I have one of the original Belkin Tunecast FM transmitters for my iPod. After I fed it one pair of AA batteries I decided I needed a better way. So, here's how I converted a car cigarette lighter cell phone charger into a method of powering my transmitter. This instructable is with a Belkin, but it will work for any transmitter, or even any battery powered thing-a-majiggy that you want to change from battery power to car/cigarette lighter power.
A bit of electrical explanation :
AA batteries put out 1.5 volts, since my FM transmitter uses two AA batteries: 2 * 1.5v = 3 volts
I used a LM317 variable output voltage regulator so I had to rig up some resistors to get the right output. If you use an LM317, the formula to determine the output is :
(Voltage out) = 1.25 * (1 + (R2/R1))
Where R1 and R2 are connected as shown later.
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Step 1: Disassemble Cigarette Lighter Adapter
Disassemble your cell phone charger and pull out the circuit board, desoldering as needed. You typically start by unscrewing the metal tip and shaking out the fuse and spring. Then, pry the two plastic halves apart. Be careful when you pull the pieces apart as there is sometimes more than one spring inside and it can go flying. Make sure you keep the fuse, spring, and connecting wires.
Step 2: Wire Voltage Regulator
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.
Step 3: Test the Circuit!
Test the circuit:
If you have a 12 volt power source at this point, you should hook it up to the inputs of the cigarette lighter adapter and measure your output with a multimeter to make sure your circuit is working correctly. If you don't have a 12v power source, you can put everything back in the charger casing and plug it in your car and test the output.
BUT BE WARNED:
If you messed up the circuit (grounded power for example) and hook it up to the car, you may blow a fuse (I admit, I've done this once or twice : ). If this happens it's not a big deal, all you'll have to do is replace your (likely) 10 Amp "Accessory" fuse, refer to your car's documentation for location.
Step 4: Diassemble FM Transmitter
Disassemble the FM transmitter by removing the battery casing and unscrewing the two screws (removing the batteries if there are any). Pull the two halves of the transmitter apart and mind the plastic clip that holds the audio cable in place as well as the small plastic piece that switches channels.
Step 5: Remove Battery Cable Connectors
Remove the circuit board (carefully) and desolder the black (ground/negative) and red (power) cables from the battery casing (as shown in first image). Now you just need to connect the power and ground from your charger cable to the power and ground cables of your transmitter.
NOTE: You may need to extend your power cables like I did by soldering on a bit of longer wire to the battery connector cables.
You really should test your power circuit (again?) before connecting it to your transmitter circuit board so you don't fry it. You should be getting a constant 3 volts out of your circuit.
Step 6: Test Everything!
You should have a complete circuit now, just a little bit sprawled out. Go ahead and try it in the car now to make sure everything works before you go through the trouble of stuffing it into that tiny package.
Test the voltage at every point (12v before regulator, 3v after regulator) and go ahead and try it in your car. Everything should work fine so all you have to do is make it look pretty.
Step 7: Cut Hole to Accomodate Power Cable
Once you've tested it, use a dremel or drill to cut a hole in the end of your transmitter casing that is slightly smaller than the cable that will be running through it. You want this to be really tight so it holds the cable in place with tension from the casing screws instead of having to glue it in place (but you can if you want). The easiest way to get this hole right is to put the pieces of the casing back together and then drill the hole at the seam.
Step 8: Clean Up and Final Test
Now, make sure you cover all connection points (where you connected circuit board wires to charger wires) with electrical tape (just a tiny bit so it will fit) and figure out how to stick it inside the casing. I routed my charger cable around the outside edge and through the hole but it's a really tight fit. Now, put everything back together, both the charger circuit and transmitter and plug it in, hook up your mp3 player, and give it a go.
If it doesn't work, you should have tested everything before you stuffed it in! Take everything apart and check all your solder connections, make sure nothing is grounding together that shouldn't (insulate it with electrical tape), and try it before putting it together again.