Week 6 - Electronics Design

Feb 28, 2018 6 min read

i’ve created new post because i needed another clean page to include the final pcb of this this week

please check the design work flow page

Schematic Design:

this is the schematic design: echo hsn sch

Component Selection:

MCU attiny44
Resistors:
- R1 this resistor will control the charging speed of the
  capacitor C4 -> 10K Ohm
- R4 this resistor will limit current to prevent LED rush current:
  minimum 330 Ohm
  5v - 1v(accros led) / 330 ohm < 12 mA
- R5 0ohm just jumper
- R6 pullup resistor, this drive the reset high to prevent mcu from reseting
  and limit the current when the programmer short it to ground…
- R7 to control the discharge of C4 when the button unreleased this resistor
  provide path to discharge capacitor, 10k Ohm
- C4 this capacitor will charge and discharge when the button (pressed/released)
  100nF

Pcb design:

this is the final pcb .. for technical details you can view the

#PCB design

pcb

drill 1/32"

Design Considerations and tips

#design-considerations-and-tips

after milling pcb echo v1 new pcb solder

v1 tx - rx

v1 programming

in this post i’ll talk about my experiment in designing the electronics circuit “Echo”
First, the goal of this assignment is to try circuit design from schematic, pcb, and produce the physical circuit

the circuit was orginally designed by Neil: Neil Echo Board

to start with i’ve downloaded Fab library for eagle which contains the available components
the library is installed in eagle/lbr folder

then the schematic is straight forward, its attiny45 microcontroller that is connected to icsp programming pins and to FTDI chip to communicate through usb

i used eagle in order to draw the schematic and the pcb

Schematic Design:

for the schematic circuit design: echo hsn sch

as can be seen in the picture the following components are used:

MCU attiny45
LED -> connected to power to detect the circuit is working
AVRISP header -> through hole package
20 MHz crystal with internal capacitors
FTDI headers -> through hole 6 pins female
PushButton circuit:
the push button is connected to a capacitor which prevents the change in voltage and slow down the signal this is because i want to elimenate the bouncing of the push button

problem 1:

the sch file is connected directly to the board file in eagle software and when i changed some compnent i got the following error: eagle board schematic error

Pcb design:

for me, most important thing in this design is to place the crystal oscillator and decoupling cap near the mcu so i started placing the components and routing unfortunately didnt take picture of the initial circuit design,

i used 3 of 0 Ohm resistors as jumpers because i used small pcb layout … pcb in order to mill the circuit, i ve to export it to PNG file for the fab module..
But Wait …
inorder to export the png file .. i’ve to export the milling part for the ¹⁄₆₄” drill bit here are the steps: click on the layer tool select the wanted layers to show ..
then File -> Export -> Image -> [1000] DPI & [x] Monochrome
dont forget to check design consideration tips error

for cutting the board and drill the holes … we use ¹⁄₃₂” drill bit .. to export that image i remove the top layer .. and export the dimensions and pad layers export holes and then i fill the black area with white in Gimp editor .. gimp edited

Design Considerations and tips

set pad names:

as can be shown below, when i export the pcb as png ..
it shows the pad numbers that have holes… pad names shown this can be problem as it could be detected as path and milled ..
in order to solve that Gimp can be used to edit the image and fill the hole
however, by researching in eagle there is setting to hide them ..

- Type “help” in eagle commands bar

this code solves the problem :
Set Pad_names off;

this is good for laptop mouse-pads or mouses without middle button:

Interface.UseCtrlForPanning

set Interface.UseCtrlForPanning 1
Panning is done by moving the mouse while holding the center mouse button (or mouse wheel) down. In older versions this was done by pressing the Ctrl key instead. If you want the old functionality back, you can set this parameter to ‘1’. Note, though, that the Ctrl key is now used for special functions in some commands, so when using these special functions (like selecting an object at its origin in MOVE) with this parameter enabled you may inadvertently pan your draw window.

DRC

for the (¹⁄₆₄” = 15.6mil) drill bit:
so i use the following parameters:
- 16 mil clearance
- 15 mil min width for track widths
- thermal pads 15 mil
- isolate for polygon: well, when you rastnest the polygon will pour the empty area, you can right click and select isolation for the polygon so it leaves some clearance with tracks and pads …

use ground plane

its a good practice to use ground plane…
there are many reasons for that i’ll not talk about it here but you can read for more details:
Wiki Ground plane
The EMI benefits of ground plane
PCB Design Guidelines

Manifacturing:

drill bit problem: there was a problem, the bit was broken and the MDX-15 head had vibration, this means that when it mill the tracks it will vibrate on the small track width and make unclean cuts .. i didnt take picture of the old PCB, however, i re-milled the PCB again

the pcb was opened in fab modules in milled in the milling machine we have…