Introduction: Re-Transmitter

Picture of Re-Transmitter

I wanted to see if I could modify one of those cheap car transmitters that people use to send music from an mp3 player to a car stereo and use it to send music from my computer to the kitchen radio.

The cheap transmitter: I picked one up that had both USB and line-in input possibilities.

These are really cheaply made, but they work well enough to transmit signals over short distances, so my initial idea was to open one up and tweak the antenna and antenna driver circuit. The first thing that I noticed when I opened it up, was that it didn’t really have an antenna… or… it had a wire that acted as the antenna… sort of. But it didn’t seem like it was designed for the kind of range that I wanted to get.

Things you'll need:
x1 cheap FM transmitter
x1 BJT transistor (2n2222 worked well)
x1 100 ohm resistor
x2 220 ohm resistors
x1 22pF capacitor
x1 10nF capacitor
x1 radio antenna
x1 good times

Step 1: Disassemble and Remove Voltage Regulator

Picture of Disassemble and Remove Voltage Regulator

The other thing that I noticed was that, even though the thing is packaged to work with 12 volts from a car cigarette lighter, the internal circuitry only needs 5 volts to operate. So we removed the internal 5-volt regulator.

Step 2: Extend Wires

Picture of Extend Wires

After removing the voltage regulator, and the plug for the cigarette lighter, we were left with this, the case with the circuits and display, and 3 wires sticking out the top. The red wire is the positive power lead where we need to connect 5 volts, the black wire is ground, and the white wire is the built-in antenna that doesn’t really work very well.

Step 3: Build Circuit

Picture of Build Circuit

I used a simple BJT amplifier circuit to boost the modulated audio signal, and drive a bigger antenna that I removed from an old radio.

Step 4: Good Times

Picture of Good Times

Put it all on a breadboard temporarily, plugged in some Daft Punk, and ... Success!! The little amplifier circuit increased the range of the transmitter from about 1 meter to about 10 meters.


celltechplus (author)2017-08-11

Contagem de Miren este tambien

'mobinessia (author)2016-08-24

Actually we only need a piece cable with proper length attached into proper pad of this gadget to increasing its range.
I already try myself and measuring the distance in the open Air, with only a piece cable I got round 80 meters, as I wrote the experiment on my blog post:

It's written in Indonesian, but there were Google Translate button if necessary.

obaidkakar (author)2016-04-23

will give u up to 2 km rang

obaidkakar (author)2016-04-23

Dear all get good idea from circuit

obaidkakar (author)2016-04-23

m_a_x (author)2016-04-19

I have got one from ebay, externally is the same but it has all in one
pcb, without usb pcb and with regulator 78M05 on the main pcb. It does not have an antenna wire and i cannot find any antenna terminal.

thiagohersan (author)m_a_x2016-04-20

yeah. I can't see a clear antenna lead or pad either...

VictorR48 (author)2016-02-16

anyone has got any results in open area ?

BlueCristals (author)2016-01-19

it is necessary to make the circuit you did?

no. sometimes just attaching an antenna helps.

I can put a cable as an antenna ???


Nathanaelmg (author)2015-06-11

Really a good idea! I got 15 meters with it! I used the same model fm transmitter.

jimvandamme (author)2015-04-24

I took mine apart and found an "ANT" terminal on the USB board, but it had nothing hooked up to it. With a 39 inch piece of wire tacked on to it, it boosted the signal over 20 dB (100 times). The scope shows a short wire receiving the broadcaster 3 feet away, vs. a local station that was of comparable power (but 20 miles away). A few dozen feet away, it's down to a similar level. Further away, back down into the noise.

chandra24 (author)2014-07-23

what did u use for power source??

i have 9 volt battery and i dont know much about resistors so can u tell me how much capacity resistor should i use?? or should use anything else instead of resistor???


thiagohersan (author)chandra242014-07-23

A 5-volt power supply from an old phone.

If you need to turn 9 volts into 5 volts, look into a LM7805 integrated circuit. I don't know how to do it with resistors...

thiagohersan (author)2014-02-14

@Coaster19, the caps might be swapped in my image... try reversing the 22pF and 10nF...

COASTER19 (author)thiagohersan2014-02-14

Unfortunately it didn't seem to change anything. Thanks for the suggestion though! I just discovered that my car's antenna is in the trunk's side, so I'll just use the larger antenna and move the whole transmitter back there.

COASTER19 (author)2014-02-14

Great ideas! I built the circuit from your schematic, however, and I couldn't make it work at all. I used a NTE123 transistor (same as a BC547). Any ideas?

Raphango (author)2014-02-03

LOL to your photo!

