Week 10

Input Device: Sensing Sound

PCB Design Software: Eagle
Machine: MODELA MDX-20 (Software: Fab modules)

This week’s assignment was to make an input device. For my final project I want to detect a person’s sneeze, so I decided to use a condensor mic.

Components and Tools

  • Scraper x 1
  • Digital Multimeter x 1
  • Soldering Iron x 1
  • Solder x 1
  • Desoldering Braid x 1
  • Tweezers x 1
  • FabISP x 1
  • Mini USB Cable x 1
  • 6 Wire Ribbon Cable with Sockets x 1
  • FTDI Cable x 1 (秋月)

Designing The Board

I designed the circuit based on Neil’s board. The image below is the Schematic.
  • Since the microphone produces small signals, one amplifier is applied to the circuit to amplify the signal.
  • R2 is a pull up resistor for the mic. The data sheet recommends a 1.0k ohm for 1.5V. In my circuit I used 10k ohm.
  • C2 is a capacitor that blocks DC voltage on the input signal and allows AC to pass through.
  • R5 and R6 resistors are for stabilising the output.
For this board I created a ground plane. Draw a polygon (on the Top layer), name it GND,
Then do ratsnest. The plane will turn red and connect all of the GNDs as shown in the image below.
(This red GND plane can be erased with ripup)

Making The Board

Some parts of the board were not milled so I cut those parts by hand with a cutter. (Later on I found out that I should have changed the tool diameter setting in Fab Modules from 0.4 to 0.3 to mill all of the narrow traces.)
After I'd finished milling the board, I noticed that a part of the GND plane was not connected. I should have noticed this when I used the command show gnd but I'd overlooked it. So I soldered a wire to bridge that section.

Programming The Board

I followed this tutorial to program my board. Since I'm using Mac, I downloaded Pyserial and installed it by typing in terminal...
  • cd the directory to the Pyserial file
  • sudo python setup.py install
  • Then, connect the mic board to the FabISP.
Download the c file, make file, and python file (right click link and Save link as…) and move them to a folder. To flash the micro-controller, type in terminal...
  • cd the directory to the folder
  • make -f hello.mic.45.make program-usbtiny
  • Type in ls /dev/tty.usb* to find the serial port you are using. Mine was FTGCKCJC.
    Now type in the following to run the python program.
  • python hello.mic.45.py /dev/tty.usbserial-FTGCKCJC