MakerGear Mendel Prusa electronics assembly notes

This post is intended as an additional commentary on the “Differences from Stock Prusa” file initially by Brock Tice. Please don’t try to connect your electronics without also reading Brock’s file.

I’ve tried to ensure that vital actions are in bold face, so they stand out in the sea of description, but read it all. Maybe a piece of description may make you realise you’ve misunderstood an instruction. Or maybe you’ll find a mistake, or a difference between my kit and yours. This is the area of the build where it is easiest to trash expensive components. Please do post a comment if you can help make this information better.

This information applies to the MakerGear metric Mendel Prusa as shipped in October 2011. Other variants may be different. Engage brain before cutting or applying voltage.

Table of Contents

  • 1.0 Power supply wiring
  • 2.0 Setting up the RAMPS system
  • 3.0 Installing endStops
  • 4.0 Heated build platform
  • 5.0 Stepper motor connections
  • 6.0 Wiring the thermistor and nozzle heater

1.0 Power supply wiring

My MakerGear kit was shipped in October 2011. The power supply included was an Eagle 600W ATX Power Supply. I have no experience with other PSUs, so my comments may only apply to the Eagle.

Unplug your PSU from the mains before starting.

(PSU stands for Power Supply Unit, and refers in this case to the whole Eagle ATX box.)

1.1 Set correct voltage

These PSUs are built to work in countries all over the world, with different national power systems. Unlike laptop power bricks, the ATX PSU can’t work out your local voltage for itself. The US uses 110 volts at 60 Hz. Australia (where I am) used to use 240 volts at 50 Hz, but the standard has changed to 230 volts, still at 50 Hz (not that the difference usually matters). I think the UK also uses 240 or 230 volts. If you aren’t sure what your local voltage is, look for a data plate on the back of  some of the bigger electrical items around you (fridges, TVs, Stereos).

Important: Set the red switch on the side opposite the mains power switch so it shows the voltage your country uses.

1.2 Identify the wires that matter

The 600W Eagle ATX power supply has a lot more connections than we need, and I cannot find a connection diagram on the web. It is said (by people who may have had the exact same model as me, or not) to have two separate 12V supplies, one which supplies the little square 4-pin connector, and the other which supplies 12V on all the other yellow wires on all the wiring harnesses.

Important: The yellow leads from the PSU supply carry 12V, the black are ground (or zero volts), and the red are 5V (which we don’t need at all).

1.3 Trick your power supply into turning on

The ATX power supplies are designed for computers. The computers provide a special signal to say “I’m awake” or “I’ve shut down now”, and we have to fake that. Luckily its easy.

The largest of the connectors on the PSU wiring harness is at the end of the cable covered in black woven braid. The MakerGear kit includes a matching white socket that this big plug fits into. Two adjacent pins on this socket need to be soldered together.

1.3.1 Find the two magic pins

Look at the photo and description in the power supply section of the ‘Differences’ file, and look at the white socket. Plug the socket onto the connector on the PSU wiring harness.

On my socket the MakerGear folks had marked the two pins with a green pen. Check that one of the marked pins lines up with the green wire on the wiring harness, and the other lines up with an adjacent black wire.  Check that against the photo in the ‘Differences’ file (anyone can have an off day…).

1.3.2 Solder the jumper wire onto the socket

If the photo and the wire colour (and the pen marks if they are present) agree with each other, take the socket off the plug and solder a piece of wire between the two marked connectors on the socket.

Triffid_Hunter tells me this is only a logic level signal, so it doesn’t need a heavy piece of wire. Regular hook-up wire will do.

(I’m not giving a soldering tutorial here. My techniques are probably 20 years out of date anyway. If you’ve never soldered, please go do some tutorials. Joint quality matters – poor quality joints heat up when large currents go through them, as well as causing other hard-to-diagnose faults. Adafruit has great free tutorials, and sells practice kits and soldering irons and everything else you need as an electronics hobbyist. Or read Adafruit’s tutorials and buy the bits from your local electronics store. I hear Radio Shack even sells Arduinos now.)

With a multimeter on resistance range test that there are no shorts between the pins you soldered and any of the ones you didn’t mean to solder.

1.2 Test that the power supply works

Plug the power supply into the mains, and switch on. If  the fan spins, it works.

Turn the PSU off again. Then unplug it as well. There are parts of the power supply that are not controlled by its mains switch. That means that any time the PSU is plugged into the mains there is voltage on some of the pins on the big connector. We protect ourselves from nasty surprises by wrapping the back of the socket (where the bare pins are, and where you just soldered onto) with electrical insulation tape.

Wrap the connector in insulating tape (I used self-amalgamating tape because I like it) to guard against accidental short-circuits.

So now you have a PSU that works when you plug it in. I bet you thought that was one thing you could rely on without soldering, but hey, you’re a maker – its your job to pervert the uses of technology!

1.3 Build the power supply cables for your RAMPS board

1.3.1 Cut your power supply cables to length

Find your 7 foot length of thick black twin-core cable.

Cut it in half. Put one of the pieces away for building the heated bed, much later on. (I haven’t built mine yet, I don’t seem to need it yet for the PLA I’m printing with.)

Cut the remaining half in half again. You now have one roughly 1 metre length put away with your heat-bed parts, and two roughly 500mm lengths for your RAMPS. One of these 500mm lengths will connect between the high current plug on the PSU and (eventually) the 11A connector on the RAMPS board. This is the cable to use for heat curing the heat-core ceramic. The other 500mm length of cable will go to the 5A connector on the RAMPS board.

