Introduction: Hacking FM Transmitter

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


RockyS21 (author)2016-05-03

Anyone experimented on a transmitter like this?

Bob.1 (author)RockyS212017-02-13

Yes. I'm ordering a 50 dB gain amplifier for mine. building a power supply for it and housing unit as well. Costs about 400 bucks. but i will never hear country music again in my life

PeterA187 (author)2017-01-11

Norway anounced its going totaly digital and there will be no more FM radio broadcasts ,FM is to be phased out..

Hello everyone, here I will reported two links to two videos posted on Youtube showing that transmits digital signals with two low-cost AM and FM transmitters.

To decode these messages, you can use various software on Linux, Windows and Android.

DanielC225 (author)2016-10-01

this is my transmitter. it has 4 channels. 88.1,88.5,88.9,89.3.

the pcb i soldered to the battery compartment and i cant find my iron.

TomT18 (author)2015-09-27

Hello, mine is similar but still has some differences tough

Its case says Wireless FM Stereo

TomT18 (author)TomT182015-09-27

Can someone help me about where to remove the resistor and where to solder the coil

ZoltánK15 (author)TomT182016-05-11

Hi! I made this:

On the picture of PCB i markd the modifyed parts.

Replace L1 to 3k3 Yellow (i don't know, is the value correct, but it works)

Replace L3 to 0R (bridge) Purple

Solder out C3 (blue, red), sold black the blue, red must be open. sold the antenna to the flying side of capa. (lambda/4, ca 0.7...0.85m)

L2 C2 is a resonant filter. (Orange) This values give ca 98MHz. You can optimize it to your selected FM freq. (I placed a 110nH and 22pF). At the resonant frequency you have much more signal. It's not easy. Dispension of an inductor can be 30%, a capacitor 5%.

GilbertoM6 (author)TomT182016-02-03

I have a transmitter with pcb identical to his, I am also having trouble connecting an antenna on it, please tell me if he found a way to do!

I thank you for any information to help me.

DavidP246 (author)TomT182015-12-30

Same boat as you. Please let us know if you find out which components to modify.

EnglishC1 (author)TomT182015-12-21


Would you please tell me if you found the solutions for your fm transmitter? I have the same model and I am looking to add external antenna to increase clarity of sound. If you found the solution I would appreciate if you send it in my email



bAss Kappa (author)2016-05-06

Are you giving instructions or narrating a story?

TomT18 (author)2015-09-27


rcs-al (author)2012-10-07

From what I see on your pic, the piece you remove from the antenna circuit is not a resistor, maybe a capacitor or a coil.

raikut (author)rcs-al2012-10-07

Yes it is a Capacitor. You are right. But i have tested it as if you remove it and add an external antenna along with a small 107pF ceramic capacitor in between the antenna and the ground, the RF signal is much more concentrated between the antenna.

I'm from Ranchi. Being a teacher wanted to start up a fm transmitter along with my colleague to air educational programmes. Heard that in India to set up radio is a hassle. Licence isn't normally given. I kindly request you to send me the details regarding it at

wa7jos (author)raikut2012-10-11

Increasing the power output of this device would be a violation of FCC rules.
These devices are limited to 100mw so as not to interfere with licensed operators.
If your modified device causes interference, you may get a nasty visit from the FCC, who can levy a fine of up to $10,000 PER DAY!

chrwei (author)wa7jos2012-10-11

Since raikut ins't in the US this may or many not apply directly.

As always, when dealing with licensed frequencies, be familiar with your local laws.

PhilKE3FL (author)chrwei2012-10-28

The FCC has jurisdiction over radio frequencies. No local laws apply, as far as I know. Not that they couldn't stop & hold someone & then inform the FCC I guess. Do you know something that I don't?

Rfmhz (author)PhilKE3FL2015-01-24

I agree there the law doesn't mind you having a transmitter but powerful ones then they will track you down .but for th wee tiny thing they don't bother about .after all it only kicks just outside your door step so no worth the trouble .

chrwei (author)PhilKE3FL2012-10-28

The FCC is local to the USA, but not to India, or Brazil, or the UK. Local doesn't always mean "the distance you can throw a rock".

