switching the christmas tree online

The first time I got interested in home automation, was when browsing the arduino libraries. That was all about X10, which communicates over power lines. There are switches that can replace the regular light switches and are pretty cheap in the US. If you want such switches here in Switzerland, you’ll be surprised how expensive they are. And routers with X10 to make it accessible from your smart phone start at CHF 400. But what is easier to get here are the plugs with 433Mhz rf controllers. I got a set with one remote and three plugs for CHF 15 at hornbach. Then I ordered two different transmitters from China. One with the nRF905 chipset and an SPI interface. And one for sort of bit banging. I thought the one with SPI was going to be easier to use. I also found some code to interface it. But first I had to know what I need to send.

My first Idea was to use the DVB-T dongle with GnuRadio to sniff the rf protocol. Continue reading “switching the christmas tree online”

software defined radio on the cheap with a DVB stick

I think I first learned about software defined radio a few years ago on the chaosradio podcast. I was totally excited about the idea, and immediately installed gnuradio not only on my computers, but at times also on the smartphone. The USRP has been on my wishlist ever since. That’s the hardware device most commonly used with gnuradio. The downside was the price tag. While the approx $ 1’000 are not much compared to commercial solutions, it was too much for just another project to toy around, and I didn’t have an idea for a project where I must have one so far.

Then in last February (2012), I read on a blog post that was probably linked from hacker news that somebody found out that a cheap DVB tuner USB stick could be used as an SDR receiver. That was exactly what I’ve been waiting for the past few years, except that a device with TX would be even cooler. Immediately I went to the local electronics shops with a compatibility list. But in most shops they had no clue, and they couldn’t even find out what chipset was in the devices they sold. In a more professional shop they tried to find out the chipset, and they had a device that looked very similar to one on the compatibility list. But it didn’t work. So, I ordered one directly from china. It took almost two months to get here, but then it took another two month before I really started experimenting with it beyond checking if I could sample something to the harddisk. Here is some background information. Continue reading “software defined radio on the cheap with a DVB stick”

Handheld 2.4GHz Spectrum Analyzer

Finally I got my Spectrum Analyzer to work.

At first I was curious if I could use the display from an old Nokia cellphone I disassembled in conjunction with the Arduino. So I was looking for anything that could help me in doing so.

Then I stumbled across Miguel A. Vallejo’s Handheld 2.4GHz Spectrum Analyzer.

Putting it into a cellphone case would be too cool, but I don’t solder SMD, and I had an Arduino Nano lying around. So I ported the project to the Arduino platform. As the Arduino Nano operates at 5V and the display and the radio both at 3.3V, I had to use voltage dividers on the signal lines.

The CYWM6935 is kind of hard to get in Switzerland. Either you pay 3 or 4 times the regular price for the device or ridiculous shipping fees or both. I ended up ordering one from Farnell for CHF 48 including shipping.

The main problem I was chasing for a while resembled to the following: I made a voltage divider for the 3.3V to power the display and radio without considering the resistance of the circuits. So the driving voltage was too low for operation. Ha, out of training in designing electronics… In the end I’m using the 3.3V from the Arduino. I thought I read somewhere that it works only when powered from USB, but in my tests it works on battery as well.

Then I used the delay() function common in arduino instead of _delay_us() in Miguel’s code. Until I figured out the difference, It took about 30 seconds for a full sweep. Now it does about five sweeps a second.

Now, It works! I can see Bluetooth, Wifi and microwave oven radiation. The only drawback is that it doesnt always pick up very short transmissions like beakons.

Here is the source code for Arduino on github. Make sure to also look at the forks that improved my design.

It’s now also on the Arduino Playground.