Power down, phone dead? Not a problem.

Since most people don’t own a backup generator (including myself though I would like one). I have been in this situation a few times in the last year due to a new building going up in my area or fires or wind between the power generation facility and the city and have lost power for more than a few hours.

In the longest lasting one I found that my phones battery was pretty close to dead so I pulled out the bread board to remedy that problem my first reaction was to use a lm7805 (datasheet) a 5V regulator that would have been nice but I didn’t have any at the time (the closest electronics store is a 20 min bike ride away (and there POS terminals would probably be dead anyway) so I couldn’t be bothered but I had a few dozen lm317t’s (datasheet) a variable regulator

With the last of my phones battery I looked up the data sheet (if I had room I would probably keep a print out of all the components data sheets I own ) and built the circuit with the light from the camera flash

The equation for the voltage regulator is Vout = 1.25v*(1+r2/r1)

I used a 240 ohm resistor for my r1 so 1.25(r2/240) and I want a near 5v out put. So 5v= ((5/1.25) -1) *240=720 and the nearest resistor I have is 750. So 1.25(1+(720/240)) =5.156 close enough! the devices internal regulator can handle an extra 160 mV. In any case its a CMOS logic device so its all 3.3V anyway. I ended up using 2 in parallel to dissipate excess heat (I didn’t have any TO-92 sized heat sinks).

As for a final product I added some low value caps (.1 uf on the input and a 10 uf on the out put) along with some protection diodes (see final schematic) . I now have it soldered on a perf-board and in an Altoids tin for use in the future. In fact the last time I went camping I brought it with me and it came in handy. The first night there pretty much killed the battery due to the cold night granted the only use I have for my phone was emergency communication and as a time piece (not an issue any more with my pebble).