1.3.2 Mark your connectors with polarity

Find the small square white 4-pin socket which came with your kit. Plug it in to the matching square 4-pin plug on the PSU wiring harness. There are only one socket and and one plug matching this description. Really plug it in, so you know you’ve got the alignment right – they are keyed so you can’t connect them the wrong way round.

Now looking at the wires on the harness end, two are yellow (12V), and two are black (Ground, 0V). You need one of the wires from your black twin-core cable soldered to a pin that lines up with a yellow wire, and the other to one that lines up with a black wire. Use a DVD marker pen to mark the white socket with a ‘+’ where the yellow wire leads to, and ‘0’ where the black leads to. (It might work to connect both the yellow pins together, and both the black pins together for extra current capacity, but I haven’t tested that. I just picked one yellow and the black nearest it).

That’s the square high-current socket marked.

Now find the black rectangular 4-pin socket with 2 slightly rounded corners (its called a Molex socket, you can Google that for an image if you can’t recognise it).  Plug it in to any of the many matching plugs on the PSU wiring harness. Again, it is keyed so it only fits on one way round.

This time, looking at the wires on the harness end, you’ve got two blacks (0V) in the middle, yellow (12V) at one end, and red (5V) at the other. With something like a paint marker or correction fluid put a + mark next to the end that has the yellow wire going to it. The pin next to that mark will be your 12V pin. Really it doesn’t matter which of the two middle 0V pins you use, but be neat and pick the one nearest the 12V pin. The photo in the ‘Differences’ file uses the other 0V pin, maybe for extra soldering clearance.

You’ve now marked both the power supply sockets.

1.3.3 Identify the +ive side of your power supply cable

Look closely at your thick black twin-core cable. You’ll see one wire has embossed writing on it, and the other has hard-to-describe lengthwise ribbing. I use the ribbing side as +12V and the other side as 0V. Make a choice and stick to it throughout your build. Getting the ends of your power cables mixed up will trash your electronics in a major – possibly fire and smoke – way.

1.3.4 Prepare for soldering

Ideally you now want to separate the twin-core wires for roughly 75mm at one end. Use a knife or a pair of side-cutters to carefully cut between the two wires for 10mm or so, then pull the ends apart with your fingers and slide a half-inch piece of the green heat shrink tubing over each wire, so you can slide it back over the joint when you’ve done soldering.

You can look at the end result in the last photo in the ‘Differences’ file. Keep the heat shrink far away from the joints as you solder them, or the heat of the soldering will shrink them so much you can’t slide them back down over the soldered joints. Which is not a disaster, it just means you will have to wrap these connectors in insulating tape too.

Strip about 10mm of insulation from both ends of both your pieces of twin-core. Twist the strands so they don’t go all frizzy and hard to solder.

1.3.5 Soldering the pins on each socket

For each of the two sockets you’ve just marked with polarity:

Unplug the socket from the PSU harness and stick it in your third hand, or desk vice or whatever.

Tin the ends of your wires.

Solder the +ive side of your twin-core to the + pin on the connector, and the 0V side to the 0V pin. It’s tricky because these connectors aren’t designed for soldering (tech perversion again). Use a multimeter to confirm there is no contact between the + and 0 pins (that could be very bad at 11 Amps, though it is more likely that the PSU protection circuit would kick in and make you think the PSU was dead).

Check that there is conduction between the pins on the socket and the corresponding wire at the other end of the twin-core.

1.4 Test your power supply cables

1.4.1 Plug the bare ends into a terminal block

Unplug your power supply sockets from the PSU’s wiring harness. (Just checking, they shouldn’t have been plugged in anyway, if you were following in sequence.)

Find the long white plastic terminal block with lots of pairs of shiny screws in two rows. Each of the screws on one side of this block connects to the screw directly opposite it, and not to any of the others. You can check that out with your multimeter’s resistance range if it sounds confusing. You cut this terminal block up later for wiring your stepper motors, but right now we will use it to temporarily connect the PSU wires to somewhere safe for testing. And to make curing the heat-core easier.

Feed the stripped ends of one of your recently soldered black power supply cables into any two adjacent holes in the terminal block and tighten the screws to lock the wires in place.

Then feed the stripped ends of the other cable into a different pair of adjacent holes on the same side as the first pair. At this point none of the wires should be electrically connected to any of the others.

Do the screws up firmly. Confirm with your multimeter’s resistance range that none of the wires are connected to each other. Confirm that the wires can’t accidentally pull out.

1.4.2 Attach cables to power supply

Plug the sockets into the PSU wiring loom. It doesn’t matter which of the rectangular Molex connectors you use, and the socket won’t connect to the wrong sort of plug.

1.4.3 Measuring voltages

Plug in and turn on the PSU.

The fan should come on.

If the fan doesn’t come on (and it did before you plugged in your newly soldered sockets), you’ve probably a) soldered a short between two or more pins, or b) you’ve screwed two wires into connected holes on the terminal block. If this happens, switch off the power supply, unplug the sockets from the power supply harness, and troubleshoot with your multimeter.

With the fan now running, use your multimeter on its voltage range to measure the voltages on the screws of the terminal block. I got a reading of 11.65V rather than 12V, so I guess PCs aren’t fussy about the accuracy of their 12V lines. This works fine.

If you get zero volts between the two ends of one of your wires, you’ve failed to connect something, or you’ve connected both bits of wire to the same voltage (say, both to black pins, or both to yellow pins).

If you get five volts between the two ends of one of your wires, you’ve probably soldered the + lead of the Molex socket to the red (5V) end instead of to the yellow (12V) end.

If you don’t have something approximating 12V on both pairs of wires at this point, you’re going to have to troubleshoot your connectors. So write down the voltages you’ve got on which wires before switching off your PSU again. Go troubleshoot.

