Building a Repstrap(7) : final electronics/motherboard

Since i was satisfied with the way the control of the repstrap was working with the temporary electronics setup (on protoboards), i started work on the permanent solution, using standalone AtMega328 (the chip powering the more recent version of Arduino boards) and a lot of electronics equipment i already had. Here is the general overview of the requirements and  what i came up with.

Requirements:

• versatile : needs to handle not only the standard reprap electronic elements (stepper control, opto end stops, extruder and build platform heating and temperature control etc), but also have spare i/o ports for the upcoming 3d laser scanning elements and more
• support the Pololu stepper drivers
• make the machine as standalone as possible : this meant a frontal control panel with lcd display, and sd card reader
• as cheap as i could (using equipment i already had or that was easy , and cheap to obtain)

The result:

1. since a single AtMega328 , even with port extensions would have been pushing things a bit on the speed side (as the stepper drivers require very fast operation , in the microsecond range for example) i decided to use two chips , communicating via  I2c
2. one AtMega controls the steppers exclusively, while the other "master" chip is in charge of all the rest (no problem since all other tasks are a lot less time critical)
3. The master chip is connected to:

• • a 74hc4094 shift register to control a group of 4051 muxers/demuxer for a total of 32 additional i/o ports (up to 64 available)
    • a second, chained 4094 shift register controls the front 2X16 LCD
    • an addtitional 4051 that checks for the states of a 4X4 button matrix
    • 3 TIp120 transistors to control the extuder head and heated build platform heaters
    • the completely diy SD card reader (using the SPI ports)

The end result :

Yes, it is quite complex,  and requires a custom firmware (currently being coded), but for now the results are encouraging , and its very cheap at around 30/40 euros (not including the stepper drivers of course)!

For reference : here is what the protoboard version looked like : VERY MESSY!

I will post the detailed schematics and additional information, as well as the firmware as soon as all this is finished and tested.

And some additional pictures: