Build A $360 Synthetic Aperture Radar With MIT’s OpenCourseware

December 18, 2012 by Brian Benchoff 38 Comments

A few profs from MIT’s Lincoln Lab are giving those poor MIT undergrads something to do over winter break: they’re teaching a three-week course on building a laptop-powered radar system capable of radar ranging, doppler, and synthetic aperture imaging. Interestingly, the radar system that teams will build for the class has a BOM totaling $360, and they’re also putting the entire class online if you’d like to follow along and build your own.

From the lecture notes from the course, the radio system is made out of an off-the-shelf LNA, oscillator, and splitter. By connecting two coffee can ‘cantennas’, it’s possible to record a .WAV file from the signal coming from the radar and use MATLAB to turn that audio signal into a doppler radar.

It’s a very ambitious project that goes deep down the rabbit hole of RF and analog design. One of the lecturers made a YouTube demo of the radar in ranging mode; you can check that out after the break.

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Electronically Controlled NFC Tag

December 17, 2012 by Brian Benchoff 20 Comments

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[Per] is replacing his car stereo with a Nexus 7 tablet. It’s a great modification to add GPS, navigation, and a good music player, but [Per] wanted to pause his tunes and tell the tablet to go to sleep with an NFC tag. This means building a an NFC tag he can turn on and off, an interesting problem to say the least.

The easiest way to do this is with a CMOS switch, but a chip like a 74HC4066 is overkill for a project this simple. What [Per] needed was a single CMOS switch, something he found and fabbed a board for.

Now, with a press of a button, [Per] can activate his NFC tag and pause the music in his new stereo. Check out the video of this electronically controllable tag after the break.

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Transmit FM Using Raspberry Pi And No Additional Hardware

December 10, 2012 by Mike Szczys 75 Comments

Now here’s a project that actually hacks the Rapsberry Pi rather than just using it as an embedded computer. [Londons Explorer] figured out how to turn the RPi into an FM transmitter. For now it’s done entirely in the user space, but we’re sure it could be improved if someone wanted to drill down further into the hardware. For those wanting to give it a try he’s rolled everything into a simple python package.

The technique requires nothing additional except a 20cm wire to serve as an antenna. The trick is to map GPIO pin number 4 to a position in memory. The clock generator is then used to toggle this pin at 100 MHz, which is the frequency to which your radio should be tuned. A fractional divider adjusts the frequency based on the sound file being transmitted.

The proof of concept for this was able to reliably transmit at a distance of about fifty meters through several walls. The problem is that this technique is limited in the amount of data which can be sent. Right now it’s only about 6-bit audio. But descending deeper through the abstraction layers to put DMA (Direct Memory Access) to use may be able to improve upon this.

[Thanks Owen via Reddit]

DCF77 Signal Filtering And Decoding

December 4, 2012 by Mike Szczys 22 Comments

[Udo] decided to build a clock using the DCF77 radio module seen above. This of course has been done before: the hardware draws a clock signal from the atomic clock in Braunschweig, Germany. So he grabbed a library for Arduino and got to work. But he was getting rather poor results and upon further investigation realized that the library had been written for 20 Hz modules and his operates at 300 Hz. This means better accuracy but the drawback is that the hardware is more susceptible to noise.

So began his journey to filter, process, and decode the DCF77 protocol. That link goes to the project overview. It will be in several parts all of which will be linked on that page. So far he has applied a low-pass filter and coded some exponential smoothing. He has yet to write the other four parts, but does mention that early testing shows that this technique will make the reception better than what is achieved with commercially available clocks. He was able to lock onto a signal that had more than 80% noise ratio. That’s impressive!

Just want to see a clock that uses a DCF77 module? Check out this PIC-based atomic clock.

Cheap Biquad Antenna Extends LAN Between Apartments

November 27, 2012 by Mike Szczys 41 Comments

[Danilo Larizza] is sharing a network connection between a couple of apartments. They are not far apart, but they are also not right next to each other so a set of external antennas is necessary. He built this 2.4 GHz biquad antenna on the cheap (translated) just to test if it improved the signal before he tried to buy a proper antenna. It turns out to work well enough that this is all that he needs.

The antenna itself is about one meter of thick wire bent into two squares which are 31mm on each side. The coaxial cable going to the router connects to the center portion of this antenna. For a bit better directional reception he added some tin foil as a reflector. Since this is outdoors he used a food storage container for protection (the antenna is mounted to the lid, the body has been removed for this picture). The whole things is perched on a stake in a flower pot with proper line of sight to the other antenna.

We’ve seen a very similar design used for an NRF 24L01+ radio. If you need more details that [Danilo] posted that would be a good project to study.

EVE Radio Breakout Board For The Raspberry Pi

November 17, 2012 by Brian Benchoff 19 Comments

The Raspberry Pi is an excellent tool to build the ‘Internet of things’ we’ve been hearing about, but there’s still the issue of connecting the Raspi to other devices. The EVE Alpha – a breakout board for several wireless radio modules for the Raspberry Pi – hopes to change that with their Kickstarter campaign.

The idea behind the EVE is to provide a link between low-power radio modules found in a few of the microcontroller projects we’ve seen and the Raspberry Pi. It does this by simply serving as a breakout board, taking the GPIO pins on the Raspi and connecting them to solder pads for a few of the many radio modules currently available.

Already the EVE supports the RFM12B wireless tranciever, a Z-Wave module, 868-915Mhz SRF modules, and has a breakout for an XBee module, allowing the EVE to communicate using one of the many different XBee boards. There’s also a battery-backed real-time clock and temperature sensor thrown in for good measure making this board the perfect building block for an outdoor weather station or solar array.

It’s an awesome idea, and if you already have a few radio modules, incredibly cheap; just the PCB is only £6, and a board with all the SMD components is only £20.

AM Chiptunes Played By A Modified Antenna Analyzer

October 26, 2012 by Mike Szczys 5 Comments

Believe it or not, this VK5JST aerial analyzer kit is going to rickroll you. [Erich] wanted to see if he could use the device in a different way. His adventure led him to use it to feed different tones to an AM radio, producing the all too familiar [Rick Astley] offering.

There’s a fair bit of math that goes into getting the correct signals to generate a given pitch. But it basically boils down to patching into the hardware early in the RF generation. This way an audio signal can be rolled into the carrier frequency. Since this kit uses a PicAXE microcontroller with available source code it is rather easy to add audio input to tweak what the chip is putting out. But there is also some hardware tinkering to be done. Read more about that at the article linked above, and don’t forget to check out the bottom of that page to hear the final results.