Busy hands | Disquiet mind

The MPU-6050 is an accelerometer and gyroscope (with added thermometer), that contains a Digital Motion Processor (DMP) to handle a lot of the heavy maths lifting of turning the raw sensor values into useful motion values before they reach the microcontroller. It seems (caveat; I'm new to this area) that the original code is only available as a binary, or as some demo code. But some very determined people have pulled a library together.

/* Source is from the InvenSense MotionApps v2 demo code. Original source is
 * unavailable, unless you happen to be amazing as decompiling binary by
 * hand (in which case, please contact me, and I'm totally serious).
 *
 * Also, I'd like to offer many, many thanks to Noah Zerkin for all of the
 * DMP reverse-engineering he did to help make this bit of wizardry
 * possible.
 */

Well, actually, it seems quite a lot of people have pulled libraries together, to different levels of functionality, with very similar names. But the one most people seem to use is this one. Which includes the MotionApps files. This is important, not just for the right function calls, but because you need other libraries from this collection as well, particularly the I2Cdev one. These libraries, although for Arduino, are not findable inside the Arduino IDE's Library Manager. You need to side-load them, old school style. Which means downloading and extracting the .zip file. Then copying the specific libraries (being the folders that contain an 'Examples' folder as a first level child) for MPU6050 and I2Cdev into your Arduino > libraries folder. And restarting the IDE.

(I've also, after the fact, found what seems to be a copy of the MPU6050 library in an ElectronicCats repo, that is in the Library Manager, and seems to have rolled the I2Cdev code into itself. But the oldest change here seems to be 6 years ago. Whereas the i2cdevlib seems to be about 13 years ago. It does seem to have more recent updates than i2cdevlib though. So the choice is yours. I'm telling you, it's a nightmare finding what's what, especially when the code linking to it won't even compile with it.)

To help us get a feel for how the boards movements are interpreted, there's a nice demo we can try out. You can find the MPUTeapot Processing project inside the MPU6050 Arduino library; MPU6050 > examples > MPU6050_DMP6 > Processing > MPUTeapot.

First step is to install Processing. I'm using P4. Copy that MPUTeapot.pde file into a new sketch (or the whole folder to wherever you save your sketches). This sketch requires another stack of libraries, the Toxi/c libraries, which we need to side-load again. Download the newest release, extract the .zip file, and copy the libraries (being the folders that contain an 'examples' folder as a first level child) into your Processing > Libraries folder. Restart Processing.

MPU6050 Circuit
Made with Fritzing

Wire up your MPU6050 to your Arduino (I'm using the Mega 2560). You need to connect the Interrupt pin to Digital pin 02 (on most Arduino boards). SDA -> SDA. SCL -> SCL. VCC -> 5V. GND -> GND.

Open the MPU6050 > MPU6050_DMP6 example, and load it onto your Arduino. By default it runs in readable yaw-pitch-roll mode. Open the Serial Monitor at 115200 baud. After finding and initialising your MPU6050 it will await char input. Send any char, and you'll get a lot of lines like;

DMP ready! Waiting for first interrupt...
ypr	0.01	-0.01	0.44
ypr	0.01	-0.01	0.44
ypr	0.01	-0.01	0.44
ypr	0.01	-0.01	0.43

Which are your yaw-pitch-roll Euler angles in degrees.

If you comment out the #define OUTPUT_READABLE_YAWPITCHROLL line, and uncomment the #define OUTPUT_TEAPOT line you'll prepare the code for use with the Processing demo. Instead of nice readable yaw-pitch-roll values the info is packed into a 42-byte FIFO packet buffer; [quat w][][quat x][][quat y][][quat z][][gyro x][][gyro y][][gyro z][][accl x][][accl y][][accl z][][]. Upload to your Arduino.

Moving over to Processing now, open the MPUTeapot demo. I find it nostalgic that this is running in OpenGL mode. If you want to increase the window size, go ahead and do that. Otherwise you just need to set the correct portName. For me that meant commenting out line 72, and uncommenting line 75, while updating line 75 to say "COM7". The demo is straight-forward, handling the drawing of the virtual plane, reading and unpacking the packets back to quaternion w,x,y,z values, and handing off the complications to the Toxi/c libraries. But when it works it's a lot of fun to manipulate the virtual plane by moving your MPU6050. And really helps you connect the board movement to what you want your project to detect/react to, in your mind.

