Intern’s Corner: Solar charger for iPhone 3GS

Every other week, MAKE’s awesome interns tell about the projects they’re building in the Make: Labs, the trouble they’ve gotten into, and what they’ll make next.

By Tyler Moskowite, engineering intern

I recently received my first smart phone, an iPhone 3GS, from my brother who just got back from Iraq. It has turned into my PDA, map, social networker, and a boatload of other stuff I didn’t even know it was capable of doing. Let’s just say simply, I love this device. Which has prompted me to want it to be on every moment of everyday, and let’s be honest, the iPhone 3GS eats through battery life at a decent rate. Being a somewhat outdoors-active person, and because I enjoy the planet that I live on, I decided to build a solar charger for my iPhone 3GS.

When I started researching best way to build one of these, the simplest way was obviously to start off with a MintyBoost USB Charger Kit v2.0. Make sure to pick up the v2.0 and not v1.2, as the iPhone 3GS will not work with a v1.2 kit. This is due to the voltage on the D+ and D- pins that the iPhone 3GS uses to connect to USB. (Malaysian student Chen Tzy Wen has posted a good guide explaining how the voltage on each pin works, and the comments have good information in them as well.) Using the two 100kΩ resistors worked fine to charge the iPhone.

This design allows for a power source to be attached to the MintyBoost to charge your iPhone 3GS via a USB cable. Included in the kit is a 2xAA battery holder, but I wanted a more renewable and longer lasting energy source.

After I settled on the MintyBoost as the right device to start my project, I came across a good guide on Instructables. (It was posted on Make: Online blog back in May.) The MightyMintyBoost is a wonderfully simple solar iPhone/iPod charger. I followed the guide to the letter, except when it came to the solar cell. The author, “Honus,” said that “Using a slightly larger solar cell (6V/250mAh), you can generate enough power to fully charge an iPhone in about 5.5 hours and an iPod Touch in 4 hours.” And so I did. I purchased my solar cell from Adafruit, maker of the MintyBoost, on sale for $25.

So here’s my MightyMintyBoost after everything is said and done, sans a pretty case for it (I’m looking to get one custom made). Sadly, the barrel plug is too large to fit the connection from the solar cell, so instead I used a JST connection. This worked out fine because I wanted a few extra feet of cord anyway; I recycled some old MP3 charger cord. The JST connection is very secure to the motherboard and will work well when I’m walking around campus charging it.

I took a trip to the beach to test out how well the charger worked. The solar panel fit really well in the sunroof of my car and quickly turned the status light red, which let me know it was charging the 3.7V battery. There was also a slight high-pitched sound because, as LadyAda explains, “sometimes the inductor resonates with the boost converter and that resonance leaks into the audible range.”

About 4 to 5 hours of sun provides enough juice to charge your iPhone completely. Just make sure you unplug your iPhone from the charger after it hits 100% because it will keep drawing power from the 3.7V battery instead of using its own.

All in all, this project was extremely easy to do, and it works extremely well. The only tools you need to build this charger are a soldering iron, solder, wire cutters, snippers, and an iPhone 3GS.

CAUTION: This project was built with the iPhone 3GS in mind. If you’re going to connect another device to the charger, please make sure it’s compatible, and won’t be damaged in any way.

Parts List:
MintyBoost USB Charger Kit v2.0
6V 200mA solar cell
3.7V – 7.0V LiPoly charger
3.7V 2000mAh polymer lithium ion batteries
Jumper wire

Update: I ended up opting out of getting a fancy case made, because a well-designed Altoids tin is much cheaper.

I made sure to order a messenger bag with side pockets large enough to fit all my parts and extra cord. Once my messenger bag finally arrived — I made sure to order one with side pockets big enough for all my parts and extra cord — I was able to start designing the mounting system that would allow me to carry the charger around campus. I wanted it to secure while I’m walking around campus, but easy to remove and attach. Velcro was the perfect choice.

All you need is about 2 feet of velcro tape, and some glue if your velcro doesn’t have glue on the back already. Start by measuring the strap width, because you’re going to create double-sided velcro to wrap around the strap to hold your solar panel securely to your bag. Now double the width you just measured, and add another inch just to be sure it fits securely enough to the bag. Secure the backs of the velcro together (I used super glue), let them dry, and then wrap them around your bag strap. Make sure you put the hook (scratchy) side facing out, and the loop (fuzzy) side pressing onto the strap.

Finally, glue 2 strips of loop (fuzzy) velcro onto the back of the solar panel, let it dry, then attach the solar panel to the bag and enjoy the free energy.