$2 FM Transmitter for Raspberry Pi

We love re-purposed consumer gear. [Tobias] sent us the link to his project to that uses a cheap, discontinued cellphone gadget to create a Raspberry Pi controlled FM radio transmitter.

The Sony-Ericsson MMR-70 radio transmitter apparently used to connect to a cell phone and broadcast music. But the Walkman cellphones in question are a little bit old in the tooth, so one can buy the transmitter units for cheap on the resale market. What makes the transmitters even more interesting is that you can activate and deactivate the radio, change frequency or output power, and even send RDS station and song information.

It turns out (link in German) that the radios have an AVR ATMega32 microcontroller and a NS73 radio transmitter module, which can be entirely controlled over I2C. (Schematic here as PDF.) The units also have handy test points strewn all around. Once the test points were mapped out, one could completely ignore the on-board AVR microcontroller and control the FM transmitter module directly using the Raspberry Pi’s I2C outputs.

And that’s where [Tobias] stepped in. He wrote an I2C daemon for the Raspberry Pi that lets you control the FM transmitter via simple commands. All you have to do is solder up a bunch of test points, install [Tobias]’s software, write a batch script, and you’re on the air. For instance, this makes building a FM radio retransmitter for online streamed audio a one-day project. You can see his working example on youtube. Of course, you’ll want a web-based remote control interface to go with that.

If you’re interested in hacking along, and don’t have a Raspberry Pi application in mind, Sparkfun used to sell the NS73 radio transmitter so you can find lots of good information about the chip. We’d love to see a stand-alone broadcasting gizmo that actually utilizes the onboard AVR chip, but our hats are off to [Tobias] for making the Raspberry Pi version so accessible.

A Dead Simple, Well Constructed FM Transmitter

[Angelo] is only 15, but that doesn’t mean his fabrication skills are limited to Lego and K’Nex. He’s built himself an amazingly well constructed FM transmitter that’s powerful enough to be received a quarter mile away.

The FM transmitter circuit itself is based off one of [Art Swan]’s builds, but instead of the solderless breadboard construction you would expect to find in a small demo circuit, [Angelo] went all the way, etching his own PCB and winding his own coil.

Using photosensitized copper clad board, [Angelo] laid out the circuit with Fritzing, etched a board, and went at it with a drill. The components found in the transmitter are pretty standard and with the exception of the trimmer cap and electret mic, can be picked up in the parts drawers of any Radio Shack. He gets bonus points for using a 1/4 – 20 bolt for winding the coil, too.

The power supply for the transmitter is a single 9V battery, the battery connector being salvaged from a dead 9V. Awesome work, and for someone so young, [Angelo] already seems to have a grasp of all the random, seemingly useless information that makes prototyping so much easier. Video below.

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THP Entry: The Everything RC Transmitter

OSRC With few exceptions, most of The Hackaday Prize are things we really haven’t seen much of before: base-3 computers that have been relegated to the history books, extremely odd 3D printers, and fancy, new IoT devices are the norm. The OSRC is not a new project to us. (UPDATE: Looks like they deleted their project page. Here is a snapshot of it from the Internet Archive) We saw it once in 2011 and again a year later. What makes the OSRC an interesting project for The Hackaday Prize isn’t the fact that it’s the most advanced RC transmitter ever created. Creating that was evidently the easy part. The OSRC could use a big financial kick in the pants, and if [Demetris] wins, we’d guess he wouldn’t be taking that ride to space. Rather, he’d be taking the cash prize to get his ultimate transmitter into large-scale manufacturing and out into the wild.

While at first glance the base model OSRC seems expensive at about $6-700 USD, consider this: a six-channel transmitter from an excellent brand costs about $120 USD. Nine channels will run you about $400. The OSRC is a forty channel radio. The sticks are capable of force feedback, and of course the ‘pro’ model of the OSRC has that wonderful screen, capable of displaying video from an FPV camera, a GPS/map overlay, or an incredibly extensive telemetry display. There are multi-thousand dollar avionics for real airplanes out there that have a smaller feature set, and that’s not hyperbole.

A few months ago, [Demetris] was interviewed by the awesome people at Flite Test. That (highly suggested) video is embedded below.

SpaceWrencherThe project featured in this post is an entry in The Hackaday Prize. Build something awesome and win a trip to space or hundreds of other prizes.