Congrats man! Now I know how to amplify my antenna signal! =D

jimvandamme (author)2013-11-23

I was going to do the same thing with the EXACT same transmitter. However, instead of taking the regulator out, I had a cigarette lighter socket and wired that to a USB plug (from a dead mouse). I just plug the USB cable into my laptop, the audio out to the audio in of the transmitter, and voila, I can play music from my laptop on the radio nearby. The regulator dropout is pretty low and the USB power runs it fine, except for the pilot light.

I haven't had a problem with range, but try putting a quarter wave wire (32 inches @ 88 MHz) in place of the existing antenna. It didn't work too well in the car, because the antenna is on the roof above the transmitter and the wave has to do some severe refraction to get there.

romulopericles (author)2013-11-23

awesome... i just my transmitter @

soshimo (author)2013-11-20

Great instructable! You should enter this in the hardware hacking contest. As long as you don't plan on marketing your product and you aren't transmitting more than a watt or so then you won't be getting a call from the FCC. A good measure of thumb for unlicensed broadcasting in the FM band is 200-300 feet range. Looking at your amplifier I don't think you will be able to get that much power coming out. With 5V and fully saturated the 2n2222 will conduct about 500ma (800ma max Ic before magic smoke is let out). Across a 1 ohm load that would be 250 mw. Also, 2n2222 are better for switching. BC547 is better for linear operation. Personally I would be more worried about the stations giving you a call regarding the rebroadcast of their broadcast. FCC has rules on THAT too! Though in essence you are acting just as a repeater, so I'm not sure about the nuances with that. At any rate this is a good hack. Great job!

thiagohersan (author)soshimo2013-11-21

Good point about the BC547. I wasn't rebroadcasting anything though... just transmitting some tunes from a USB stick (or a computer) to a radio.

soshimo (author)thiagohersan2013-11-22

Ah, I missed that fine point. I think you are fine then. Like I said, as long as you stay within 200-300 feet you won't incur the wrath of the FCC (within that range - other frequency bands have different regulations of course).

fazgard (author)soshimo2013-11-20

Great tips, looks interesting.

MaskMarvl (author)2013-11-20

Pretty neat... :) Although, in your instructable you stated that you were trying to send music from your computer but with this setup you're not using your computer. And if you're not using your computer you could have just placed the transmitter next to the radio in the kitchen. (Just saying...) :D

thiagohersan (author)MaskMarvl2013-11-21

This was just the test setup. I was using the USB reader on the transmitter, but it's very unreliable; doesn't read certain files, etc. The final setup would have my computer plugged into the transmitter's 1/8" jack.

MaskMarvl (author)thiagohersan2013-11-21

Ahhh... I get it. It all makes sense now... :D

ASCAS (author)2013-11-20

Excellent Job! Keep up the good work!

I made something similar to this, about a year ago. In our country, a store named "CD-r king", sells similar transmitters for 50php ($1). Instead of having the single transistor amplifier, I made a 4 phase FM amp, fitted it in a project box, added a preamp for the mic and simple audio mixer circuit that lets you speak on the mic while playing music from the iPhone (broadcasting). You could give it a try :)) I tested it from our school's football field and worked @long range.

spark master (author)2013-11-20

Ok now that we all know becoming a radio station w/o a ticket is a violation of FCC rules, however, the chances are seriously below possible. You mod this unit and you have a better chance of hitting the lottery. UNLESS your dad is a Fed, works at FCC or your neighbors do and you obliterate their TV reception during the Super Bowl.

DO this with a 1, 000 watt amplifier or even a 20 watt amp and it could happen, easily. Intermittent use makes it hard to find.

If you are going to do a Jpole , why not a copper catus, this IS , AFTERALL, Instructables

http ://ww,94

http ://ww


PhilKE3FL (author)2013-11-20

Nice and that should work. Remember you are amplifying the FM frequency. Also, in the USA at least, there are rules & laws concerning FM band transmitters and the power output. The little devices are made to only work within a car and to NOT interfere with a car right next to you, very important, at least in the USA. Also as one person noted the FM TXer as it comes can reach up to about 10 meters perhaps even more, when conditions and power cord are aligned just right. The antenna is usually one of the power cords in this design.

I made a similar mod to one I bought, different design it had a "real" extendable antenna, by simply making a real FM band antenna and adding a real external antenna jack. I used a BNC connector. I was then able to pick up the signal with a radio in my kitchen about 20 meters away and through the house. I was in my work room in the basement on the other side of the house from the kitchen window where the radio was located. The reception in this case was possible but marginal until...