MPUTeapot demo
Gif by chebe

I went to an event at the weekend. Which meant I needed an event badge. Time to dust off my HalloWing badge. I finished the last post saying I would update once I got the badge reading images from the SD card shield. Well, dear reader, I was pleasantly surprised to discover that at some point I had gotten it working! At least to the point of auto-rotating through all the images on the card.

I don't really remember what I did to get the SD card working, but from the code the main bits are;

#include <SPI.h>
#include <SD.h>

const int chipSelect = 10;
File root;

void setup(void) {
  if (!SD.begin(chipSelect)) {
    Serial.println("initialization failed!");
    while (1);
  }
  Serial.println("initialization done.");

  // List files on the SD card
  root = SD.open("/");
  printDirectory(root, 0);
  root.close();
}

printDirectory(root, 0) uses dir.openNextFile(); which sorts through your files alphabetically. bmpDraw(...) (see code below) is what what reads the bmp file and translates it into what the tft wants. Nice.

But I wanted more. First thing you need to know is that Adafruit have moved on and really want us to use the Arcada library to work with the badges in Arduino. But if I wanted a simple life I would have gone with the CircuitPython option. I travelled the frustrating path instead.

I set up two of the buttons to 'scroll' left/right through the images. The images have overlays that display matching usernames. One of the buttons toggles on/off the tft backlight, and the other button toggles on/off the on-board NeoPixel. For a while things weren't working, and after a lot of digging I discovered that Adafruit had made breaking changes, including deprecating a function they had used in the examples I was working from.

setAddrWindow(...) was changed, from (start_x, start_y, end_x, end_y) to (start_x, start_y, width, height). So the change was from;
tft.setAddrWindow(x, y, x+w-1, y+h-1);
to;
tft.startWrite();
tft.setAddrWindow(x, y, w, h);
tft.endWrite();

pushColor(...) is deprecated. After experimentation I discovered that;
tft.pushColor(tft.color565(r,g,b));
needs to be replaced with;
tft.startWrite();
tft.writePixel(col, row, tft.color565(r,g,b));
tft.endWrite();

For the buttons a library is required;
#include <Adafruit_FreeTouch.h>
Then you need to create each of the four buttons (A2, A3, A4, A5);
Adafruit_FreeTouch qt_1 = Adafruit_FreeTouch(A2, OVERSAMPLE_4, RESISTOR_50K, FREQ_MODE_NONE);
Each of which you qt_1.begin(); in setup(void), and then read with qt_1.measure() as in;
if (qt_1.measure() > 700) { /* do stuff */ }

The backlight and NeoPixel parts were straight-forward. And the only other thing I did was add an extender to the battery JST connector so it's easier to reach. I wrapped the leads around the shield, and tucked the battery between the shield and board.

Back of the badge, with SD card shield in place, and battery tucked between
Photo by chebe

Front of the badge, displaying an image with overlay, and NeoPixel backlit
Photo by chebe

( Messy code dump cobbled together from various tutorials )

An Arduino microcontroller can generate a square wave on PWM pins to simulate an analog wave. But the wave alternates between 5V and 0V, it doesn't change polarity.

If you have a component that expects an AC signal you need to change the 0V to 5V signal to a -5V to 5V signal.

One simple way to do this is with a SparkFun Transceiver Breakout. Connect the Power pin to either 3.3V or 5V on the Arduino, GND to GND, T2IN to a PWM pin, and T2OUT to your component. analogWrite() your required frequency to control your component.

Back of SparkFun Transceiver Breakout with labelled pins visible
Photo by chebe

I finally opened up my Mega Pro Mini 3.3V, long retired it turns out, for something. Luckily I had bought the necessary connectors at the same time, and I always have an FTDI breakout board handy. But things I needed to figure out;

You need to add Sparkfun to your "Additional Boards Manager URLs" in the Arduino IDE;
https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json

Then select your Board as "SparkFun Mega Pro", and Processor as "ATmega2560 (3.3V / 8 MHz)".

But also, that each of the pins in output mode only supply about 40ma. Not the 200ma these lamps require. *sigh* Back to LEDs!