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.
- 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)
- 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
- 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)
- 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: