Turn a Raspberry Pi into an FM transmitter
makezine.com
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All of my research showed that most people just pony up and replace the mobo for $200. Unwilling to give up I kept researching and found that you can turn your Raspberry Pi into a flashrom to restore your bios. http://flashrom.org/RaspberryPi
Waiting on female to female GPIO connectors to come in mail before I can hook my raspi up to the SPI sockets on my mobo and reflash the bios. Got my fingers crossed, if this works it will be a great win!
It's really neat how easy this kind of thing has become recently with all of the hobby electronics guys driving the market and opening up doors that 10-15 years ago looked bleak at the time (though that could just be my memory coloring it).
I have used similar stuff to fix up routers (though haven't had to interface with the flashrom directly) and install openwrt.
A lot of PC repair shops in China will reflash BIOSes really cheaply too; they usually use a clothespin-like clip to attach to the chip so they can do it quickly without having to solder anything.
With the right software, plus favorable ionospheric conditions, you can get really, really good range. This guy did 7,600km, eastern Austria to south Florida: http://gerolfziegenhain.wordpress.com/2013/04/13/raspi-as-ws...
QRP is tough. I'm a licensed HAM radio operator and my Tx has a 5 W final stage, so I fit right into the QRP limits. It's tough. You're at the mercy of the ionosphere all the time.
100 W amplifiers are cheap. :)
http://en.wikipedia.org/wiki/WSPR_%28amateur_radio_software%...
This was into a poorly tuned dipole at 35ft.
For 10mW, sure, but I've done Michigan to France on 5W with a plain old 40m loop 15-20 feet off the ground. In hindsight, I should have tried it on 3W if only to get 1000mi/W on a cloud warmer.
But I have some concerns about its signal characteristics, since you're essentially abusing digital hardware (GPIO) to produce a modulated output it wasn't designed to produce. Someone with a spectrum analyzer should check to see if it produces any nasty harmonics, or similar noise, that could interfere with other stations.
Thanks for the link! That's a clever way of using harmonics to your advantage.
We used an FPGA though, so with a 100 Mhz fundamental frequency we had harmonics well into the 1.5 Ghz. We called it CPU-SDR. Video here:
https://sites.google.com/a/groundworkstech.com/index/blog/rs...
Bonus: Using mutiple PCB traces, you have a basic Yagi-like directional antenna. Feeding different traces you can modify the RF radiation pattern it in real-time.
But messing with the phases should be possible.
I don't doubt that the spectrum coming out of that thing is pretty horrific. There are no filters, and who knows what the wave form is (rectangular?).
Disclaimer: not an active HAM, used to clean seagull crap off 500KHz distress aerials on small ships in the last century.
Output from the digital pin is very rectangular. But you have many sinusoidal harmonics ready to be fired into the air if you plug a small antenna on it.
Of course it can. A square wave is just a series of discrete sine components, each of which will happily propagate through the air. If this were not true, WiFi wouldn't work -- it also relies on multiple, coordinated frequencies and complex modulation.
The problem being that any real circuit has a finite band it operates on. It will filter the components of the wave that are outside of its bandwidth, leading to 'ringing' artifacts (Gibbs phenomenon).
On the contrary. An ideal square wave's higher harmonics decline rather quickly:
http://en.wikipedia.org/wiki/Square_wave#Examining_the_squar...
In other words, there's enough energy to cause problems at 2, 6, 10 ... times f, but at rapidly declining levels.
> It will filter the components of the wave that are outside of its bandwidth, leading to 'ringing' artifacts ...
Ringing artifacts are present in a system whose bandwidth rolls off abruptly. It's less significant in a system with a gradual drop-off in response, like a piece of wire acting as a broadband antenna.
A true square wave would require an infinite series of discrete sine components, wouldn't it?
Yes, in principle, it represents an infinite series:
http://en.wikipedia.org/wiki/Square_wave#Examining_the_squar...
But as you can see, the higher harmonics decline quickly in energy.
According to a guy named Fourier, a rectangular wave is really just a bunch of sine waves working together.
"[...] a committee consisting of Lagrange, Laplace, Monge and Lacroix was set up to report on the work. Now this memoir is very highly regarded but at the time it caused controversy.
There were two reasons for the committee to feel unhappy with the work. The first objection, made by Lagrange and Laplace in 1808, was to Fourier's expansions of functions as trigonometrical series, what we now call Fourier series. Further clarification by Fourier still failed to convince them. As is pointed out in [4]:-
All these are written with such exemplary clarity - from a logical as opposed to calligraphic point of view - that their inability to persuade Laplace and Lagrange ... provides a good index of the originality of Fourier's views."
http://www.instructables.com/id/Raspberry-Pi-security-Slow-S...
The guys who run it are extremely knowledgeable about RF and there's some really cool stuff being done with it [2][3].
[1]: http://nuand.com/
On the other hand, for what it does, that x40 is actually quite cheap.
It's very hard and expensive to transmit signal on a single frequency. And very easy to pollute everything.
FM transmitters for MP3 players are cheap and work well (within a very short range, but that's their use-case), see for example http://www.amazon.com/FM-Transmitters-MP3-Player-Accessories...
So surely it can't be that hard or expensive - unless it changes when you try to broadcast to more than a few feet away?
So I don't think you can assume they are particularly clean transmissions.
Note that the news here is only the Make project on it, as this has been known and working for well over a year. The current PiFM software uses DMA from the userspace to play without clicks and without taking the whole CPU. The later versions also appear to support stereo, so I can't make much sense of this sentence in Make's tutorial (step 5):
> Each file is then re-encoded into a mono format the PiFM radio can handle.
If their pre-configured image is really doing this, it is doing it wrong, I think. Worth noting that the PiFM script is capable of reading from stdout, and not just play WAV files, so the possibilities go well beyond playing music off a SD card.
By the way, here is the original page in a Wiki (of which Make shows a screenshot, but I found no link): http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspb...
edits: add information
Interesting idea... but wouldn't you need to increase the broadcast range for that to work effectively?
[1] http://www.mcmanis.com/chuck/robotics/projects/eelb.html
Could this be used to create a simple, low bandwidth wireless connection between 2 pis?
Exactly what concerns do you have?
What does that mean? What needs to warm up?
The pi started to respond to ICMP packets in 19 seconds
(I love Vagrant/Virtualbox but sometimes, after putting the laptop to sleep, it stops working and I have to reboot).
http://adereth.github.io/blog/2014/01/06/benchmarking-mathem...