Step 2: Designing circuit boards

I designed total three boards - main circuit board with Attiny44 and 2 axis accelerometer, sub circuit board for mechanical switch, and sub three boards for LEDs. By utilizing knowledges you learn from series of electronics design and production, you should be able to design above circuit boards pretty comfortably. The most important part when you use accelerometer and bluetooth module is figuring out appropriate pin assignment reading datasheets of those and Attiny. Talking about bluetooth, you have to make sure you connect Rx and Tx pins of bluetooth module to TX and RX pins of Attiny. Also MXD6235M was not in the Eagle Library, I learned how to design and add new parts to the library from this tutorial.

Step 3: Designing exterior

To build working prototype as soon as possible, I decided to build a simple box as exterior. I designed it using SolidWorks and saved a 2D drawing which contains all components as DXF file to allow CorelCAD to read the file. I also wanted to put MIT logo on the top of my product so I downloaded a png file and converted it into DXF file using Inkscape. This tutorial would help you to learn how to do that.

Step 4: Fabricating circuit boards

As experiencing several iteration of circuit board fabrications, this process went smooth as well. Neil sometimes mentioned that "design and make new PCBs rather than using those you made during previous assignment," which is so true. Through several iterations you could learn how to solder beautifully and quickly, and most importantly would spend less time on debugging circuit boards due to stupid mistakes. After I build PCBs, I checked connection using multimeter and they looked all good.

Step 5: Fabricating a case

I cut plywood with laser cutting machine in IDC. After cleaning up a lense of laser cutter, I turned on air compressor and laser cutter. I opened 2D components drawing saved in DXF file type in milimeter (I designed the model in units of milimeter), imported MIT logo and arrange it on one of the component, and defined all line as hairline. When you do printer setting, open preference and make sure you load appropriate setting from "advance" tab. I chose 6mm plywood sheet for my 5mm plywood sheet, which works perfectly for that. I assembled all components and got a box.

Step 6: Programming Attiny44

This is one of the moments of highlight I struggled with. I used Arduino IDE to program the Attiny44 for the first place to use SoftwareSerial. I found Arduino libraries consumed so much memory storage that I couldn't use float or double for variables that were voltage values an accelerometer sends out. I ended up using int variables without casting values into float just using analogRead() function. Even if I used only two integer variables with SoftwareSerial, my Arduino sketch reached to 3800 bytes.

After I powered on the circuit board and connected to my Mac, I found I got only value of 0 or 1023 from the circuit board with bluetooth module, monitoring them at Terminal. This happened because of specification of the accelerometer's output signals. MXD6235M provides two digital outputs that are set to 50% duty cycle at zero g acceleration. The outputs are digital with duty cycles (ratio of pulse width to period) that are proportional to acceleration. This means we need to compute magnitude of accerelation from frequency of HIGH/LOW state switching per unit time. As I mentioned, I had have already realized that 4K internal memory of Attiny44 was not enough to add additional variables and functions. So my only option is programming the microcontroller using C with minimum required libraries and basic functions. I went back to lecture about Input devices, and looked at Neil's sample code. I modified the program and programmed Attiny to send some computed values from the pulse outputs, and turned on three LEDs while the device is on. Surprisingly the program only consumed 596 bytes!! I succefully got magnitude of acceleration using python code to monitor them on my Mac.

