Re-engineering Some FM Transmitter Firmware

[Furrteck] had a little adventure with this FM transmitter he picked up on eBay. It worked alright, but he wanted to be able to scan through the frequencies, and to have the device return to the same settings after power cycling. He cracked it open and got to work to achieve all of his goals.

The device is driven by an ATmega48, and there’s a 6-pin ISP header on the board. An initial read of the chip wouldn’t work, and he soon discovered the unstable power supply was to blame. After connecting his own regulated source he could read the chip id without a hitch, but the code is locked so no dumping was possible. Fortunately he managed to trace out the board, and includes a full schematic in his write up. With this in hand he erased the chip and started programming his own firmware from the ground up.

The video after the break shows off the completed project. He can now scan through frequencies with audio feedback to let he know when he’s found a station to hijack. The new code will also write a tuned station to EEPROM for use the next time the rig is powered up.

17 thoughts on “Re-engineering Some FM Transmitter Firmware

  1. That’s cool.Like really cool. In college I had this exact idea but to use to hijack the professor’s mike (auditorium) to inject music but people on forums were clueless on how to do that,with this write up,everything is covered. Added to the to do list.

    Thx HaD and Thx Furrteck!

  2. Nice, I have the same transmitter that I use in the car since typical itrips don’t have the power to overcome all the intermod crap all across the FM band.

    One thing, is it possible to have it not transmit as changing frequancy. One thing that has bothered me with these is that its constantly transmitting as I change freq past actual stations.

    1. Keep in mind this is a low power device.. but way over the legal part 15 limits..

      How much power are you pushing on yours? I’ve seen this model up to 10 Watts you could end up with RF burns if your not careful.

      If you in the usa your also committing a felony, the PLL on this thing is shit so your also probably shitting all over the WFM band causing all kinds of interference as you drive around.

      1. 50mW is what mine is supposed to be. A friend got the 500mW one that looks the same.

        I get about 100m from mine to a radio in a mobile phone in the city line of site before all the noise starts to overcome it. In the whops with only about 10-15 FM stations it will get about 400m to the same mobile.

        I dont feel bad about it since the poor band planning and excessive space between stations (.8MHz between some, come on) means that there is basically nothing left to use for itrips etc.

      2. Agreed, the output of these needs to be filtered to prevent wideband interference… this kind of device will splatter all over the dial. Also, Part 15 is there for a reason, it’s the law.

        To everyone: If you can’t accurately measure the output of your FM transmitter, you can’t guarantee that you’re in compliance with the law. If you can accurately measure your output to verify compliance with the maximum radiation limit, congratulations, you’re actually qualified to operate the device! If you can’t/won’t accurately measure the output of your FM transmitter, you have no business operating it into anything but a dummy load. Experimenting and learning are both very good things, but staying within the bounds of the law is essential to your continued ability to experiment and learn! Learning the applicable laws for your chosen field of experimenting/study is just as much a part of the process as actually building/testing/operating/etc… your project. As scientists, we still have responsibility for our actions.

        Note to richms: just because you don’t understand FM allocation and just because you don’t like the fact that unlicensed FM tramsitters sometime have a difficult time transmitting cleanly doesn’t mean you have the right to violate the law.

  3. Oops, fixed the schematic link. Always screw up the relative paths, thanks Chris. And glad you guys like it !

    Don’t hesitate to share your code modifications, I can add links to it on the page.

    1. This type of FM transmitter is illegal in the united states, you cannot get a license for this type of LPFM station. Its actually a federal offense even if you don’t cause interference.

      Take my word on it as a previous owner of the same device.

      That being said its too bad I nolonger have it as I never thought to modify the firmware for any purpose.

  4. All vibrations come free as a birthright from mother nature . Any person or group that seeks to control or limit these for their own peronal gain are in the wrong . I don’t seek to control or rip you off . Why do you feel a need or a right to rip me off?

    1. No one’s trying to rip anyone off. Rules & structure are essential to the functioning of society. Not all rules are good, but having no rules at all is worse. Also, the purple cosmic bunny of enlightenment would like to have a word with you.

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