Sometime all you need is a better antenna. Or in this case, a "real" antenna :) The best antenna for FM transmitting would be a simple J-pole or even a dipole and in fact I tried connecting to my ham radio 2-meter J-pole antenna on the outside of the house and that improved reception all through the house on a radio anywhere I was in the house. the radio in the kitchen was now nice and clear as well. I also checked outside to see how far it got and it only got close to the end of my property so the neighbors would not have a problem. I use this all the time to re-transmit stuff on my iPod or coming from a computer online radio station.

Enjoy! And try improving your antenna to get even better range. My web site has building and tuning instructions for a J-pole antenna for the ham radio 2-meter band which is a bit higher in frequency than the FM broadcast band. It is at then search for "J-pole" and then click on the "J-pole" link to see the on-line version or the [click here] link to download the file with a javaScript program and the documentation for building and tuning the antenna.

SugarCluster (author)2013-11-15

I think I had that one...while it had broken LCD screen.

An interesting and creative project.

That 10 nF capacitor to ground is only 0.16 ohms at 100 MHz. Perhaps more RF would get to the antenna if the 10 nF capacitor were moved to the other end of the 220 ohm resistor that goes to +5 volts.

I have to agree that the use of those plug-in breadboards have parasitic inductances and capacitances. Perversely, those parasitics could explain why the amplifier seems to work well in spite of the 10 nF capacitor shunting the signal to "ground".

rusty0101 (author)2013-11-19

Do be careful with this type of project. There are legal reasons that these transmitters have such crappy antennas and amplifiers. The manufacturer has to limit emitted radio power to below certain levels in various countries. I know that in the USA, transmitting with greater power than that, without appropriate licenses, can result in some rather hefty fines. I presume that in Canada and through most of Europe there are similar rules. (I'm pretty sure that it would be less expensive to have B&O come in and wire up a custom system with different content for each room, than it would be to pay the fines the FCC will impose.)

That said, nice instructable.

thiagohersan (author)rusty01012013-11-19

Yes! Very good point.

Some information regarding non-licensed broadcast in the US:

mybluevan (author)rusty01012013-11-19

rusty0101 is exactly right. For the US you can see the specific rules for these devices at the FCC's website: Changes to the transmitter's antenna or an amplification circuit will change the device's maximum effective radiated power and likely make the device illegal and subject to fines in the 5 digit range as well as possible criminal charges.

mybluevan (author)mybluevan2013-11-19

That being said, I highly encourage you to look into an amateur radio licence and learning to experiment with radio legally and responsibly. Radio experimentation is a great field, just please do so legally.

dwsworks (author)2013-11-19

I have made a smarter way to use it, I soldered two wires made a small hole on the top side 2 cms behind so that the fm transmitter can also go in car. This wire which I soldered I had taken from a creative speaker that had a male female connector to its adapter with + & - on it for polarity. I soldered shorter part from transformer inside the transmitter and longer to transformer. now I can use fm transmitter both in my car and at my home. I use it to hear songs from my sd card, usb in4o my old kenwood music syst3m that was lying idle as it had cd and cassette player both which are outdated. now my system is usb, sd card driven. even I can connect mobile through stereo wire that I can connect to fm transmitter. Also then idea struck why not add more functionality to my old music system, so I added bluetooth also to it using my bluetooth headset without distroying it. I just soldered a smalll stereo headphone socket inside the bluetooth headphones. wala, my music system is upgraded. I have gramaphone attached to itto enjoy my old records.

thiagohersan (author)dwsworks2013-11-19


I didn't mention it anywhere, but, as you pointed out, if you do it like I did and remove the voltage regulator, then you can't use it in your car again...

SolidRaven (author)2013-11-19

You sure it makes any difference? I can't say I ever successfully routed a signal above 40 MHz over a breadboard without running into some serious loss. Actually been meaning to measure just how much with a network analyser for a while now. (*Puts on to-do list*)

May I advice you just get a piece of copper clad and do it on that instead, should perform a lot better, you can just glue everything to it and it'll cause less interference. Plus the 2N2222 isn't the best transistor for this sort of stuff, Infineon makes pretty nice (and cheap) stuff that seems to be specially made for the FM band.

foobear (author)2013-11-18

Nice hack. How does it sound? vid plz?

thiagohersan (author)foobear2013-11-18

It sounds ok if you're listening to The Hood Internet or Skrillex... might not be so good for Vivaldi. I suspect there's a bit of distortion around the edges.

I left these in Brazil when I moved, and I should've made a video, but unfortunately all I have is this animated gif : )

radznjason2011 (author)2013-11-16

I have one of these. Looks exactly the same. Out of the box it transmitted about 10 meters.

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