When you do have something approximating 12V on both pairs of wires, switch off your PSU again, then unplug the power supply sockets from the PSU wiring harness.

1.4.4 Insulating the joints

Insulate the soldered joints with the heat shrink tubes you installed earlier, or by wrapping them with insulation tape if you can’t move the heat shrink. If you’ve never used heat shrink before, please look up some advice on the internet. I used a small hot-air blower intended for setting high-temperature embossing compound for rubber stamp crafting, but there are other, more conventional, ways to do it. I used to use the barrel of my soldering iron.

You now have two working 12V power cables with bare wires plugged into the white terminal block. This is more than enough to heat-cure your heat-core ceramic.

1.4.5 Power Supply Strangeness

I had a problem, which I have written about elsewhere. In summary,

When you are heat-curing the heat-core, only plug one of your two wonderful new black power cables into the PSU’s harness. Leave the other cable completely disconnected from the PSU. I know it seems like it shouldn’t make any difference, but for me, the PSU wouldn’t run with both sockets plugged in if the ends of one of them were unconnected to anything.

The problem goes away completely when you have both cables wired up to the RAMPS board, its only an issue when we leave one cable’s end unconnected. Its probably a very clever protection circuit designed to shut the PSU down if wires come loose.

1.5 Using the PSU for heat-curing the heat core ceramic

This step happens part-way through the construction of the MakerGear extruder head.

1.5.1 Connecting the heat-core ends of the power cable

Plug the square white 12V connector into the PSU wiring harness.

Turn on the PSU. The fan should spin. Use your multimeter (on voltage setting this time) to check that you’ve got 12V (or eleven point something) on the screws of your terminal block. Yay! This is enough to cure your heat-core. Yes, it seems I’ve got you plugging and unplugging unnecessarily, and I do. But if any step fails it really helps to know that it was something you’ve just done, not an old problem you hadn’t noticed yet. Its standard troubleshooting behaviour. Turn off the PSU again.

Put the pins from your heat-core wires into the holes in the screw-terminals opposite the ends of your black twin-core cable. Gently tighten the screws til the pins are held firmly but not squished. You don’t want to deform them or they won’t fit into the connector they are destined for.

1.5.2 Start heat-curing

Here is the procedure (repeat til the new ceramic matches the old ceramic’s colour):

  1. Start your stopwatch
  2. Turn on power
  3. Wait two minutes
  4. Turn off power
  5. Let cool for 10 minutes.

When I started the curing process I measured the voltage on the white screw terminals going to the heat-core. There was voltage, and it had dropped from 11.65V to, I think, about 10.5 V. This was a good sign because it meant the heat-core was soaking up current. After a minute or so I could feel the air getting warmer and smell a plastic odour. When the stopwatch reached two minutes, I turned off the power supply. Yay. Iteration one completed.

After about 10 minutes I gave it another 2 minute burst. Repeat til the new ceramic you applied looks the same colour as the original ceramic on the heat-core. For me, I think 4 heating cycles got the colour matched.

Now you’ve finished the heat-curing, turn off the PSU.

Unscrew the heat-core from the terminal block.

1.6 Attach RAMPS plugs to power supply cables

1.6.1 Mark the polarity and current on the plugs

Find the strange green two-pin plugs that fit into the RAMPS 11A and 5A sockets on the RAMPS board (the plugs are the same colour as the sockets).

Insert both the plugs into the sockets, and look at the writing on the PC board next to the sockets. The size of the huge + and – signs is a hint that you need to get this right! On my board, looking down from above with the sockets nearest me, + is on the left and – is on the right. The left-most socket is 11A, the rightmost is 5A.

Label the tops of the plugs (11A, +, -) and (5A, + -) so you’ll never get them mixed up.

Remove the plugs you’ve just labelled from the RAMPS board.

1.6.2 Insert wires in plugs

Unscrew both the black power supply cables from the big screw-terminal block.

Plug the wire from the square white power socket into the back of the plug that you’ve just labelled (11A + -). Remember which side of the cable you chose as +. The other side is 0V (or -, compared to the +. We don’t have any true negative voltages in this system, and people familiar with electronics often use ground, GND, 0V and – interchangeably. And sometimes even ‘Earth’, but that’s really old-fashioned)

Now you’ve got half of your power supply needs wired up.

Plug the wire from the black rectangular Molex power socket into back of the  plug that you’ve just labelled (5A + –). Again, check polarity. The Molex connector can supply enough current for the stepper motors and extruder head, but not enough for the heat-bed. The heat-bed will connect to the square white connector, which can supply 11 amps.

Now you’ve wired up the power plugs. Tighten the screws so the wires can’t pull out of the plugs.

1.6.3 Test voltage and polarity on the plugs

With the green plugs not inserted into the RAMPS board, turn on the PSU again and do a last paranoid check that you’ve got the right voltages, and they are the right way round (matching your labels). That is, use your multimeter’s voltage range to check that each plug has approximately 12V on it, and check the polarity matches the labels.

Then turn off the PSU again, and leave the green plugs unconnected to the RAMPS until you are ready to test your extruder or motors.

You now have a working power supply to drive your printer. Brock explained that in about a page and two photographs. I think he’s smarter than me.

2.0 Setting up the RAMPS system

2.1 Soldering up a RAMPS from little tiny bits (or not)

Brock’s ‘Differences’ file has half a page of tips on this, and a pointer to a more detailed set of instructions. I paid the extra $50 for MakerGear’s fully assembled RAMPS v1.4 option. Well worth the money. So I have no comments of any help to people building their own.

