Introduction: FM Transmitter Running Off Wall Power With Extended Range.
This is part of the iTrip Universal Plus.
A long time ago, in a galaxy far,far away this little device was purchased for a whopping $50. Little did it know that it's time on this world was not long. (A bit too much?... Never!)
This little guy served me well for a couple months, but eventually a new car stereo with an aux in banished the FM transmitter to the realm of abandoned electronics.
It toiled there until one day it's power source was brutally cut off and given new life as a USB power source in the car.
Now the iTrip lays mangled and forgotten, or rather it did, until the other day when I decided to re-purpose it. Yay!
Ok, so here's the plan: Give it a new power source and bring this puppy back to life.
Step 1: Tools of the Trade.
What you are going to need to do this yourself:
Obviously the keystone for this project is a FM Transmitter.
There are many different ones out there and many of you probably have one laying around, but even if you don't they can be found for a lot less than the $50 that I paid in a music store a couple years ago.
Regarding tools, all I really used is in the picture below.
A sharp blade, your favorite wire cutters, an implement for poking and prodding, long needelnose pliers, and some electrical tape; maybe some heatshrink tube to make it pretty, you know, if that's your thing. Pretty...
Last but not least, you will need some sacrificial wire, and a wall wart that outputs the same voltage as the original power source. We'll get more into that later.
Step 2: Examine the Original Power Source to Determine Power Requirements.
For me this was pretty much a no brainer. The device was originally powered by a cigarette lighter adapter for the car, so safe bet is that it runs off 12 volt power.
Just to be sure though, I took apart the power adapter. (Pic 2)
Following the wires to the board and then where they went on the board told me two things: First, the transmitter is indeed powered by the roughly 12 - 15 volts supplied to the cigarette lighter outlet when the car is on. Second, the green wire is +12V and the purple one is -12/Ground. (Pic 1, Notes A and C)
Since there is nothing between the power connection of the adapter and the wires besides a 2Amp fuse (Pic 1 Note B) I know that the same 12-15 volts are being sent along through the wires to the transmitter, and the 2A fuse indicates the maximum amperage the device can handle.
Step 3: Choosing a New Power Source.
I took a trip back to the realm of abandon electronics; one of my many tubs, boxes, and drawers containing all the sorted electronics and wires that I've collected over the years waiting in limbo to one day be either used, re-purposed, or thrown away; and found a candidate for this project.
This particular wall wart outputs 12 volts, which is what is needed for the transmitter, and 750mA (miliamps) or 0.75 amps, which is well within the 2 amp cap indicated my the fuse.
A higher amperage may have given the transmitter more power, but this is what I had on hand.
With the 2A fuse in mind I would not recommend going over 1.5A since the function of a fuse is to cut power to the device if there is a power spike to protect the circuitry I wouldn't think operating the device right at or near it's upper limit would be very good for it.
Without a supply of various wall warts to choose from, other options for acquiring one might include a trip to the local thrift shop or hardware store to find one that will work.
Another option would be to make your own. Now, should you choose to go this route, more power to you. There are ample resources out there on the interwebs that would instruct a more capable person on how to do it, but that goes beyond the scope of the ible and this author.
Step 4: Finished... or Is It?
At this point the project could be nearly finished. All that is left is to attach the wires from the wall wart to the power wires of the transmitter.
I did this to test the device, and found that the range and clarity were woefully lacking. So it was time to draw upon the wealth of knowledge of the millions of internet users out there, in other words it was time to start googling.
It turns out that there are gobs of sites describing how to 'boost fm transmitter range' (search term I used) and the general consensus is that there are basically three different ways to accomplish this goal.
1) Add or extend the antenna.
This is probably the most simple method and the one I choose.
2) Bypass the power limiter.
Some MP3 player FM transmitters made for the car have a little chip somewhere on the circuit board which limits the power of the signal being sent to the antenna in order to comply with FCC regulations. Unfortunately I could find no such thing in my device.
3) Build a signal booster.
Again, outside the scope of this ible.
So it's time to figure out how to build a better mouse trap, I mean antenna. Wait, what?
Nevermind, just click next.
Step 5: Identify the Antenna and Decide How to Improve It.
The model I am working with actually had a decent antenna already, but bigger is better right?
While taking the car power adapter I realized that there was a blue wire which just terminated inside the case, not connected to anything, so it must be the antenna.
Upon further investigation my theory was proven. On the board for the transmitter the blue wire's connection is marked with an A for antenna. (Pic 2)
Now it's time to extend the antenna. You could just splice a chunk of wire onto the end of the antenna where the power from the wall wart is going to be connected to the power to the transmitter, but I opted to put an extension between the wall wart and the transmitter using a cable that had at least three wires so that one could be connected to the antenna in order to be more in keeping with the original design.
After some deliberation, I ended up going with a length of USB cable, but ideally something with three wires of similar gauge to the wires of the transmitter and wall wart would be preferable.
Step 6: Hooking Up the Antenna Extension and Power.
After cutting into the USB cable I found that it had two thicker power wires, two thinner data wires, and was shielded. My first inclination was to try and find another cord, since the shielding would defeat the purpose but then I thought to myself, "Self, why not just the shielding it's self as the antenna." and so I did.
This is how a telephone line extension box came into play. Strictly speaking it is not necessary for this project, but it made it easier to connect the itsy-bitsy antenna wire to the fat glob of wires that made up the shielding.
Now all that is left to do is connect the other end of the USB cable to the wires coming off the wall wart.
Step 7: Finished Product.
Here it is in all it's glory, plugged into the wall and working.
The lengthened antenna definitely helped increase the range of this beauty as well as the clarity over more of the range.
The length of the chords also makes it possible to get the antenna up near the ceiling and away from stuff that will cause interference.
It now produces a fairly clear signal around 60 feet away, and i'm hoping to better that when I find a better frequency to set it to so it doesn't have to compete with other radio stations.
Now I basically have a 'home' range RF transmitter that just plugs into the wall and can hookup to nearly any audio source since the interface is just regular headphone jack.
I intended this device for use with my computer as the input, but use your imagination and the sky is the limit.
Step 8: Caveats and Addendums.
There are regulations governing transmissions such as those in the FM band which are enforced by the Federal Communications Commission, the FCC, and I take no responsibility if they come knocking on your door after you attempt a similar project.
That said, from what I was able to gleam in doing research for this project it seems that the way the FCC defines a pirate radio station is based on the strength of the transmission at a distance of 200 meters, about 650 feet, from the source. This could serve as a guideline as to how much is too much; however, there may be other regulations that must be taken into account depending on where you live, and of course the FCC regulations only apply within the US.
On the other hand, I seriously doubt that one of these little FM transmitters could even approach that kind of range since their stock range is usually something like 2-10 feet.
It's just something to keep in mind should you decide to try something like this project.
As for any project, you already have everything you need; the support of fellow diy-ers, a little ingenuity, and of course time and patience.
We have a be nice policy.
Please be positive and constructive.