build a good speaker enclosure, how to work with balsa wood, etc.
Here are some more pictures of the project.
Amp. battery glued, but not connected yet.
The lid, with just the panels for the amp.
Preliminary version, testing with a different mp3 player.
All the wires connected, ready to go. Notice the two battery packs, one for the amp, one for the mp3 player.
Same, just a different angle.