Confirming Brock’s comments, I didn’t use ‘the diode’ (of which there are two in the kit, in a tiny plastic bag), because I’m happy running the RAMPS Arduino off the USB connection from my laptop. At least until I build an SD card reader add-on, which is way down my list of fun things to do.

The assembled RAMPS has its jumpers set for 1/16th micro-stepping, and the jumpers are hidden under the tiny Pololu driver boards, so don’t try to change them. Flashing the RAMPS with Sprinter (which defaults to 1/16th steps, and is more recent than Tonokip anyway) is much easier and less prone to physical damage than unplugging and replugging tiny circuit boards. And you have to learn how to flash the driver software anyway before you can print anything, because you have to change some default values to match your printer.

2.2 Attaching the RAMPS board to the Arduino board

The RAMPS board and the Arduino board are sourced from different companies. Mine came in separate anti-static bags. Don’t shuffle your plastic-soled shoes on carpet while touching these boards. If your air is really dry it’s a good idea to touch something large and metal (like a filing cabinet or your beer fridge Winking smile ) if it is in reach from your desk, or use an anti-static wrist strap. Electronics are more static-resistant now than 20 years ago, but if you are getting static zaps from doorknobs you need to take precautions.

2.2.1 Attaching the Arduino to its wooden baseplate

Find the laser cut wooden plate with rectangular notches cut out of the top edge. This is the baseplate that all the electronics are attached to. Find the four small white hollow plastic cylinders (spacers) that will hold the Arduino up off the baseplate.

Put a small hex-headed bolt into each of the mounting holes near the corners of the Arduino (don’t use washers), pass them through the plastic spacers and through the holes in the plywood base, then fasten them with a washer and nut. Don’t over tighten or you risk cracking the Arduino circuit board.

2.2.2 Squeezing the RAMPS board onto the Arduino

When you’ve got both anti-static bags open, you’ll see that the Arduino has a rim of black plastic sockets around the top of the board on three sides. Looking at the RAMPS you’ll see a corresponding set of pins pointing downwards on three sides.

You have to carefully line up all the downward pointing pins on the RAMPS with the sockets on the Arduino. They are fragile – don’t bend them. If you do bend them, straighten them very carefully with a small pair of pliers before trying again.

Put the Arduino on it’s baseplate on the bench top. Align the RAMPS pins. Gently apply pressure until the tips of all the pins are in the right sockets, and the two boards are parallel to each other. Then gently squeeze the boards together by their edges a little at a time, working your way around the edges until the RAMPS board is touching the Arduino sockets on all three sides. Don’t press on the little boards on top of the RAMPS, squeeze the boards together by their edges, directly over the sockets and pins. Yes, it may hurt a bit, the tops of those metal pins are a bit sharp.

Now that’s what I call assembly – press fit FTW!

2.3 Attaching the RAMPS board to the printer’s frame

With the major electronics unit together, you can attach it to the printer’s frame. If you look at lots of photos of MakerGear Prusas you’ll see one edge of the electronics baseboard lies on top of one of the lower front-to-back threaded rods. The edge with the two rectangular cut-outs neatly fits between the threaded z-rod (not quite touching the inside of the bearing, but supported by the outside of it) and the bolts that hold the Z-support rod centred. The end of the RAMPS with the power supply and USB connectors faces away from the Z-rods. <insert explanatory photo here>.

Once you’ve worked out where it goes, fix the baseboard in place with cable ties through the laser cut slots and around the threaded rods. Cable ties are cheap, but they are not reusable – if you need to move the RAMPS for some reason, cut the cable ties with diagonal cutters or a knife, and re-attach with new cable ties later. I bought a packet of small black cable ties from a hardware store in addition to the red ones supplied with the kit, and used the black ones when I ran out of red ones. They are also great for holding cables out of the way of moving parts.

3.0 Installing endstops

Endstops are the things that let the RAMPS firmware (the code on the RAMPS board that controls the motors and extruders) know where the nozzle is. They are the only direct feedback that the software has as to where the nozzle is, everything else is done by dead-reckoning (counting motor step pulses, and hoping the motor did what you told it).

3.1 Types of endstop

There are two main types of endstop – optical and mechanical:

Optical endstops need a small circuit board and some components, and are triggered when some part of the printer breaks a beam of light. Ideally they are better than mechanical endstops because they are non-contact, so they don’t wear out or get thumped by moving parts. If I sound unconvinced, that is because I am.

Mechanical endstops are just a micro-switch mounted somewhere so that a moving piece of printer presses against the activation arm of the switch. Simple, direct, easy to buy replacements.

Some people are talking about the possibility of hall-effect endstops. MakerGear doesn’t offer them as an option, so you’d be on your own. It’s even possible to print without endstops, but you’ll have to be very careful to never issue a ‘home’ G-Code command either directly or through running someone else’s G-Code unedited. Otherwise the print-head would just keep trundling towards, and past, zero til something broke or you cut the power to the motors.

I chose the mechanical endstops because they are simpler, more robust, and maybe less prone to electrical interference. And they are cheaper to replace from your local electronics store. If you chose the optical endstops you’ll have to follow the advice on the MakerGear website without benefit of my lengthy commentary Smile

3.2 Number of endstops

RAMPS 1.4 has the potential to listen to six endstops, but the kit only includes switches for three, which is fine. Most people set them up as minimum endstops, which means that you put them at the zero end of their axes, and when they trigger they tell the firmware that the nozzle has reached zero on that axis. So, for example, if x-minimum triggers, the nozzle is at zero on the x axis (left-to-right). Maximum endstops trigger when the nozzle has gone as far away from zero as it can get.

I’ll be assuming you are using minimum endstops.

