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Having designed 3 of my 3D printers, 2 of which use 5 volt 28BYJ-48 steppers , there is one thing that does not add up in the design of ToyRep that has bothered me since I first read about ToyRep, and that is the Bipolar hack.

If you use the 28BYJ-48 stepper motors as unipolar stepper motors they produce more torque than if modified for bipolar operation. Thus I ask why does ToyRep use drivers designed for Bipolar steppers, and why do the bipolar hack?

Both of the printers of mine that use the 28BYJ-48 steppers use them as unipolar steppers. When you write the firmware for the MCU of choice control the the printer (interpret the G-Code, read an SD-card, com with computer, and drive the printer) it is easy to assign the needed pins to drive unipolar steppers with ULN2803's or ULN2003's, as the current drivers. As such it makes no sense to use bipolar drivers when using the 28BYJ-48 stepper, it is much better to use ULN2803/ULN2003 drivers with an MCU that has at least the 16-GPIO's left over to run the 4 sets of steppers (X, Y, Z, E), so at least 20 GPIO's without a heated bed, and at least 23 GPIO's with a heated bed.

Also why use a Arduino based controller? Is it just to increase the price? Just an ATMEGA + Crystal will do the job for under $2 for the MCU, the ULN2803 drivers cost about $1 each and each will drive 2 sets of steppers, thus needing only 3 of them total (one for X,Y, one for Z,E, and one for PWM to control the hotend temp and to power the fans), then some simple passives to do the rest of the interfacing, add a simple board to wire everything up and it costs a total of about $7 including SD-Card socket to make a good working controller board. Even if you chose a Parallax P8X32A Propeller MCU it is still lower cost than an Arduino.