PhilKE3FL (author)chrwei2012-10-29

True & India might be different as far as local governments having some jurisdiction over radio frequency usage. raikut would have to tell us that.

raikut (author)PhilKE3FL2012-10-29

Here in India, it is not like in the US.
The FM frequency spectrum is not much occupied at the place where i live. There are only 5 stations and one only broadcast in the morning.Therefore in total there are only 4 FM radio stations in Siliguri(QTH) And more or less there is no harm in transmitting on other free frequency. I had the urgency to make this hack because i had already told that my workplace is situated at a basement and there aren't any FM frequency penetrating, there fore i can use it here without any problem as it will not stray out from my workplace.
And yes, indeed here in India we do have regulations and guidelines that has to be followed. And there are consequences if not followed

ac-dc (author)chrwei2012-10-11

Waste of time in this case, and most... you can't just solder on an antenna and make much difference. Duh.

raikut (author)ac-dc2012-10-11

Well if you think it like that no hard feelings. I am living in a small village at a place in India, where not many are in the field of electronics. My love for electronics and elctronics gadgets are more that i can imagine. The work that i do for a living is a simple job of accounting, and the amount that i get paid here is nothing for you people there. It's just you would spend in a day. Life here in India is Hard, and i could tell better than anyone because i have faced it. Even after having such a hard life here in India, i like to share whatever knowledge that i have to people all around the world, and i have found instructables the best website ever to happen in my life. Whatever i have learnt is from by myself and i have never ever had the chance to learn electronics from any Colleges or institutes. I love reading lot of electronics books and if one tells me to stay all day having a Soldering Rod in you had i would be the happiest person to do so.
Please don't get offended as i wanted to explain that i am not a super hi-fi tech person who could write a really super instructable. I just tried and i am very much sorry if i have failed.
I have given a 100% in telling and reflecting whatever i had done and if you think it is a waste of time may be it is. I am very sorry that i was not up to your mark.

PhilKE3FL (author)raikut2012-10-28


Congrats on your ham license! I case you didn't catch it my call sign is in my user name, PhilKE3FL - KE3FL. Also, Instructables is here just for people like you who have figured something out that works for you.

Could it be done better? Probably. Does it matter if the transmitter now allows more frequencies out than it should? Not if you're the only one listening to the radio & it doesn't "bother" anyone else.

Good luck with the ham radio & I too hope your license comes soon, I know about the waiting, at lest when I got my license it only took three months. Now in the USA we can have a license the day after passing the test!

Fred82664 (author)raikut2012-10-12

well get your HAM ticket ,,,,play in the world of radio with this the right way

raikut (author)Fred826642012-10-13

Yes Sir!!! Actually i have recently passed my Ham Operator License and i am waiting for my License to come up. As here in India, the process of obtaining a license takes usually more than a year, and i am on the line. I am praying that my license will arrive soon, so finger cross. If i do have this License than i will be the first in my city to have one.

Fred82664 (author)raikut2012-10-13

Vary Good,,,I will be happy fro your QSO send me a message here with your call letters. at this point in time I can only run on Digital Moods like EchoLink . Apartment living has antenna restrictions so HF is not assessable for me right now

chandshar (author)raikut2012-10-11

Raikut please do not be put off by some of the comments your work is a hack and you have enjoyed sharing your work with us carry on with the good work and keep posting I realise that you are making do with the hardship you have
Best of luck from UK

sudharma (author)chandshar2012-10-11

Do not get disheartened by some comments. I am only a physics graduate and learned Electronics all by myself. I also do automobile repairs, mechanical repairs, Electrical repairs etc. All I learned by myself. My friends and relatives tells me that I could get these works done by some one else, why take the trouble. I tell them that there is nothing to beat the mental satisfaction you gets from doing some thing all by yourselves successfully.
So keep doing what you are doing and be happy with it. When you are successful, Post it here for all others benefit.
One word of warning..
1) Please take appropriate precautions and follow safety procedures when doing any job yourselves.
2) Always use proper tools appropriate for that particular job.

wsecomp (author)raikut2012-10-11

You may want to think about joining the thousands of other East Indian immigrants that have come to the US because of the finances in India. I just heard this morning that the average income for a person with an engineering (i.e., electronic engineering, such as is your interest) degree is $92,000US (4,845,640 ₨) per year.

nfarrow (author)raikut2012-10-11

Thank you Raikut for this instructable.

PhilKE3FL (author)ac-dc2012-10-28

Oh but you can. His will improve things a little but as it was said, if it solved his problem, served his purpose, then that's all that matters.

I once removed a small antenna & attached a BNC connector. I then plugged in my 144 - 440 MHz J-pole antenna on the roof of my house to a similar small transmitter. The difference was that with the J-pole antenna I was able to transmit all over my house & about half of my yard. With the antenna that came with the unit it could get to a few radios in the house but NOT to the kitchen & not any place outside.

I still have that little modified transmitter & it still works well but I wanted something a little more powerful. I wanted something where I could keep the antenna inside the house. The little transmitter I built can be adjusted from something around 1 mW or lower (I can't measure that low) & goes up to 25 mW was the correct choice for my situation.

scott5134 (author)wa7jos2012-10-11

It is a receiver, boosting the receive power means nothing, you can boost all you want, if it was a transmitter, that would be different

ac-dc (author)wa7jos2012-10-11

He isn't increasing the power output by much, this instructable is fairly redundant based on prior hacks by others, except to restate or remind that if you don't mind an external antenna you can add one of the right length for the frequency for a little gain.

Let's get serious here, there is no possible way he can produce enough power from 5V, let alone 3V, to have anyone else complaining and even if they did the FCC has better things to do than track down someone with a mW range FM transmitter. It's just not going to happen and even modified it will still produce lower output than other off the shelf FCC approved products, UNLESS there was an additional booster stage added with a higher drive voltage which is another hack for another day...

What I mean is, this is something people have done for years to their MP3 player add-on transmitters but with more detail than presented here.

To build a regulated power supply for a generic cheap Chinese transmitter seems like overkill, but on the other hand if the finished hack suits a need then it serves its purpose. I'd just start with something higher quality before spending more in time than the transmitter is worth.

The capacitor that was removed should stay there, it is used to filter undesirable frequencies.

Lastly, think about putting this in a shielded case with only the antenna outside.

quatch (author)ac-dc2012-10-11

Has anyone tried setting the gain higher using the i2c (note5 on p9)?

raikut (author)wa7jos2012-10-11

Well you are correct,
But as the title correctly says it is a Hack, and i suppose hacking are sometimes illegal. If you are able to find a free frequency in the FM spectrum at you place then no issues. You can just Google and find it out for you place. So you should be cautious while transmitting that you will not be interfering with local FM stations.

quatch (author)raikut2012-10-11

page 4 of the datasheet shows it to be an inductor. I don't know what that would provide here, not an RF engineer :( Maybe some frequency stability, or voltage modification.

jimvandamme (author)quatch2012-10-28

An inductor makes a piece of wire electrically longer. It's called a loading coil. If you have a quarter wave (a couple feet at FM freq) antenna you don't need it, but if it's only a couple inches like the original, the inductor increases the voltage on the antenna so more gets radiated.

Rfmhz (author)2015-01-24

Aye all you have to do is break a component off and attach your antenna there you have more power .

Rfmhz (author)2015-01-24

Aye all you have to do is break a component off and attach your antenna there you have more power .

ivmilicevic (author)2014-10-19

Is it possible to add line output to cheap fm transmitter, to use as mp3 player in DIY amplifier?

achaudhury99 (author)2014-08-26

I have a FM Transmitter Zebronics Vivo which uses a KST-6705T board. I have added its image. There are similar products from other brands which I feel may use same board. Links to these products from ebay follows. There are some nice videos in youtube which shows how to use them. Links follows. Its transmitter
Frequency is FM86MHZ.

Is it possible to increase the transmission range of this transmitter. Is it possible to change the transmitter frequency to some other for eg. 89MHz, so that I can tune my mobile phone to 89MHz and receive the signal transmitted from this transmitter.

5 in 1 Wireless Headphone

KST-670B-2 (MH2001)

Zebronics Vivo Wireless Headphone:

Zebronics Airwalk Diamond Series Headphone

Intex Wireless Roaming Over-Ear Headphone

You tube How to use videos:

ilovemoarK (author)2014-08-18

It killed mine (or I did not assemble properly) but this is very smart and creative!

samander (author)2014-02-23

Raikut, what is the range of this hack? Thanks in advance.

123mohan (author)2013-09-28

please visit this link.....for low power FM transmitter .... you love it..


microsurfer (author)2013-06-02


I have one similiar. I 've found that if you have a 35mm jack extension the signal improves with less interference. Save opening one up.

raikut (author)microsurfer2013-06-03

The range may extend if you add an extension as the wire itself will act as an antenna, but you would not get enough range as predicted, it may increase to couple of more feet, but not more than that, if you add an external antenna, than you will see the difference, so go ahead and try, you will be amazed by the range the antenna will offer you

phmanzano (author)2013-04-22

I finally hacked my FM transmitter with the help of this tutorial but found some differences on my model, see for details (wire antenna, hidden screws, bypass, ...). Hope this helps too. Since there's no FCC rules here I welcome better hack to improve my range :-)

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