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Easily Turn Your Raspberry Pi into an FM Transmitter

Have you ever wanted to be your own radio DJ? [Kevin] has made it easier than ever with his Raspberry Pi FM Transmitter program. The program is written in C. [Kevin] has made source code is available along with a compiled binary.

PIFM allows you to load up any audio file and specify a frequency to transmit. The program will then use PWM to modulate the audio sample through the Pi’s GPIO4 pin. [Kevin] claims that the RasPi alone will only transmit around a 10 cm distance. He says that making a simple antenna out of a jumper wire can increase the distance to around 100 meters. All you have to do is hook up the wire to the GPIO4 pin to drastically increase the range.

The legality of such a transmitter will vary from place to place, so be sure to check out your local regulations before you go transmitting audio on regulated frequencies. If this kind of thing is interesting to you, you may want to investigate ham radio. It’s not all Morse code and old fogies. Some people claim it’s a hacker’s paradise.

[via Reddit]

A 555-Based, Two-Channel Remote Control Circuit


[fahadshihab], a young tinkerer, shared his circuit design for a simple remote control using 555 timers.  Using a 555 calculator, he designed a clock circuit that would run at 11.99 Hz. Two transistors are connected to inputs (presumably button switches). One sends the plain clock signal, and one sends the inverted clock signal. A matching circuit at the other end will separate the channels. All it requires is connecting the two circuits in order to synchronize them. It would be easy enough to interface this with an oscillator, an IR LED, or a laser for long-range control.

The great thing about this circuit is its simplicity. It’s often so easy to throw a microcontroller into the mix, that we forget how effective a setup like this can be. It could also be a great starter circuit for a kid’s workshop, demonstrating basic circuits, timers, and even a NOT gate. Of course, it would be a good refresher for those without a lot of circuit knowledge too. Once you’ve mastered this, perhaps an AM transmitter is next?

Amateur Radio Transmits 1000 Miles On Voice Power

Many of us tried the old “Two tin cans connected by a string” experiment as kids. [Michael Rainey, AA1TJ] never quite forgot it.  Back in 2009, he built “El Silbo”, a ham radio transmitter powered entirely by his voice. El Silbo is a Double Side Band (DSB) transmitter for 75 meters. While voice is used to excite the transmitter, it doesn’t actually transmit voice. El Silbo is a CW affair, so you should bone up on your Morse Code a bit before building one. Like many QRP transmitters El Silbo’s circuit is rather simple. A junk box loudspeaker is installed at the bottom of the can to convert voice power to electrical power. The signal is passed through a step up transformer, and used to excite a 75m crystal. Two NPN transistors (in this case MPS6521) pass the signal on through a second transformer. The signal is then routed through an LC network to the antenna.

Back in 2009, [Michael] brought El Silbo to the Maine coast in an attempt to make a transatlantic contact. This isn’t as far-fetched as it sounds – [Michael] has “crossed the pond” on less power. While the attempt wasn’t successful, [Michael] has made connections as far as 1486km, or 923 miles. That’s quite a distance for simply yelling into a tin can! One of [Michael’s] favorite El Silbo stories is a 109KM conversation (QSO) he had with W1PID. [Michael] found that the signal was so good, he didn’t have to yell at all. He reduced power by dropping to his normal speaking voice for the “dits and dahs”. The two were able to converse for 17 minutes with [Michael] only using his speaking voice for power. We think this is an amazing achievement, and once more proof that you don’t need a multi-thousand dollar shack to make contacts as a ham.

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A DIY Solution for Controlling Robots and Quadcopters


RC transmitters used for controlling robots, quadcopters, airplanes, and cars really aren’t that complex. There are a few switches, pots, a screen and a radio transmitter. The maker toolbox already has all these components, so it only makes sense someone would try to build their own RC transmitter.

[Oscar]’s project started by gathering a bunch of toggle switches, 2-axis joysticks, pots, tact switches, an Arduino, LCD, and a Ciseco XRF wireless module. These were attached to a front panel made of polystyrene and work on the communications protocol began.

It should be noted that microcontroller-powered RC transmitters with XBees is nothing new. There was a Kickstarter for one last year, but the final product turned out to be bit janky and full of fail wiring, We’re really glad to see [Oscar]’s attempt at a DIY RC transmitter, and hopefully we’ll see this project taken up and improved by others.

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