3.3 Wiring Endstops

3.3.1 Selecting the wires

My kit had six wires about 500mm long, each with a small crimp already attached to one end (hooray for every wire I don’t have to crimp or solder). They were coloured yellow and green, purple and brown, and grey and black.

You need two wires for each mechanical endstop, and I recommend pairing them up the way I listed them. That way it is easy to remember which is the ground connection of each pair. The darker ones in each pair – green, brown and black – are ground, and the brighter ones -yellow, purple and grey – are Signal.

3.3.2 Soldering wires to the endstop micro-switches

For each micro-switch:

Read the teeny tiny, black on black lettering next to the connectors. You are looking for the markings ‘NO’, ‘NC’, and ‘C’.

‘C’ stands for Common. Find your 1k resistors and solder one end of a 1k resistor to the ‘C’ contact on the micro-switch. Don’t try to bend the resistor body – they are brittle and will snap in half. Next strip 10mm from the non-crimped end of one of your ground-coloured wires (green, brown or black), and solder the stripped wire end to the other end of the 1k resistor. Cut a piece of heat shrink tubing just thick enough to go over the resistor, and long enough to cover the resistor and all the solder and bare metal. Heat the heat shrink so it insulates all the bare metal, and mechanically strengthens the joints at each end of the resistor.

‘NO’ stands for Normally Open. It means that this contact is open (unconnected to Common) when the switch is un-pressed, and closed (connected to Common) when pressed. Strip 10mm from the end of one of your signal-coloured wires (yellow, purple or grey), and solder the wire to the ‘NO’ contact on the micro-switch. To insulate the joint, heat a short piece of heat shrink tubing over it. Using your multimeter’s resistance range, test the switch. There should be 1k of resistance between the Common and Ground wires when the switch is pressed, and no contact at all when the switch is not pressed.

‘NC’ stands for Normally Closed. Don’t connect anything to this contact.

3.3.3 Locate the endstop connectors on the RAMPS board

Look at the RAMPS board from above, with the big green power sockets and the USB socket to the left. In the top right corner of the board, find the little rectangular forest of upright pins, six pins across by three pins down.

Find the three (or possibly four, if you are given a spare) small black two-pin push-on connectors that hopefully were near your coloured wires. To be sure you’ve got the right ones, try them on the RAMPS board endstop forest before installing the endstop wires. If they don’t fit, you’ve got the wrong connectors, and you’ll have to look in more plastic bags til you find the right ones.

3.3.4 Attaching endstop wires to push-on connectors

This is easy.

Pick up an endstop micro-switch. Plug the crimps from the endstop wires into the push-on connectors til they click and lock firmly into place. They’ll only fit in the right holes. Twist the wires together, starting at the micro-switch end but not twisting the resistor, obviously. Twisting the wires keeps things much neater, makes them less floppy, reduces the chance of wires getting caught in motors or belts, and reduces some forms of electrical interference.

Repeat for the other two endstops.

Now you have three endstop switches, each firmly wired to a small push-on connector.

3.3.5 Plugging the endstops into the RAMPS board

Look at the endstop forest again.

The top row of six pins is labelled ‘S’ for Signal, the middle row of six is labelled ‘-’ for ground (and corresponds to Common on the endstops) and the bottom row is labelled ‘+’ (and we don’t connect anything to it).

The columns are labelled below the pins, corresponding to the following layout. As you can guess from the names, the  x-min, y-min and z-min columns are the ones we’re setting up here.

   x-min x-max y-min y-max z-min z-max
S   +     nc    +     nc    +     nc
-   -     nc    -     nc    -     nc
+   nc    nc    nc    nc    nc    nc

Plug one endstop connector in so it bridges the ‘S’ and ‘-‘ pins of the x-min column, with the ground wire (green, brown or black) over the ‘-’ pin. This is now your x-min endstop.

Plug the next endstop connector in so it bridges the ‘S’ and ‘-‘ pins of the y-min column, with the ground wire (green, brown or black) over the ‘-’ pin. This is now your y-min endstop.

Plug the last endstop connector in so it bridges the ‘S’ and ‘-‘ pins of the z-min column, with the ground wire (green, brown or black) over the ‘-’ pin. This is now your z-min endstop.

There will be no connectors in the max columns or on the ‘+’ row.

3.3.6 Mounting the endstops switches to the frame

Mount the endstop switches on the endstop holders as shown in the photo captioned ‘End Stop micro-switch mounting’ in Brock’s ‘Differences’ file. For some reason I had run out of the right sort of bolt at this point, so I fudged it with some less than ideal replacements. Hope you have better luck!

Now you really have to work out which way round you want your printer. You already know where the RAMPS board is, since you’ve already cable-tied it down. Having the power and USB at the back of the printer helps keep clutter away from the front.

Since building mine I’ve found that having it ‘home’ the x and y axes at the end of each print pushes the bed (with the printed object on it) out where the print is easy to remove. So I ended up turning my printer around so that (zero,zero) is in the far right as I look at the printer, next to the RAMPS board. That way the printer almost hands me my prints when they are finished. I think this may be the way MakerGear Rick runs his printer.

The downside to this arrangement is that the bed is rotated 180 degrees from normal co-ordinates. This makes no difference at all to printing, but it does mean I have to think harder when manually moving the nozzle with pronterface.

If you follow my arrangement (RAMPS board at back-right, Y-motor at back, X-motor at left) then this is generally where to put your endstops:

X-endstop – goes on the back x-smooth rod on the far right, pretty much touching the x-end-idler. Its switch lever faces  the extruder motor. Set it up so that when the extruder motor moves fully right, the end of the motor opposite the extruder presses the micro-switch lever before hitting the x-end-idler.

