$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.

Hackaday Prize Finalist: A Network of Satellite Ground Stations

There are astonishing things you can do with a network of sensors spread across the globe, all connected to the Internet. Thousands of people have already installed hardware to detect lightning and flightaware gives out subscriptions to their premium service to anyone who will listen in to airplane transponders and send data back to their servers. The folks behind SatNOGS, one of the five finalists for The Hackaday Prize are using this same crowdsourced data collection for something that is literally out of this world: listening to the ever-increasing number of amateur satellites orbiting the planet.

There are dozens of cubesats and other amateur satellites flying every year, and they have become an extremely popular way of experimenting in a space environment, giving some budding engineers an awesome project in school, and testing out some technologies that are just too weird for national space agencies. The problem with sending one of these birds up is getting the data back down; a satellite will pass above the horizon of a single location only a few times a day, and even then for only minutes at a time. The SatNOGS team hopes to change that by planting receivers all around the globe, connecting them to the Internet, and hopefully providing real-time telemetry from dozens of orbiting satellites.

[Pierros] from the SatNOGS team was kind enough to sit down and answer a few questions for us about his entry to The Hackaday Prize. That’s below, right after their finalist video. Some of the SatNOGS team will also be at our Munich event where we announce the winner of the Prize.

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The Internet of Things Chip Gets a New Spectrum

Last year we learned about Weightless, an Internet of Things chip that solves all the problems of current wireless solutions. It’s low power and has a 10-year battery life (one AA cell), the hardware should cost around $2 per module, and the range of the Weightless devices range from 5+km in urban environments to 20-30km in rural environments. There haven’t been many public announcements from the Weightless SIG since the specification was announced, but today they’re announcing Weightless will include an additional spectrum, the 868/915 MHz ISM spectrum.


The original plan for Weightless was to use the spectrum left behind by UHF TV – between 470 and 790MHz. Regulatory agencies haven’t been moving as fast as members of the Weightless SIG would have hoped, so now they’re working on a slightly different design that uses the already-allocated ISM bands. They’re not giving up on the TV whitespace spectrum; that’s still part of the plan to put radio modules in everything. The new Weightless-N will be available sooner, though, with the first publicly available base station, module, and SDK arriving sometime next spring.

Weightless has put up a video describing their new Weightless-N hardware; you can check that out below. If you want the TL;DR of how Weightless can claim such a long battery life and huge range from an Internet of Things radio module, here’s an overly simplified explanation: power, range, and bandwidth. Pick any two.

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Dusty Junk-bin Downconverter Receives FM on an AM Radio

This amateur radio hack is not for the faint of heart! With only three transistors (and a drawer-full of passive parts), [Peter Parker, vk3ye] is able to use a broken-looking AM car radio to receive FM radio signals (YouTube link) on 2 meters, an entirely different band.

There are two things going on here. First, a home-made frequency downconverter shifts the 147 MHz signal down to the 1 MHz neighborhood where the AM radio can deal with it. Then, the AM radio is tuned just slightly off the right frequency and the FM signal is slope detected.

The downconverter consists of a local tuned oscillator and a mixer. The local oscillator generates an approximate 146 MHz signal from an 18 MHz crystal, accounting for two of the three transistors. Then this 146 MHz signal and the approximately 147 MHz signal that he wants to listen to are multiplied together (mixed) using the third transistor.

If you’re not up on your radio theory, a frequency mixer takes in two signals at different frequencies and produces an output signal that has various sums and differences of the two input signals in it. It’s this 147 MHz – 146 MHz = 1 MHz FM signal, right in the middle of the AM radio band’s frequency range, that’s passed on to the AM radio.

Next, the AM radio slope detects the frequency-modulated (FM) signal as if it were amplitude modulated (AM). This works as follows: FM radio encodes audio as changes in frequency, while AM radios encode the audio signal in the amplitude, or volume, of the radio signal. Instead of tracking the changing frequency as an FM radio would, slope detectors stick on a single frequency that’s tuned just slightly off from the FM carrier frequency. As the FM signal gets closer to or farther away from this fixed frequency, the received signal gets louder or quieter, and FM is detected as AM.

