My Pi Description

My Experiences With the Raspberry Pi -- Tracking My Learning -- My Pi Projects

Saturday, August 10, 2013

Gertboard - Camera Remote Control - Finding The Patterns

Continued from last post
There are two common types of remote controls. The remote control for your TV, DVD player, and audio system are generally infrared devices. They are good for short range, line of sight, applications. There is not much that can interfere with their signal, but they must be aimed to function. My camera remote control is a radio frequency device, which does not have to be aimed, operates over a longer distance, and can communicate through walls. Generally, there are two carrier frequencies (the constant, high frequency, that carries the useful information) in use for these devices, 315MHz and 433MHz. My remote uses 433MHz (I see references to both 433MHz and 434MHz for the same hardware - I use both freely in my blogs because I can't keep track of which number I used last).
My project really comes down to replacing that transmitter you see in the photo with my Pi/Gertboard/Atmega. To do that I have to eavesdrop on the transmitter when it talks to the receiver. I did not develop the means to do that, the technique was passed down to me by my son on his blog. You can see his post here. My setup, and a diagram of the connections follows:
The 433MHz receiver, from Sparkfun, is the green rectangular printed circuit board sitting on the breadboard. The black square thing hanging off the breadboard is the 3.5mm stereo jack. It can be purchased from Sparkfun. The jack's pins would not fit into the holes of the breadboard so I soldered two wires to the jack's pins and plugged those two wires into the breadboard. Looking at the bottom of the jack with the four pins to the back (like in the photograph) the left most pin connects to the 1Mohm resistor. I soldered a wire across the ground connection at the front to connect to the circuit ground. That connection is fairly obvious from the photo.
I added the capacitor from habit of adding bypass capacitors to circuits whether they need them or not. It's purpose is filter out any garbage on the 5V power connection to the receiver. It's probably not doing much so if you don't have one don't bother about it.
The receiver requires 5V plus or minus 0.1V. My 5V is supplied from a 5.1V, 0.7A, cell phone charger I happened to have lying around. The charger has a USB connector so I used Sparkfun's Hydra cable to connect to the binding posts of the breadboard assembly. It has a USB on one end a several connectors on the other end. I used the cable's alligator clips to clip onto the binding posts of my breadboard.
Be very careful with the Hydra cable assembly. It would be very easy for the two alligator clips to short to each other, or for the positive 5V alligator clip to short to the sleeve of the barrel connector, thus shorting out your power source. Always connect the alligator clips to the circuit before applying power. I have binding posts on the breadboard assembly so it is easy to keep the alligator clips apart from each other and I'm careful not to move the assembly while under power to avoid one of both of the alligator clips getting loose.
The antenna is optional. It's easy to add, just stick a wire into the breadboard. If you wish to use one, the length of the wire is fairly important. Make it a quarter wavelength long. To figure the length divide the speed of light, 300,000,000 meters per second by the frequency and divide this by 4. This gives you the length in meters. For my receiver, the calculation is (300,000,000/434,000,000)/4. This computes to 0.173 meters or 6.8 inches.
OK, so what do we do with this receiver setup? We take a 3.5mm stereo cable and plug it into the receiver circuit and into the line input of your PC or MAC. Then, you launch an audio recorder and editor program called Audacity. It's free, open source, software, and it's great. Here is a link to the site where you can download it. When the RF receiver receives the modulated signal from the Canon remote control transmitter it demodulates it (removes the 433MHz carrier) and outputs the intelligence it received from the remote control transmitter. This signal is within the audio range, so can be recorded by Audacity. Audacity, while capturing the signal displays the waveform like you would see on an oscilloscope. Once captured, you can zoom into any portion of the waveform. So let's look at some waveforms (Click on an image to enlarge):
The waveform above shows the receiver output zoomed all the way out. About 5 seconds after I started the recorder I pressed the exposure button on the camera remote control transmitter. What you see in the waveform is mostly a constant amplitude of random noise. Where it says "Pattern In Here" you can see a slight difference in the display. After a time, the noise reappears.
Above shows two separate data captures where we have zoomed in to show about 50ms. of the captured data. This is within the "Pattern In Here" area. Each waveform shows one complete sequence of the camera's remote control transmit pattern. The bottom waveform, is the result of pushing the exposure button all the way down as if you were actually taking a photograph. The top waveform shows the pattern resulting in pressing the button partially down as if you were previewing the camera data or waiting for just the right moment to push the button all the way down to make the exposure. This preview mode is not particularly useful for my project. I just wanted to show that different patterns are possible. The difference in the patterns is slight. Look at the location at the black vertical line, and look at the very end of the pattern.
In the above graphic, we see the a pattern captured when the camera remote control button is pressed (top waveform) compared with the waveform captured from my project's 433MHz transmitter driven by the ATmega microcontroller on my Gertboard. As you can see the two patterns are identical.
The two figures above were made possible because Audacity allows you to display multiple captures on the screen. You can then slide either capture left or right to line them up to see the differences or similarities between the waveforms. If you were a musician, each capture could be a track of your music. You can play one or more tracks while recording a new track. You could save the combination of multiple tracks as your song. Great stuff.
Now that we can see the pattern we have to analyze it. It was not particularity easy because, as you can see from the waveforms, the vertical edges were not very vertical. My method was to establish the time for the shortest pulse width. The beginning of the pattern has a low going pulse followed by a high going pulse, then low, high, and low going pulses. These five pulses are the shortest pulses. So everything is computed in relation to the time of these pulses. This pulse width is calculated to be 284 microseconds.
Rather than trying to break the pattern down to 1's and 0's, I resolved the pattern down to 6 sub-patterns. They are pretty well enumerated from the figure above. The only sub-pattern not shown is the long high going pulse at the end of the pattern. This "T" pattern is the length of 34 basic pulses. Therefore, the entire pattern can be expressed as:
Now that we have the patterns we can write some code. That's for the next posts.

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