Here is a code I used for Attiny44 programming, modifiying sample code

<?xml version="1.0" encoding="utf-8"?> // // hello.accel.44.c // // accelerometer MXD6235MP // 9600 baud bluetooth Serial // #include <avr/io.h> #include <util/delay.h> //#include <SoftwareSerial.h> #define output(directions,pin) (directions |= pin) // set port direction for output #define set(port,pin) (port |= pin) // set port pin #define clear(port,pin) (port &= (~pin)) // clear port pin #define pin_test(pins,pin) (pins & pin) // test for port pin #define bit_test(byte,bit) (byte & (1 << bit)) // test for bit set #define bit_delay_time 102 // bit delay for 9600 with overhead #define bit_delay() _delay_us(bit_delay_time) // RS232 bit delay #define half_bit_delay() _delay_us(bit_delay_time/2) // RS232 half bit delay #define serial_port PORTA #define serial_direction DDRA #define serial_pin_out (1 << PA5) #define in_port PORTA #define in_direction DDRA #define in_pins PINA #define x_pin (1 << PA1) #define y_pin (1 << PA0) #define led_port PORTA #define led_direction DDRA #define led_1 (1 << PA2) #define led_2 (1 << PA3) #define led_3 (1 << PA6) #define NLOOP 3 void put_char(volatile unsigned char *port, unsigned char pin, char txchar) { // // send character in txchar on port pin // assumes line driver (inverts bits) // // start bit // clear(*port,pin); bit_delay(); // // unrolled loop to write data bits // if bit_test(txchar,0) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,1) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,2) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,3) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,4) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,5) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,6) set(*port,pin); else clear(*port,pin); bit_delay(); if bit_test(txchar,7) set(*port,pin); else clear(*port,pin); bit_delay(); // // stop bit // set(*port,pin); bit_delay(); // // char delay // bit_delay(); } int main(void) { // // main // static unsigned char i; static uint16_t xoff,xon,yoff,yon; // // set clock divider to /1 (8 MHz) // CLKPR = (1 << CLKPCE); CLKPR = (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0); // // initialize output pins // set(serial_port, serial_pin_out); output(serial_direction, serial_pin_out); // // initialize led pins DDRA |= _BV(PA2); //Enable output on LED pin 3 DDRA |= _BV(PA3); //Enable output on LED pin 3 DDRA |= _BV(PA6); //Enable output on LED pin 3 // // main loop // while (1) { // // read x (approximate loop timing) // // LEDs ON PORTA |= _BV(PA2); PORTA |= _BV(PA3); PORTA |= _BV(PA6); // xoff = 0; xon = 0; for (i = 0; i < NLOOP; ++i) { // // wait for x high // while (0 == pin_test(in_pins,x_pin)) ; // // wait for x low // while (0 != pin_test(in_pins,x_pin)) ; // // time x low // while (0 == pin_test(in_pins,x_pin)) xoff += 1; // // time x high // while (0 != pin_test(in_pins,x_pin)) xon += 1; } // // read y (approximate loop timing) // yoff = 0; yon = 0; for (i = 0; i < NLOOP; ++i) { // // wait for y high // while (0 == pin_test(in_pins,y_pin)) ; // // wait for y low // while (0 != pin_test(in_pins,y_pin)) ; // // time y low // while (0 == pin_test(in_pins,y_pin)) yoff += 1; // // time y high // while (0 != pin_test(in_pins,y_pin)) yon += 1; } // // send framing // put_char(&serial_port, serial_pin_out, 1); put_char(&serial_port, serial_pin_out, 2); put_char(&serial_port, serial_pin_out, 3); put_char(&serial_port, serial_pin_out, 4); // // send x // put_char(&serial_port, serial_pin_out, (xoff & 255)); put_char(&serial_port, serial_pin_out, ((xoff >> 8) & 255)); put_char(&serial_port, serial_pin_out, (xon & 255)); put_char(&serial_port, serial_pin_out, ((xon >> 8) & 255)); // // send y // put_char(&serial_port, serial_pin_out, (yoff & 255)); put_char(&serial_port, serial_pin_out, ((yoff >> 8) & 255)); put_char(&serial_port, serial_pin_out, (yon & 255)); put_char(&serial_port, serial_pin_out, ((yon >> 8) & 255)); } }

Step 7: Programming Android App: Failed, then triage...sigh

This step is the most challenging part for me. For the week of Network and Application, I created the very best my first mobile App that can turn on/off a LED light on circuit board with bluetooth serial communication. I thought I got familiar with Android App development as a beginner, so I tried to create App that can receive bytes from my circuit board. As I did two weeks ago, I used Android Studio, which is official Android IDE. You can download the package from here. I referred python code for pulse output monitoring that I mentioned above, but had challenges on how to receive bytes and find framing. Due to my lack of knowledge about Serial communication and Java programming, eventually I couldn't debug my code, and I gave up to create it in a given time frame for final presentaiton. I will leave this to my IAP project.

Step 8: Programming GUI on laptop by python

Instead of creating Android App, I decided to code monitoring GUI for laptop trying to realize what I wanted to do on Android phones. Main feature I wanted to realize was giving notification to a user when my device detect abnormal acceleration. What the GUI program needs to do are 1) getting bytes from the circuit board with Bluetooth Serial communication, 2) computing absolute value of acceleration, and 3) giving popup notification to user if the value exceeds threshold. By modifying Neil's sample python code, I achieved #1 quite easily, and computed delta of absolute value of acceleration. #3 was achieved by using tkMessageBox.

Here is GUI code.

