I had an idea that's been eating at me for a little while and I decided to do something about it. I wanted something that would allow me to listen to music from my phone without headphones and be small enough to be portable, like a Jambox. I know, why didn't I just go out and buy a cheap boombox with an audio in, or blow some cash and get a Jambox? Because I also wanted it to be rechargable from either a 12v source or an integrated solar panel. This would allow me to charge it in the car, at my desk, or a campsite. I also wanted it to be able to charge my phone while I'm playing music, so I wanted a USB port, or two. It also needed to be relatively small but with good if not high quality sound.
Here's what I came with:
I started with driver selection and came up with the Dayton Audio ND-65 2-1/2" (more like 2") full range driver which has a Fs of 80Hz but many people rave about the bass this little guy can put out.
For the case I planned around the driver selection and response but also the size of the solar panel. I shot for an Fb of 50Hz, which was optimistic, but the solar panel size should just fit on the top. The box design I came up with was 17"L x 6-1/2"D x 4"H, or about 3 liters inside with a divider, made out of 1/2" birch plywood.
The solar panel I wanted was a 1.5W 12V used for keeping a car battery topped up and puts out a constant current of .125A. I checked max voltage, unloaded it was over 20V, under load it was more around 16-17V. The dimensions in the manual I downloaded said it was 14.75"x6.5", but in actuality is only 5.5" wide.
The electronics were the real challenge. I initially wanted 10 AA batteries but decided to only use 8 since the voltage per cell of a AA NiMh battery is 1.1 to 1.4V (fully charged) and still provided me enough voltage to run a small 2x15W amplifier. The other consideration was the charging circuit for the batteries, which requires 2V minimum above the output. So, for 10 cells I'd need at least 16V and my solar panel will only do that under optimal conditions. The charging circuit is the most basic design using an LM317 adjustable voltage regulator with just a couple of resistors and a diode to keep the battery voltage from backfeeding into the panel or the 12v wall wart. The USB ports are powered by an LM7805, which really needs a heatsink and a couple of capacitors but I haven't put them in yet. Thanks to Steven Wilcoxon for all the help, guidance and a few parts.
After knocking it all together this weekend I found that my charging circuit was .7V lower than I expected. It was pointed out that I forgot to account for the voltage drop across the diode, so I threw another resistor in to adjust the voltage up and it's putting out around 11.39V. Perfect. The USB ports work but the LM7805 gets hot and there's an audible hum and occassional pop if the phone is playing and charging. Probably just needs a couple of caps.
Listening impressions: The sound is actually pretty good, I would have liked more bass but it isn't bad getting down to about 100Hz before really dropping off. The highs are a bit loud but I haven't run the drivers in, they'll tone down after several hours of play.
I'll probably work on a new case for it, one that's quite a bit smaller and lighter. I'll take the solar panel out of its housing and save a couple inches in either direction. I may even try the laser cutter out and make it out of clear polycarbonate or something.
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