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 Post subject: Portable rechargable tunes
PostPosted: Tue Sep 10, 2013 10:17 am 
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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.


Attachments:
portable.jpg
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portable2.jpg
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portable3.jpg
portable3.jpg [ 180.52 KiB | Viewed 1001 times ]

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 Post subject: Re: Portable rechargable tunes
PostPosted: Thu Sep 12, 2013 8:44 am 
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I've already started planning the smaller box, it'll be 3.25"H x 4.25"D x 12.25"L, which has a volume of 82.25 cu. in. as opposed to the first version which was 5.75" x 4" x 17" with a volume of 221 cu. in. I figure this is a minimum size as the disassembled solar panel is exactly 4" x 12" and will sit nicely on top. I'll print a couple of mounts to hold it in place, eventually. The box will be tuned to 75Hz using 3/4" PVC 4" long. Hopefully, this shouldn't interfere with either the battery pack nor the driver magnet. I'll leave out the center divider too as the box is too small for the backwaves to really interfere with each other.

I'll be making it in 1/2" plywood, again, as I have some on hand, but the final version will be plastic cut on the laser cutter. I've been toying with the idea of making the housing large enough to accomodate the solar panel being mounted inside the plastic but I think that makes it just a bit too large.

I'll also be playing with a new adjustable dc/dc voltage regulator for the USB charger. I hope it will get rid of the hum that occurs while charging and playing.

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 Post subject: Re: Portable rechargable tunes
PostPosted: Tue Sep 24, 2013 2:43 pm 
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Version 2.

I made the enclosure a lot smaller, almost too small. It's 3.25"H x 4.25"D x 12.25"L as planned but components didn't fit quite the way I wanted them to. It's much easier to carry around the sound quality is decent but the mids and highs are still a bit louder than the lows, some dampening material on the back wall might help with that. The volume gets quite loud. I haven't used it enough to know how much practical use I can get out of it before the batteries die, but I've had it going at low to moderate levels for a couple hours and it was fine.

The finish is a rubberized automotive undercoating and wasn't very nice to put on. It tended to glob and/or bubble up and took way longer than the can suggested to dry.

Also, did a test of the solar panel this weekend and I would call it a success. I didn't realize that the solar panel voltage output would be conditional on the battery voltage. It stayed slightly over 2 volts above the batteries and crept up as the batteries charged. I didn't get to a full charge but I'm sure given a good location it should have no problem recharging the batteries in a day and at only 0.125 Amps I'm not worried about gassing the batteries if I leave it too long, even though the voltage is regulated to a max of 11.4V.

I'll have a few more modifications to it before I make the final version, but at least I'm happy with it. One improvement is the addition of a volume knob since my co-workers complained about the noise coming from my cube and my phone was on it's lowest volume setting before being mute. Oh, well, at least it works and it does everything I wanted it to do, and I enjoy the sound.

*Steve, you were right there is a dip at about 5kHz.


Attachments:
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IMAG0273.jpg
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IMAG0272.jpg
IMAG0272.jpg [ 217.74 KiB | Viewed 956 times ]

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 Post subject: Re: Portable rechargable tunes
PostPosted: Tue Sep 24, 2013 3:02 pm 
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The reason the solar panel voltage slowly climbs is the charger circuit is wanting to draw more power then the panels are supplying in order to feed the batteries.

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 Post subject: Re: Portable rechargable tunes
PostPosted: Tue Sep 24, 2013 7:23 pm 
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When I say slowly I mean over the course of hours. I believe solar panels are constant current sources and it's the voltage that will vary. I like that the voltage difference across the charging circuit remains consistent at 2V, including the diode. It means I can measure the voltage at the panel and get a fair idea of the battery state of charge. I imagine that the voltage difference will rise further than 2V as the batteries near full and the charge circuit stops at 11.4V, or the panel voltage could also stop at about 13.4V.

Honestly, this is the first time I've done any sort of data gathering on a solar panel. And being a neophyte electronics wannabee I'm tickled that I was able to create the charging circuit from scratch and it works... and I haven't burned the house down. Thanks again for your guidance, Steven.

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 Post subject: Re: Portable rechargable tunes
PostPosted: Thu Sep 26, 2013 7:42 am 
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Minor disappointment last night, I turned on the power for the box and no sound came out. I checked the battery voltage and it was 7.8V, which means somehow the batteries had completely discharged in the span of 3 days. Strange, because it sat for a week and maintained a charge last week. I suppose it's possible I have a short or something. The only thing that stays connected to the batteries is the USB charger but nothing was plugged in. I did measure a very small voltage across the power switch last week when it was turned off, not sure why that is. So maybe the switch is passing a small load from the amp. It's baffling because it didn't do it last week.

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 Post subject: Re: Portable rechargable tunes
PostPosted: Mon Oct 07, 2013 10:22 am 
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Big thanks go to Kelly for spending some time pointing in the right direction. Steven you were correct, there is a 5mA draw coming from the 7805. I'll install a switch next to the USB ports to turn it off, for now.

Otherwise, as the speakers get broken in the sound is getting better and better, the highs are mellowing and in an open space or outside the lows open up a bit.

On to the next project!

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 Post subject: Re: Portable rechargable tunes
PostPosted: Sat Nov 09, 2013 8:12 am 
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Thanks Lee.

Even if you're still in the design phase, post it up here. I've found that you can get some great ideas and/or improve your design from help or comments from some of the members here. And I would like to see this forum get used more. It's sort of an obvious problem, nobody uses the forum because nobody uses the forum. If we get more folks using the forum I'm sure we'll get more folks using the forum. If you get what I mean.

For instance, this project came about because another member mentioned something about music, camping, and dead cell phones. I had the same problems but had never thought about it in the same train of thought until that moment. Since I was really in the mood to do a new speaker project, I decided to start designing it. A week later I had the prototype and a week after that I had the finished unit. Still needs some tweaks but for the most part it works for its intended purpose.

I'd love to see your project sometime.

Jon

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