Instructables
Picture of Solar powered Tunes and phone charger
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This project stemmed from a discussion with a friend that said he wanted to be able to listen to music while he was camping but he didn't care for the sound of the portable boxes out there and they didn't have any way to charge them while in the wilderness. I wanted to make a solar phone charger since my phone always seems to die a couple hours into a camping trip. I decided we could combine them and have something that would please both of us. 


I went looking for an affordable tiny full range driver with decent bass and found the Dayton Audio ND-65 at Parts Express. I love this driver, with it's aluminum cone it can better withstand getting stuffed into a pack. I already had a TA2024 2x15w amplifier but had to design a charging circuit to round out the required electronics. I went through 2 iterations on the box design, the first one was to test how low the speakers would get in a small enclosure which seems to drop off right around 105hz. But I found I could really shrink the enclosure and tune it to about 75Hz without any major impact on the 105Hz I was getting before. The final enclosure ended up being 12.25"L x 4.25"W x 3.25"H (outside dimensions) of 1/2" birch plywood. The length and width were decided by the size of the solar panel I wanted to use, the height was dictated by the height of the driver.

The exterior was painted with a rubberized undercoating for cars to help withstand some abuse and the elements.

Power is supplied by 8 AA NiMh 2200mAh batteries which are in turn charged by a 12v 1.5A solar panel. The batteries are fully charged at 1.45V per cell or 11.6V total. Open voltage on the solar panel indicated 22V in full sunlight, so I needed to regulate it down to my maximum voltage of 11.6V. This was accomplished by using an LM317 voltage regulator with two resistors and included a diode. The value of the resistors will vary somewhat as you'll need to account for a 0.7V drop across the diode. I believe my actual resistors wound up being 240 and 2100.

The USB charging circuit is another simple circuit using only an LM7805 to output 5V to a pair of UBS ports. This works really well but tends to get warm if you have a really power hungry device, since it's only really good for about 1 amp. I'll be installing a switch on the LM7805 since it's always on and draining the batteries with a tiny current draw.

The audio input is via an 1/8" audio jack from the headphone jack on the phone. This may eventually get replaced with a bluetooth input device since audio fidelity isn't of paramount importance. But as it is I can have my phone plugged in and charging and listen to some tunes at the same time.

My overall listening impressions are that it is way better than one of those small bluetooth portable speakers. It still doesn't get as deep as I'd like but isn't bad for a 2-1/2" driver. Mids are really good and the highs are bit bright but it's smoothing out with use. It does get rather loud, volume is currently completely controlled by your device, though I may add a volume knob because the lowest volume on my phone is still too loud for my cube neighbors at work.

With a full charge I had this playing at near max volume for about 8 hours, the phone was not being charged, and the solar panel wasn't connected.
 
 
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ourgangmn made it!8 months ago

I love your idea. I am trying to make one, but using a Lepai amplifier from Parts Express that includes a volume control and 3 1/2" speakers. I also am concealing the edges of the solar panel in grooves at the top of the wood sides and ends, making the solar panel the top of the box. I put in a 2k resistor along with the 240. My voltage is 11.1V. Is my resistor too large?

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Gyvven (author)  ourgangmn8 months ago

Hey ourgangmn,

Nice work. I'm considering doing another project using the 3.5", myself. I like your idea of using the solar panel for the top.

Your voltage ideally should be about 11.1 up to 11.6 after the diode. Remember the diode will drop about 0.7V. Your11.1V should be adequate, you'll get the cells up to about 1.38V each and not overcharge them.

Below is the mk2, or maybe it's mk3, Where I've incorporated a cheap BT/FM tuner/SD reader and USB. I still have the Aux input and an extra USB charger on it and it's about 40% smaller than the one in the instructable. I haven't noticed any ill effects from the smaller size (10.5"x3.25"x3" outside, 1/4" mdf), but the Fb of the box is about 80Hz, well below where I expect it to roll off. It's still not pocket sized, but it's much easier to haul around. I'm also using a cheap Boost/Buck regulator off ebay. It'll take any voltage from 3-35 and output the 11.6 I want. Though it gets a bit warm when plugged into the wall, because it's only rated for about 0.6A. I'm sure my small solar panel won't cause that problem. I also really like my new laser cut box, I can draw my template and have it cut exactly the way I want it and not have to rely on my dubious woodworking skills. I'll eventually get around to making an update to this instructable about this.

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ourgangmn Gyvven8 months ago

I'm not quite sure what I'm doing wrong, but my batteries do not appear to be charging. My circuit is basically what you posted, but as soon as I unplug form the wall or bring it inside...no power.

Gyvven (author)  ourgangmn8 months ago

And you verified you're getting 11.1V between +V and Gnd after the charging circuit? Check this without the batteries connected. Also, double check your diode isn't turned around.

You should be able to run your box while charging. If you can do that but you're not getting power when just on batteries I would suspect a bad connection in your battery holder (which I've come across in mine lately). I had to use my multimeter to track it down and I found one of the tabs was slightly off the + side of the battery.

ourgangmn Gyvven8 months ago

OK, I discovered my battery connections were a bit suspect. Both battery holders have one battery that can keep the spring compressed on the negative end without pushing the positive end of the battery against its terminal. Also had to repair my negative lead since the glue that was holding my battery holders in place let go and the negative wire pulled away from the board when the battery pack moved. Fixed both problems and...it works!