<?xml version="1.0" encoding="utf-8"?> # # desktop_thief_detection.44.py # # modified receive and display acceleration # hello.accel.45.py serial_port # # Keiichi Onishi 12/15/14 # (c) Massachusetts Institute of Technology 2014 # # This work may be reproduced, modified, distributed, # performed, and displayed for any purpose. Copyright is # retained and must be preserved. The work is provided # as is; no warranty is provided, and users accept all # liability. # from Tkinter import * import serial import tkMessageBox import math WINDOW = 600 # window size magvalue_old = 0.60 def idle(parent,canvas): global filter, eps, magvalue_old # # idle routine # byte2 = 0 byte3 = 0 byte4 = 0 ser.flush() while 1: # # find framing # byte1 = byte2 byte2 = byte3 byte3 = byte4 byte4 = ord(ser.read()) if ((byte1 == 1) & (byte2 == 2) & (byte3 == 3) & (byte4 == 4)): break xoffl = ord(ser.read()) xoffh= ord(ser.read()) xonl = ord(ser.read()) xonh= ord(ser.read()) yoffl = ord(ser.read()) yoffh= ord(ser.read()) yonl = ord(ser.read()) yonh= ord(ser.read()) xoff = 256*xoffh+ xoffl xon = 256*xonh+ xonl xvalue = xoff/float(xon+xoff) x = int(.2*WINDOW + (.9-.2)*WINDOW*xvalue) canvas.itemconfigure("xtext",text="X %.2f"%xvalue) canvas.coords('xrect1',.2*WINDOW,.05*WINDOW,x,.2*WINDOW) canvas.coords('xrect2',x,.05*WINDOW,.9*WINDOW,.2*WINDOW) yoff = 256*yoffh+ yoffl yon = 256*yonh+ yonl yvalue = yoff/float(yon+yoff) y = int(.2*WINDOW + (.9-.2)*WINDOW*yvalue) canvas.itemconfigure("ytext",text="Y %.2f"%yvalue) canvas.coords('yrect1',.2*WINDOW,.3*WINDOW,y,.45*WINDOW) canvas.coords('yrect2',y,.3*WINDOW,.9*WINDOW,.45*WINDOW) magvalue = math.sqrt(xvalue**2+yvalue**2) mag = int(.2*WINDOW + (.9-.2)*WINDOW*magvalue) canvas.itemconfigure("magtext",text="ACCEL %.2f"%magvalue) canvas.coords('magrect1',.2*WINDOW,.55*WINDOW,y,.7*WINDOW) canvas.coords('magrect2',y,.55*WINDOW,.9*WINDOW,.7*WINDOW) difference_magvalue=math.fabs(magvalue-magvalue_old) magvalue_old = magvalue canvas.itemconfigure("magdeftext",text="Delta %.2f"%difference_magvalue) canvas.coords('magdefrect1',.2*WINDOW,.8*WINDOW,y,.95*WINDOW) canvas.coords('magdefrect2',y,.8*WINDOW,.9*WINDOW,.95*WINDOW) canvas.update() if (difference_magvalue > .03): tkMessageBox.showwarning("Theif Allert", "YOUR LAPTOP IS IN DANGER!") parent.after_idle(idle,parent,canvas) # # # check command line arguments # if (len(sys.argv) != 2): print "command line: hello.accel.45.py serial_port" sys.exit() port = sys.argv[1] # # open serial port # ser = serial.Serial(port,9600) ser.setDTR() # # set up GUI # root = Tk() root.title('hello.accel.45.py (q to exit)') root.bind('q','exit') canvas = Canvas(root, width=WINDOW, height=1.00*WINDOW, background='white') canvas.create_text(.1*WINDOW,.125*WINDOW,text=".33",font=("Helvetica", 24),tags="xtext",fill="#0000b0") canvas.create_rectangle(.2*WINDOW,.05*WINDOW,.3*WINDOW,.2*WINDOW, tags='xrect1', fill='#b00000') canvas.create_rectangle(.3*WINDOW,.05*WINDOW,.9*WINDOW,.2*WINDOW, tags='xrect2', fill='#0000b0') canvas.create_text(.1*WINDOW,.375*WINDOW,text=".33",font=("Helvetica", 24),tags="ytext",fill="#0000b0") canvas.create_rectangle(.2*WINDOW,.3*WINDOW,.3*WINDOW,.45*WINDOW, tags='yrect1', fill='#b00000') canvas.create_rectangle(.3*WINDOW,.3*WINDOW,.9*WINDOW,.45*WINDOW, tags='yrect2', fill='#0000b0') canvas.create_text(.1*WINDOW,.625*WINDOW,text=".33",font=("Helvetica", 24),tags="magtext",fill="#0000b0") canvas.create_rectangle(.2*WINDOW,.55*WINDOW,.3*WINDOW,.7*WINDOW, tags='magrect1', fill='#b00000') canvas.create_rectangle(.3*WINDOW,.55*WINDOW,.9*WINDOW,.7*WINDOW, tags='magrect2', fill='#0000b0') canvas.create_text(.1*WINDOW,.875*WINDOW,text=".33",font=("Helvetica", 24),tags="magdeftext",fill="#0000b0") canvas.create_rectangle(.2*WINDOW,.8*WINDOW,.3*WINDOW,.95*WINDOW, tags='magdefrect1', fill='#b00000') canvas.create_rectangle(.3*WINDOW,.8*WINDOW,.9*WINDOW,.95*WINDOW, tags='magdefrect2', fill='#0000b0') canvas.pack() # # start idle loop # root.after(100,idle,root,canvas) root.mainloop()

Step 9: Assembly

After I completed all development including hardware, electronics, input and output device, network, and application, I assmebled all into one system. Bringing all componets into one was an exciting experience. One thing I forgot to think about was open/close mechanical structure of the case. I couldn't quite close the case due to no locking mechanism. I need to redesign it for the future.

Step 10: Test

I tested the device and it worked properly! I put my device on a laptop and connected to another laptop with bluetooth running python-coded GUI. When I try to remove the laptop, the device senses the movement and successfuly send the information to the laptop and provides notification to me. I would say the system worked well in terms of its function.

What I could do better are 1) adding physical or software switch to turn on/off the device because once I hook up battery to circuit board it would keep consuming electricity, 2) creating Android App sticking to original plan and use case!, 3) making the device smaller, and 4) setting appropriate threshold to detect acceleration of the device eliminating noises. These four items would be definitely my IAP project.