Introduction: Hacking FM Transmitter
In this instructable i am going to shed light on a FM transmitter i had purchased from ebay a month ago. After purchasing this gadget i was happy for a couple of days until i found out that there was some discrepancy in this gadget. Firstly i found out that the Gadget was powered by AAA batteries which didn't lasted longer. Secondly most important the range that it was covering was very much shameful. Like about 100 feet in open space and if you were inside a building about 50 feet. Therefore i thought of doing something. After some research and gaining some inputs from Google. I found out that no such description were given for the particular model, as it was a Chinese made.
But indeed i got some information about the gadget. Therefore i would like to share with Instructable.
My primary Objectives would be as Follows:-
1. The power source should be compromised.
2. It should be capable of adapting with both the adapter as well as with the batteries.
3. The ease of access should be incorporated like it should have the ability to get along with any of the multimedia device.
4. Could be carried along while travelling.
5. Finally the most important objective, the range should well be increased to about more than the regular given specification.
Step 1: Opening the Shell
As i was shown various ways in instructable itself on how to open the transmitter, i was a bit surprised with my gadget. I had seen that in instructables posted to instructables it was shown that the FM Transmitter was hold up by only one screw and rest were just popping up the case.
But in my case i was surprised because there weren't any screws holding the Transmitter. I tried to pop it open but was unsuccessful. I tried and saw various opening to find if there were any hidden screws holding the unit. To my frustration at the end i found out none. Then by hook or by Crook i had to get inside therefore i made up my mind that i would open it, if also i had to break it.
Then i steady put up a wedge inside the shell and forced it to make a gap. I put a second wedge and again forced to make a more bigger gap. Still i was not able to see any screws from the gap. I continued till there was a breaking point. Then after a final push, there was large pop sound. Here i found out that this gadget was not held by any screws.
The Company had made this product so that it was not serviceable, that no body would be able to open this product. I could see that the legs holding the gadget had broken, and in no way i wold be able to put it back like it would have been before. I had to think for that also. But on the other had i had successfully opened the shell.
I guess this process of fixing it permanent was done deliberately so that no one like me could open it. But nothing was going to hold me back. I had successful opened the shell. But i would like to caution to the reader that this was process that i followed was done in a very slow and steady process, don't hurry up as you may end up damaging the PCB of the Transmitter.
Step 2: The Discovery Inside
After successfully opening the shell. I was able to see the PCB of the transmitter. I have given the photo's of both the back and Front side. It was totally different from the PCB that i had seen in other Instructables. I noted the parts No. that were printed on, especially the IC Nos. I took a look at the PCB with a much closer vision and kept note of the various inputs and outputs. Like i noted the audion in wires and the Battery connection and kept a note. I then removed all the wires from the board. Then i was back on to Internet finding the parts for details.
I could distinctly see two IC on the board.
I was not able to find the datesheet for the first IC but luckly i was able to find out the details of the second IC, i have also attached the link for further clarification. I was able to find the classfied documents of the transmitter chip IC and i have attached it along here. Follow the link.
What i found out from the datesheet that the Second IC was the brains of the gadget. It was the transmitter IC. The date sheet is also not given in detailed but i found out what the pins are configured for. and particularly the pin for the antennae which was most important for me. It was pin No. 5. After following the track i was able to find out the RF pin out and also found out that the antennae was just a coil of PCB in built wound up tracks which was linked to the earth of the 3.5mm socket. Might be it may be helpful for the projection of the RF signal..
Step 3: Removing the Holdup
After exactly finding out the limiting resistor for RF, i decided to remove it from the board permanently. I would suggest to all the readers to do to remove it permantly as i think it may cause some RF loss. As if the resistor would be kept there itself, the RF would be splitted to go to the antennae as well as through the resistor, this would definately cause a loss in the RF outpur. Therefor if you would like put as much juce to the antennae, i would recommend to remove it fully. I used a micro soldering Iron gifted by my sister on my Birthday to lift it off. Kindly take a look at the images. I started by dislocating the lower end of the resistor then going up. After successful removal of the resistor. I took an idea from one of the instructables for the Antennae part.
Step 4: Attaching the Antennae
Here come the delicate part of soldering the antennae part to the board. Try to get a thin wire, not as thin as a thread and solder on to where i have soldered. It should be perfectly soldered as when moved around it should not get disconnected. Then at one end attach a ring or a small washer of a kind. After successful attaching the antennae wire along with the holder, in the PCB fix it to the transmitter as if there should be minimum movement. I did it for myself by attaching a binding wire to the antennae and fixing it in the shell itself.
Just take a look at the pictures.
It should be remembered that as Antennae is the important part and for the objective to be completed successfully you should follow the steps here very precisely as this is the primary hack for the project.
Now finally to solder the Audio jack to the FM transmitter.
Step 5: Attaching the Audio Input Jack
This is a stereo transmitter and it has the capabilities to transmit audio in two channels simultaniously. Therefore it is very much important to solder the audio jacks in their respectively places. In my case there color of the wires were WHITE, RED and BLACK.
White = Left channel
Red = Right channel
Black = Ground & Antennae
I connected the White and Red terminal to the board but i didn't connect to the Internal antennae as i thought the RF would be splitted to the on-board antenna.
Step 6: Adding a Seperate Power Supply
I thought of adding a separate power supply to the board as i wanted to make it more handy as i didn't wanted my device to operated via a small batteries.
I had found out in the manual of the IC that it was not able to handle more that 5V, therefore i had to make my power supply as specified. Then with the help of a simple voltage regulator IC 7805 i made a power supply, along with some filter capacitors, which was also not needed but i thought of adding it. I made it in such a way that i didn't needed a transformer in one end.
I had a lot of switching adapters lying in my house mainly of 12V and i just made a socket, where in i was able to plug in the 12V directly and from the end of the regulator i would get a steady 5V.
Simple as it sounds.
And wala my FM transmitter is ready for the long range coverage.