Project #1:
At work it is sometimes so quite that even at the lowest setting my ipod is louder than I want, especially when I listen for 5 hours in a row. So I decided to make a set of headphones with some attenuation. My first try was to add 120 ohms in series with each side. This worked reasonably well, but as a coworker pointed out, it really wasn't the right thing to do since headphone impedance is frequency dependent. Then I realized the correct attenuator for a headphone that is typically 32 ohms. So I made another set by putting 33 ohms in series and 3.3 ohms to ground on each side. This is a 10x attenuation, has a 3 ohm drive impedance and looks like approximately 36 ohms to the amplifier. Yes, ideally there are slightly better resistor values, but welcome to the world of standard 5% values.
I did this by buying an empty headphone jack at frys ($0.99). Then I put the resistors in the headphone jack and pulled out some wires. I soldered those wires to the headphone jack on an old set of ipod headphones. Then covered the three wires with some electrical tape. Not amazing looking, but does the trick. A better choice would probably have been to buy a female headphone connector so I could just have an attenuator to add inline without permanently attaching to the headphones. But oh well. I could have cut the headphone wires and inserted the resistors there. But I bought a cheap set of headphones to experiment and the wires were super thin so I figured it was better to not have to deal with them.
Project #2:
I'm working on building a backup battery for my iphone. The first attempt involved a 9V battery and a linear regulator. But that only gave me about 10 minutes on my old ipod mini. Then I noticed that someone online just put 4 AA batteries in series and used that. It is a bit scary since that's 6V, but USB is only supposed to put out 4.75 to 5.25V. But it worked, well for the ipod. After a couple hours of playing off the battery pack I got bored. At first it didn't work for the iphone, but then I found a tip online to short the two data wires together which worked. There were other people who had a more complex (and power burning) way to get it to work, but luckily just shorting the two wires works. I'm still not very comfortable with the over voltage issue - seems to work for iphones/ipods (wasn't too worred about them since they also work off firewire which can be up to 10V), but I want it to work for any phone that charges off USB. So I'm going to keep thinking about that. And ideally it would fit in an Altoids tin, but I haven't gotten that worked out yet. The battery holder is just a bit too wide and I don't have the skillz to cut it up and remake it as some sites recommend.
9V and linear regulator version:
4 AA in series version:
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