Electronics
I based my electronics on a existing commercial board called USB2AX. You can find all the information about it here. It was basically done with the request to be as small as possible to use in robotics projects, together with Dynamixel motors. The protocol used is half-duplex TTl (3 pins connection).
What I intended to do is to take this board as a reference and try to simplify it using the component on the Fab inventory, so that everyone in the FabLab environment could replicate it and improve it.
To do so I used the eagle file from the original file (everything is OpenSource, so it was posted in the website) and analysed which components might be taken off and which not.
Bill Of Materials:
1 X ATMEGA32U2-MU
1 X CRYSTAL-SMD-3.2×2.5 16MhZ
1 X USB Type A Plug through hole
1 X Dynamixel Connector Right Angle (SPOX2.5-3WAY)
1 X 4.7µF USB decoupling capacitor
2 X 0.1µF ATmega decoupling capacitors
1 X 0.1µF Capacitor for UCAP
2 X 18pF Load capacitor for quartz
1 X 22R USB line resistor array
2 X 150R DATA line protection and current limiting resistor array for LEDs
1 X 100R PTC resistor for DATA overvoltage protection
1 X 5.1V SOD-923 Zener diode for DATA overvoltage protection
1 X Green LED
1 X Red LED
1 X IP4282CZ6,115 ESD protection
1 X Heat-shrink tubing (diam:12,7mm)
I then used the old schematic to start from scratch positioning the new components, placing them and routing them.
I contacted Nicolas Saugnier, who is the creator of USB2AX, and advised me on which components migh be critical to remove. Somehow the Zener Diode and the Crystal seemed to be the most delicate components. I then changed the rest for Faba components, and reduce the security system near the USB plug (nowadays all the computers have their own security systems to avoid over voltage). This is the final schematic.
And this is the Bill Of Materials:
1 X ATMEGA32U2-AU
1 X CRYSTAL 16MhZ 8pf SMD
1 X USB MiniB
1 X PIN HEADER 1X3/90
1 X 10µF C1206FAB capacitor
3 X 1µF C1206FAB capacitors
2 X 8pF C1206FAB capacitors
1 X Diode Zener 5.1V SOD123
1 X Green LED
1 X Red LED
2 X 22R R1206FAB resistors
3 X 150R R1206FAB resistors
1 X 100R R1206FAB resistor
1 X 0R R1206FAB resistor
As space wasn't really an issue for me in this project I prioritized that the board could be milled with a single 1/64 mill bit, the standard mill used in all FabLabs.
(download eagle board here) (download eagle schematics here)
Here you have the final png file which I converted with FabModules to mill it.
(download here)
(download here)
pass: fab
This is the PCB already milled, I had to mill it twice as the first attempt didn't remove the copper all the way around and wasn't working properly.
I then soldered all the components, being really patient and accurate, specially with ATMEGAU32U microcontroller. It took me about an hour to solder everything but I was quite happy with the overall result
