Instructables

Convert Belkin FM Transmitter from Battery Power to Car Power

Step 2: Wire voltage regulator

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Now, wire up your voltage regulator, the center pole of the cigarette lighter is the positive (12v) and the outside is the ground. So, according to the schematic that came with your voltage regulator (or the LM317 diagram), connect the center pole to the positive input of the voltage regulator (see closeup images), and the outside connector (ground/negative) to the resistor connected to the adjust pin (see closeup image).

As mentioned earlier, here is the formula for determining what resistors you need for a an LM317 regulator:
(Voltage out) = 1.25 * (1 + (R2/R1))

Since we want apprx. 3v out I decided to use R2 = 150ohm and R1 = 100ohm which results in:
(Vout) = 1.25 * (1 + (150/100)) = 1.25 * 2.5 = 3.125v

3.125v is a little more than needed, but not enough to matter.
I'll leave the math to you if you need a different output (system of two equations anyone?)

Pick two wires that run through the cable to the phone and hi-jack them for your own uses : )
I picked the red and black wires, just because that makes sense for power and ground. Solder your chosen power wire to the output of your voltage regulator (see closeup image), and your chosen ground wire to the ground/outer pole you already connected to the voltage regulator. Make sure you cover any metal surfaces that may contact others with electrical tape.

Now, on the other end of the charge cable, snip off the old charger cable end and strip the wires you chose for your power and ground.
 
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