Solar charging a Nokia N95

One of the coolest parts of working on the Yahoo Purple Pedals project was figuring out how we could get the bikes as far off the grid as possible. Once we decided on using the N95 as the brain for the bikes, we had to figure out how much juice it takes to run the code, how often the code was going to run, and how we could prevent our users fromhaving to plug-in the bike everyday like a cell phone.
Solar charging a Nokia N95 - schematic

After thinking through and testing out a few different ideas, we settled on solar power as the simple, inexpensive, green, and aesthetically pleasing solution. In the following walk-through I am going to explain the design process and how we got from the idea of using solar to creating a solar replenishing backup power system that can run off the grid for weeks on end. The first step was figuring out how the phone charged. Power can be a tricky business and the phones weren`t cheap so we didn`t want to accidentally fry one in testing. Luckily, this step turned out to be a lot easier than we expected. The factory charger was a regular 5V AC adapter which output about 7. 2 volts DC at 880 milliamps. The next step was to figure out the minimum and the maximum power levels the N95 could take in at the charge port and still charge correctly. Our idea was to cut the tip off the power adapter, test the polarity of the wires inside, mate it to a variable power supply, and see what happened to the N95 as we cranked up and down the juice. The first challenge came when we went to strip the wires on our newly cut adapter wire. The N95 charger uses a flat cable to connect from the tip to the body, which means you can`t use regular wire strippers to get at the copper inside. We devised a technique of splitting the wire in half with an exacto knife to create two independent wires which were then possible to strip with regular tools. We...

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