Y-endstop – goes on the front of the right-hand Y smooth rod. Set it so that the print bed passes over it, and the front of the y-carriage presses the switch just before the y-carriage touches the front idler wheel, or the Y-belt touches the underside of the y-carriage.

Z-endstop – goes on the Z smooth rod on the far right of  the printer, below the x-end-idler. The switch lever faces upwards, where it will be pressed by the underside of the X-end Idler as it descends.You can’t set this at all accurately til you’ve got the motors running. Z height is much more critical than X or Y location, so there’s a whole procedure for setting it that has to wait til you’ve levelled the print bed. Install the endstop now, but don’t bother adjusting it yet.

The Z-endstop has, as the ‘Differences’ file notes, a hole in the bottom prong of the endstop mount, to hold a bolt. The bolt allows for fine adjustment of the Z-height by changing the distance between the two prongs. I wouldn’t try to make large Z-height adjustments this way for fear of snapping the Z-endstop mount like a chicken wishbone.

4.0 Heated build platform

I haven’t built this yet, so I have no comments. There are still several things I don’t understand about using the PCB heater plate. I’ve just bolted it down onto the plywood build platform with bolts and washers, and covered it with blue painter’s tape. No wiring at all.

5.0 Stepper motor connections

5.1 Identify the motors and wires

The MakerGear kit includes five stepper motors. Four are identical, with wires that already have crimped connectors on the end. One of these is your X motor, one is your Y motor, and two are your Z motors. The fifth stepper motor has a gearbox mounted on it, and it’s wires don’t have crimps on them. If you’ve already built the frame and extruder, you already knew all this.

All five motors have the same coloured wires, and the sequence to remember is Red – Green – Yellow – Blue. When looking down at the RAMPS board with the USB cable to the left, all of the stepper motor wire connectors must have this sequence of wires, reading from left to right. Its easy to check, so don’t be one of the folks who have motor problems because they’ve got the sequence wrong.

5.2 Identify the pins on the RAMPS board

Look at the RAMPS board from above again, USB to the left. There are four small PC boards sitting on top of the main RAMPS board, three along the bottom, and one in the top left corner. These are the stepper motor drivers. Above each driver board on the main RAMPS board is a horizontal line of four vertical pins. These are what you plug the stepper motor wiring connectors into.

It may be hard to read the writing on the board with it already cable-tied to the frame, so remember that the bottom left driver is X, the bottom middle is Y, the bottom right is Z, and the top left is E (for extruder).

5.3 Wiring the motor connectors

Because I hate crimping connectors, I’ve done the wiring slightly differently to Brock’s description in the ‘Differences’ file.

5.3.1 Connecting the Y motor

This is the easiest, because the wire connected to the motor is long enough to comfortably reach the RAMPS board.

Straighten the wires out then loosely braid them, starting at the motor end (or just twist them, if you can’t be bothered doing a basic braid – though braiding looks better and doesn’t untwist). Find one of the four-wire sockets which is just like the ones you used for the endstops, only twice as wide.

Plug the red wire’s crimped connector into the left side of the socket til it clicks.

Plug the green wire’s crimped connector into the space next to the red wire.

Plug the yellow wire next to the green wire.

Finally, plug the blue wire next to the yellow wire.

Check that all four wires are firmly seated. Having motor wires come loose when the motors are powered can damage the motor drivers, so pull gently at each wire to be sure they are solidly locked in the socket.

Now you have a connector with wires in the sequence Red-Green-Yellow-Blue. This order is important! (Have I said that enough yet?) The motors will not work right if you mix up the order.

Plug the socket onto the pins above the Y motor driver board, with Red to the left.

5.3.2 Connecting the Extruder motor

This would be as easy as the Y motor, except that the Extruder motor wires don’t have crimps on them.

Strip 5mm of insulation from the end of each wire, and twist to reduce fraying.

Crimp pins onto the ends of each wire.

Straighten and braid the wires.

Plug the crimps into a connector, Red – Green – Yellow – Blue from left to right. Check seating.

Plug the connector onto the RAMPS board, on the pins above the E motor (at the top left), with Red to the left.

5.3.3 Connecting the X Motor

The wires on the X motor are not long enough to reach the RAMPS board, and also the X motor moves up and down, so we have to allow for that. We need to extend the wires. We do this using the grey four-strand wire and the white screw-terminal block (which we used earlier for testing the power supply and curing the heat-core).

Find the long white screw-terminal block. With a small saw or strong knife, cut off a section of screw-terminal with four pairs of screws. While you are at it, cut off a second section also with four pairs of screws. One will be for the X motor, the other for the Z motors.

Cut off the ends of the wires to the X motor, about 100mm from the crimped ends (I hate wasting perfectly good crimps. That’s why I suggest cutting 100mm away from the crimps. Maybe the crimped offcuts will be useful for another project.)

Strip 10mm of insulation off the ends of the X motor wires, and twist the bare ends to stop them fraying.

Braid all four wires together into a cable.

Insert each wire end into a different hole on the screw terminal piece you cut off earlier, starting with Red on the left, then Green, then Yellow, then BlueTighten the screws to hold the wires firmly.

This screw terminal will be attached to the left-most lower threaded rod, behind the Z support rod. We need to connect it to the RAMPS board using the grey cable with four wires inside it.

Measure out a length of the grey cable that will reach from the screw terminal, under the printer along the Z support rod, under the RAMPS board then back over the top of the RAMPS board, finally arriving at the stepper motor driver pins. Leave yourself plenty of slack, at least 200mm. You have lots more grey cable than you need for the printer, so don’t be stingy. When you’re sure it will be long enough, cut off your measured length.

