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RDB Build Documentation
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RDB Reprap Development Board

Release status: experimental

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Generic reprap extension for generic development board
CAD Models
External Link

This board is meant to take GPIO lines (wires,cables,connectors etc) from *any* development board (3.3v or 5v) and provide the support components required for basic operation of a 3d printer or cnc machine.

  • Pcb size: 160x100mm
  • Voltage regulator: 3, fixed
  • Support components for functions: 5 pololu drivers, 4 thermistors, 3 endstops, and 4 switches (low side mosfets with dedicated drivers and optoisolators).
  • optimized for DIY: single sided, no bridges, large clearance 0.5mm, min track width 1.5mm, format and traces "bulky" enough so board could be made with any diy technique.

Power input 1, for steppers, dev board, logic level

  • this connector supplies steppers, has a voltage regulator output meant to supply the dev board itself, and another voltage regulator providing the local logic level voltage
  • input simple reverse polarity protection with high current diode and fuse
  • input simple peak voltage burner with zenner and transistor meant to flat off peaks made by steppers switching or bemf
  • 12v-32v input voltage, 24v recommended (max input voltage depends on the burner zenner voltage, regulators inputs and stepper drivers max input)

Power input 2, for low side mosfets

  • this connector supplies the low side mosfets (switches)
  • input simple reverse polarity protection with high current diode and fuse
  • this can be supplied from a different psu, other than first one, as long as optoisolators ICs are equipped (to provide galvanic isolation)
  • 12v-32v input voltage, 12v recommended for existing heaters compliance (max input voltage depends on hexfet breakdown Vbr, regulators inputs and most likely the resistive loads/heaters used and the relevant wattage calculations)


  • Outputs from two voltage regulators
    • Output for development board supply
    • Output of logic level
  • Steppers
    • This board is meant to support pololu carrier stepper drivers, currently using A4988 or DRV8825 stepper driver ICs. Further documentation on pololu carrier drivers at pololu website and Pololu_stepper_driver_board and other places
    • Each driver has a logic input connector with Step/Dir/Enable, and outputs with (bidirectional) leds indicators.
    • Stepper inputs truth table: the board has a pullup resistor on enable line. In case of a4988 outputs will be default off and EN enable line needs to be actively driven low to enable output. This was considered a safer approach. Step and direction lines have no changes on rdb, these are just passing through, so their status is as they are on the carrier boards.
  • Thermistors
  • Endstops
  • Switches (vdc, low side mosfets)
    • Switches are driven by mosfet drivers which in their turn are driven by optoisolators ICs. This provides strongest drive and offers flexibility, in that the pheripheral loads can be driven with different psu.
    • Each mosfet connector provide directly both V+ (individually fused) and switched gnd. Truth table: by default (undriven) outputs are off, and when input pin is low outputs are off. Only when the input pin is driven high then the mosfet gnd is active.
    • Switches are low side mosfets and are identical, so any switch can control any type of load, hotend, bed, fan, in any combinations. Highest consummers (heated bed) should be closest to the input connector (to minimize track length).


Soldering tips and tricks:

  • ensuring consistent alignment of the carrier headers: if headers are soldered individually would end up having different angles, and later on it will be hard (mechanically) to switch carriers from one header to another; two 40 pins rows of male '100 pins can be used to align all carriers, and 3 carriers can be soldered on these, while ensuring 90 degrees angles and that all headers align to each other.
  • ensuring the bus bars soldering does not warp the pcb (too much): when soldering the bus bars get heated (expand in length) and later on when cooling down they shrink and will tend to warp the pcb; to avoid this, either solder the bus bars with the pcb bent in the opposite direction, or try create sharp bends in the bus bars to relieve the dilatation effect when it takes place; also use just 1mm2 solid conductor, exagerating its size makes the warping worse.
  • if using the pictured type of fuse holders with separate headers, before soldering put a fuse in them just to ensure their position does align with each other and the fuse has good contact on both sides; the pictured glass fuses are there for the sole reason to align the headers before soldering.
  • after headers and bus bars, soldering priority goes first to all smd components on the back and then to all other through hole components which are not specially sensitive; the through hole ICs should be soldered last;
  • on the other hand the big through hole electrolitic capacitors can be soldered at any time: because this board does not have any soldering on the front then big components dont restrict iron access to anything.


Most of the board can be tested without connecting any pheripherals except power inputs. The board has a logic level regulator onboard which can provide the logic level for testing its own inputs. From the logic level output connector, take a red and a black wire, and solder in series ~270ohms resistors for the purpose to limit the current in case of accidental short circuits. With these (resistance-limited) wires you can test the board logic level inputs, accordingly to each input truth table.

  • Test conditions:
    • all components are equipped and all soldering operations are completed (do not attempt to test parts of the board before completion)
    • power supplies connected to both power input 1 and power input 2
    • the green leds (power indicators) should be light up
    • nothing else connected
  • testing steppers: the output leds will be off, when the en (enable) input is changed, then output leds should light up; normally this test should be enough to conclude that the stepper carrier is working correctly; further tests can be done similarly with stp and dir inputs, but its not recommended and most likely not needed.
  • testing switches: same thing as above, changing the state of the input switches input pin according to the truth table, should make each mosfet led light up


  • remember to set the microstepping level using solder bridges (solder "jumpers") on the back of the pcb.
  • for stepper driver outputs indicators one could use 4 normal leds instead of 2 bidirectional ones, but these have to be the 1mm type which are very slim and narrow (in picture top right carrier header, the 4 yellow leds)
  • no need to use '100 headers for board inputs like stepper stp/dir/en or any other inputs aswell, one can safely solder the digital wires in place of connectors and that would probably be better anyway.
  • in case the power input 2 is supplied from the same psu as power input 1, then the optoisolators ICs can be bypassed (in pictures top DIP8 package), and one could avoid purchasing them in the first place. Just solder the digital line that comes from dev board to the input of the fet drivers. Or even more in this case plus the condition that the logic voltage is 5v (not 3.3v boards), then the fet drivers can also be bypassed alltogether, and the digital line can go soldered to the fet driver output which goes to the mosfet gate pin directly. Just a note, except for purpose of lowering cost, there is no other motivation to bypass these. Bypassing does not bring any benefit, so it is not recommended to bypass these components, but however it is possible under those conditions. Again, cost reasons only. Both optocouplers and mosfet drivers use positive logic and/or are non inverting, which makes the above possible.
  • logic level 3.3v or 5v: the board is actually meant to work with 3.3v logic level, all it requires is using a 3.3v regulator in place of the 5v one (in datasheet is 7805: 5v just because it has to be written something there one way or the other). The step/dir/en can be at 3.3v logic voltage as long as its supported by the carrier drivers. Optocouplers also can work with either. If optocouplers get bypassed, then mosfet drivers inputs same thing, can work with either.
  • board can be physically split in two along the middle holes, for whatever reason, but it is required to supply vcc-gnd from the stepper side to the thermistors/endstops/optos input. This goes from the vcc-gnd output of the steppers side to the vcc-gnd input near the endstops/thermistors side (otherwise this connector should remain unused).

Source files

Note: files should be opened with a kicad version equal or higher than the version used to make the sources.

Name Size (mm) Comments Layout pdf Schematic pdf Kicad zip Kicad version Date
RDB-STPTS-002-DIY 160*100 ~ [1]pcb [2]sch [3] github bzr-5139 2014 Sept