At 5:23, [vk3ye] steps through the circuit diagram. As he mentions, these are old tricks from circa 50 years ago, but it’s very nice to see a junk-box hack working so well with so few parts and receiving (very) high frequency FM on an old AM car radio. A circuit like this could make a versatile front end for an SDR setup. It makes us want to warm up the soldering iron.

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Reverse Engineering a Wireless Studio Lighting Remote

If you want to take a photograph with a professional look, proper lighting is going to be critical. [Richard] has been using a commercial lighting solution in his studio. His Lencarta UltraPro 300 studio strobes provide adequate lighting and also have the ability to have various settings adjusted remotely. A single remote can control different lights setting each to its own parameters. [Richard] likes to automate as much as possible in his studio, so he thought that maybe he would be able to reverse engineer the remote control so he can more easily control his lighting.

[Richard] started by opening up the remote and taking a look at the radio circuitry. He discovered the circuit uses a nRF24L01+ chip. He had previously picked up a couple of these on eBay, so his first thought was to just promiscuously snoop on the communications over the air. Unfortunately the chips can only listen in on up to six addresses at a time, and with a 40-bit address, this approach may have taken a while.

Not one to give up easily, [Richard] chose a new method of attack. First, he knew that the radio chip communicates to a master microcontroller via SPI. Second, he knew that the radio chip had no built-in memory. Therefore, the microcontroller must save the address in its own memory and then send it to the radio chip via the SPI bus. [Richard] figured if he could snoop on the SPI bus, he could find the address of the remote. With that information, he would be able to build another radio circuit to listen in over the air.

Using an Open Logic Sniffer, [Richard] was able to capture some of the SPI communications. Then, using the datasheet as a reference, he was able to isolate the communications that stored information int the radio chip’s address register. This same technique was used to decipher the radio channel. There was a bit more trial and error involved, as [Richard] later discovered that there were a few other important registers. He also discovered that the remote changed the address when actually transmitting data, so he had to update his receiver code to reflect this.

The receiver was built using another nRF24L01+ chip and an Arduino. Once the address and other registers were configured properly, [Richard’s] custom radio was able to pick up the radio commands being sent from the lighting remote. All [Richard] had to do at this point was press each button and record the communications data which resulted. The Arduino code for the receiver is available on the project page.

[Richard] took it an extra step and wrote his own library to talk to the flashes. He has made his library available on github for anyone who is interested.

ESP8266 Distance Testing

With progress slowly being made on turning the ESP8266 UART to WiFi module into something great, there is still the question of what the range is for the radio in this tiny IoT wonder. [CNLohr] has some test results for you, and the results are surprisingly good.

Connecting to the WiFi module through a TPLink WR841N router, [CN] as able to ping the module at 479 meters with a huge rubber duck antenna soldered on, or 366 meters with the PCB antenna. Wanting to test out the maximum range, [CN] and his friends dug out a Ubiquiti M2 dish and were able to drive 4.28 kilometers away from the module and still ping it.

Using a dish and a rubber duck antenna is an exercise in excess, though: no one is going to use a dish for an Internet of Things thing, but if you want to carry this experiment to its logical conclusion, there’s no reason to think an ESP8266 won’t connect, so long as you have line of sight and a huge antenna.

There’s still a lot of work to be done on this module. It’s capable of running custom code, and since you can pick this module up for less than $5 USD, it’s an interesting platform for whatever WiFi project you have in mind.

THP Semifinalist: A Continuous Wave Radar

There aren’t many Hackaday Prize entries playing around in RF, save for the handful of projects using off the shelf radio modules. That’s a little surprising to us, considering radio is one of the domains where garage-based tinkerers have always been very active. [Luke] is bucking the trend with a FM continuous wave radar, to be used in experiments with autonomous aircraft, altitude finding, and synthetic aperture radar imaging.

[Luke]’s radar operates around 5.8-6 GHz, and is supposed to be an introduction to microwave electronics. It’s an extremely modular system built around a few VCOs, mixers, and amplifiers from Hittite, all connected with coax.

So far, [Luke] has all his modules put together, a great pair of cans for the antennas, everything confirmed as working on his scope, and a lot of commits to his git repo.

You can check out [Luke]’s demo video is available below.

SpaceWrencherThe project featured in this post is a quarterfinalist in The Hackaday Prize.

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