Strip 30mm of the grey outer coating from both ends. Remove the thin metal foil and cut off the bits of string. With each of the four coloured wires now revealed, strip 10mm of insulation off one end, and 5mm of insulation off the other. Twist the bare ends to stop them fraying.

Plug the 10mm ends into the screw terminal block that connects to the X motor. The colours of the grey cable don’t match the stepper motor wire colours, so we have to choose how to align them. Red to Red, Green to Green, Yellow to White, Blue to Black. Tighten the Screws.

At the other end of the cable, with 5mm of bare metal on each wire, you need to crimp the little pins onto the wires. Check Nophead’s video again on the Stepper Plastruder page if you’ve forgotten how you crimped all those connections. (I think its called repressed memory…)

With the crimps on the wires, insert the crimps into a black 4-pin stepper motor connector, starting with Red on the left, then Green, then White, then Black.

Route the cable under the printer and around the RAMPS until it is over the pins for the X stepper motor, and plug the connector in with red to the left.

5.3.4 Connecting the Z stepper motors

There are two Z motors, one at the top of each Z threaded rod. We want them to move exactly together, so we can send them the same signals. That means using the other four wire/eight screw terminal block we cut earlier. I mounted my terminal block at the top of the frame above the RAMPS board.

Cut just the pins off the wires from the left-hand side Z motor. Strip 10mm of insulation from the ends. Braid the four wires for neatness. Insert the stripped ends into the terminal block in the sequence Red, Green, Yellow, Blue from left to right. Loosely screw in place.

Notice that the right hand side Z motor is almost touching the screw terminal block. So cut the wires from the right hand side Z motor 150mm from the motor. The offcuts  should be long enough to reach from the screw terminal block to the RAMPS board, and they are already crimped! Yay!

Strip 10mm from the bare wires of the right hand side Z motor,  twist to reduce fraying, and insert into the same holes that the left side Z motor wires go into. It might be easiest to undo the screws (one at a time) to release the left side wires, twist the matching wire ends together, then insert into the screw terminal and tighten the screws. You end up with a screw terminal block with two red wires going into the leftmost hole, then two green, then two yellow, then two blue.

If you’ve been lucky, the remains of the wires from the right hand side Z motor are long enough to reach from the screw terminals to the RAMPS. If so, insert the bare ends into the screw terminal on the other side from the motor wires, matching wire colours red to red, green to green etc. Insert the crimps into a connector, Red on the left, then Green, then Yellow, then Blue. Plug the connector onto the RAMPS board above the Z stepper motor driver, with Red on the left.

If the wire remains were too short, you’ll have to rig up a piece of the grey four-wire cable just the way you did for the X motor, using the same colour correspondences.

That’s your motors wired up.

6.0 Wiring the thermistor and nozzle heater

The extruder nozzle heat-core and the thermistor are both types of resistors. The nozzle is a tightly wound coil of low-resistance nichrome wire that heats up dramatically when enough current runs through it. The thermistor is a tiny glass bead-like thing that changes resistance in a predictable way when its temperature changes. Between them they form a feedback loop, with the Arduino using the thermistor to tell whether to pump more or less current through the nozzle’s heat-core.

As the ‘Differences’ file says, find the two black connectors that match the ones on the heat-core and on the thermistor. Note that although the same connector types are used for both the thermistor and the heat-core, the genders are reversed. What is male on one is female on the other. This is good, because it guarantees we can’t accidentally plug the thermistor into the heat-core’s cable or vice versa. It means we really have to pay attention now, while wiring things up, but we’ll be safe ever after.

6.1 Routing and cutting the hot-end cable

Find your grey four-core cable again. Consider that the extruder head has to be able to move unrestricted from all four corners of the print bed, at both fully lowered and fully raised positions of the Z axis.

I figure you need enough cable to run from the RAMPS board, up the back diagonal threaded rod to the top of the frame, then at least another 450mm.

Try it out – tape the grey cord in place from the RAMPS to the top of the frame, then wiggle the loose end around to everywhere you’ll want the extruder nozzle to go. When you are sure you’ve allowed enough cable, add 100mm to your estimate and cut a piece of grey cable to length.

Strip 40mm of grey insulation off each end. Remove foil and string.

Strip 5mm of insulation off the end of each coloured wire, at each end.

6.2 Attaching the connectors for the thermistor and heater core

6.2.1 Assign colour codes

I used Red and Green for the heat-core, because I associate red and green with dangerous mains wiring (yes, I started in electronics before people worried about colour blindness), and the heat-core uses lots of current. And Black and White for the thermistor, because they seem like extremes on a thermometer’s temperature scale (icy snow to burned black).

6.2.2 Attach grey cable to thermistor and heat-core

This next bit is a little tricky, because of the evil crimp pins again, and I can’t test my instructions because my connectors are already fully wired.

Find the connector that mates with the heat-core’s connector. Plug the two connectors together to prove you have the alignment right. The end that doesn’t mate with the heat-core’s connector must be where the crimps go on the red and green wires. Check that the crimp pins fit (even though you haven’t crimped any wires to them yet). I have vague memories of different sizes and genders of crimp pins. Once you are confident you have the right crimp pins for this connector, crimp the pins onto the red and green wires. Polarity does not matter, as resistors are unpolarised. Unplug the two connectors again, then insert the crimped wires, ensuring that they lock tightly and won’t come out. Plug the two connectors back together; – now your heat-core is connected.

Now find the connector that mates with the thermistor’s connector, and do the same with the thermistor’s connector (that is, plugging, checking, crimping, unplugging, inserting, and plugging again). Though using the black and white wires, of course. Now you have the thermistor connected too.

