Raspberry Pi RTL-SDR Frequency Scanner

Being one to never miss a bargain I grabbed a handful of Raspberry Pi Model B’s off of eBay a while back for an absolute steal when the new model was released. Since then I’ve been trying to put them to good use. The Tiny Python Panadapter was one project. The other is a RTL-SDR Frequency Scanner inspired by the Adafruit project Freq Show: Raspberry Pi RTL-SDR Scanner – See what’s in the radio waves around you using software-defined radio and a Raspberry”

https://learn.adafruit.com/freq-show-raspberry-pi-rtl-sdr-scanner/overview

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Now to get this thing working you need to add a 3.5″ TFT LCD display to the Raspberry Pi. It’s a nice ergonomic screen which fits perfectly over the mainboard and is nice and stable preventing it from being bent or snagged.

The Adafruit project uses a PiTFT screen which is nice, expensive and has been out of stock for quite a while. Now, foolishly to a degree, surfing eBay for alternatives may seem like a good idea but be warned. There are some very nice 3.5″ screens out there for an absolute snip but as to what you actually get when the dice are rolled is another matter!

My £13 landed a Waveshare Spotpear 3.5

http://www.wvshare.com/product/3.5inch-RPi-LCD-A.htm

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This screen came with a driver disc which supposedly has a pre-configured disc image which supports the screen out of the box, simply needing to be burnt to an SD card. The Pi boots, screen works and everyone is happy. Until the image on the disc has a corrupted file! If it helps anyone in a similar situation, when extracting the .img file an error is reported by 7-Zip stating that a single file within the archive is corrupt. As to which one, take your pick it could be anything (harmless or harmful).

Now I must stress that this screen didn’t come directly from Waveshare, which is probably why the disc is more use as a drinks mat than anything else. The fact the vendor has disappeared into the ether when I’ve emailed and asked for a replacement or directions to a genuine disc image did make me wonder if I’d been well and truly seen off!

So, with fingers crossed that it was a simple help file that was broken rather than anything more vital, I blundered on and burnt the image to the SD card and booted up, or at least tried to. The boot process hung with all sorts of errors namely to do with not being able to mount the root partition. Nothing I tried overcame this problem.

My money was now definitely on the “been seen off” side of the Roulette table!

Now a lot of time went into Google research in an attempt to overcome the “out of the box” solution, only to find a whole community of people who were regretting the day they bought a Waveshare Spotpear 3.5!

I resigned myself to having to build a custom kernel based around the advice of various forums out there, but again and again people were reporting lack of support for this specific device, which is never a good sign. That said in the middle of a huge forum post I found this link

https://docs.google.com/uc?id=0BxTKwT1nnLaoM001MDhyTGJIZU0&export=download

Someone had posted this as a working solution for the Waveshare Spotpear 3.5

It is thankfully, a boot image which actually works. Once burnt to an SD card, the Pi boots, the screen springs to life and all is good.

As proof that this thing is real and does actually do what it’s supposed to do, here’s a snap of it in action.

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The support documentation with this image is actually very good and helps with calibration and use with other peripherals such as cameras. All I need to do is calibrate the screen and the touchscreen functionality and we’re in business.

As an aside, to prevent the image becoming unusable after performing any system update make sure you do the following –

sudo apt-mark hold raspberrypi-bootloader
sudo apt-get update
sudo apt-get upgrade

The Adafruit article requires the screen side of the project to be running correctly before the RTL-SDR elements are installed, so watch this space.

24 hours on and I was able to get this project finished. With the screen calibrated and working perfectly it was a simple case of following the Adafruit project through and then starting the python script. There was a degree of apprehension when the screen went blank, but a few seconds later it sprang into life!

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After tweaking a few settings we were in business. The trace shows The Chris Evans Breakfast Show on Radio 2.

 

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To tidy things up I salvaged yet another piece of metal from the D-Link switch case which I used to make my dummy load and fashioned an angled viewing bracket for the Pi. A light blast through with a spray can and a few rubber feet finished it off nicely. The Pi has two holes in the PCB and with a few brass spacers and some screws everything was bolted together.

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It gives it a nice angle of tilt to make the screen readable plus keeps it still. The temporary prop made from a block of notelets wasn’t cutting the mustard and it was sliding all over the shop, plus it keeps the back of the main PCB away from any harm!

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