6.2.3 Attach the other end of the hot-end cable to the RAMPS

Put crimp pins on the ends of the black and white wires, and insert them into a two-pin connector like the ones you used for the endstop RAMPS connections. Look down on the RAMPS board, with USB to the left, and find a horizontal row of six pins directly above the Z stepper motor connector. The two left-most pins are labelled ‘T0’, for Thermistor zero. Plug the black and white wire’s connector onto the two pins labelled T0.

The heat-core doesn’t use crimps to connect it to the RAMPS board. Directly above the USB connector on the left edge of the RAMPS board you will find three pairs of green screw connectors labelled (D10, +), (D9, +) and (D8, +). These are high-power outputs, for the heat-core nozzle, a heat-bed, and either another nozzle (dual extrusion, whoo!) or a fan.

Insert the green wire into the hole labelled D10, and tighten the screw. Insert the red wire into the hole next to it, labelled ‘+’, and tighten the screw.

That’s the electronics build done. Congratulations. Celebrate before installing the software and testing everything.

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13 Responses to MakerGear Mendel Prusa electronics assembly notes

  1. Gary says:

    Outstanding! Dished up in bite-sized pieces, Verb-Noun consistency, valuable tidbits of advice, high (pertinent information)/(word) ratio.. Certain to save me hours and $$ when I get to that stage.

    I have one recommendation. Whenever execution of one instruction in a series requires the reader to move attention away from the text, then that series shoutd be bulleted. Example:

    1.3.2 Mark your connectors with polarity
    * Find the small square white 4-pin socket which came with your kit. (There are only one socket and one plug matching this description.)
    * Plug it in to the matching square 4-pin plug on the PSU wiring harness. (Really plug it in, so you know you’ve got the alignment right – they are keyed so you can’t connect them the wrong way.

    Why? Maybe only applies to old folks like me with ADHD, but I suspect most people would benefit. It may take me a while to “Find the small sqare …”, and once I do, I need to quickly resume reading where I left off.

    Please keep up the good work.

    • BrazenArtifice says:

      Hi Gary
      thanks for the kind words. I guess I’m surprised that verb-nound consistency is rare enough on the web to be worth commenting on 🙂

      I take your point about attention being shifted from the text to the physical and back. I’ve tried to help with that by bolding the physical steps, so you can quickly find the last bit you acted upon. Clearly I haven’t got the balance right yet. I’ll read it again with fresh eyes this evening and see what I can do.

      If I bullet every step, there are too many bullets! I don’t believe in more than seven or so bullets in a list, so I’d have to step down to subheading level four, or even five, to get that right. I don’t think WordPress handles those heading levels well.

      Still, I’ll keep massaging it. Hopefully when you are at that stage of the build, you’ll have printed the post out and be scribbling your own pencilled notes on it, as you work out what I reallymeant to say.

      Thanks again for the thoughtful comment

  2. Mark C says:

    Now that I’m finally to this point, I have a question: is there a reason there are two rows of connectors for the Z-motor on the RAMPS board? In other words, can I just plug both Z-motors onto the board, one on each row, provided that my cables can reach and skip using the terminal block? Intuitively, it appears to me that’s what the two rows are there for, but my intuition does some really weird things sometimes…

    • BrazenArtifice says:

      Yes, if your z-motor cables are long enough, that is just what the duplicate z-stepper connectors on the RAMPS board are for. I’m jealous, mine wouldn’t both reach.

      Great to hear your build is progressing.

      • Mark C says:

        Slowly, but albeit progressing! Also thought you’d like to know that I’ve been using only your blog as a guide now, it’s so far more concise and clear than the Prusa standard instructions with the differences guide!

  3. Pingback: Prusa Mendel – Câblage de l’alimentation

  4. draj1968 says:

    Fantastic , open source works ! only because of people like you exists, Thank you very much and hats off for your dedication……..

    • BrazenArtifice says:

      Thank you for the kind words. I’d never have been able to get my printer working if not for all the other folks who have written up their experiences. We all learn from each other, and knowledge advances!

  5. lateralg2 says:

    I echo draj1968’s comment.

    Another option for Z motor wiring is:
    Use the excess length of the Y motor wire to splice into the right Z motor wire. This requires cutting wires at the proper point to achieve:
    * Avoidance of assembling the dreaded Molex connectors.
    * Tidy Y motor wire length.
    * Sufficient Z motor wire length.
    * Adequate length for soldered splice with heat-shrink.

    • BrazenArtifice says:

      Yes, I guess that would work. My approach to wiring the Z-motors (using a spare piece of screw-terminal block up on top of the A-frame, above the RAMPS board) means you don’t need to solder, and you don’t need to crimp either! And you can take it apart again easily if you get something wrong.

  6. Ray M. says:

    Thank you for your excellent instructions!
    On the Z motor wiring, there is two different grey multi coductor wires included in the kit. One is lighter grey and the wires within are 24 gauge, the other is a darker grey and the wires are 22 gauge. I recommend using the 22 gauge wire for the Z steppers.

    I am working with a high school robotics team to get this built for them. I t has been somewhat frustrating trying to find all of the best instructions and pulling it all together, but it has made an ideal learning experience for the team! We should have it done before school starts next week.

    Thank you!!

    Ray M.

    • Hi Ray!
      I’m glad to help. You are so right about the “learning experience” value of assembling this machine. It’s probably the most complicated single project I’ve ever undertaken, and the greatest sense of triumph at the end. Though now that my printer is so reliable, I’m finding that designing things worth printing is the hard bit!
      Best of luck with the rest of the build.

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