I've been a bit busy lately and mainly working on other people's projects, so I haven't had a lot to post lately, but I have been working on the R3 (RepRap Router) again.  My laser-strap prototype has taken up residence in the new CNC room at Hamerspace and is now sporting new endstops and tool mounts including a boom for its powerful little flex shaft spindle.  I got in a quick test of the tool mount and spindle last meeting and grabbed some video:

Here's some additional pics:

Next on my list of stuff I need to do for this prototype is to get the hold down system in place and mill the sacrificial top flat.  I plan on embedding some extruded aluminum T-slot that works with 1/4" hex bots into the MDF work surface like the bigger CNC setup at the space and using standard woodworking hold down clamps for now and maybe design some milled or 3D printed ones as the project progresses.

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First off, peel the masking off all your laser cut parts.

Next, you'll mount the BeagleBone to the bottom using 14mm M3 bolts, M3 nuts, and 1/4" OD 1/4" high nylon standoffs.  Make sure to mind the resistors on the back side of the board around the power connect (labeled R150 and R189).

Next, you need to partially thread all the remaining M3x14 bolts into the T-slots of the spanning pieces and the end caps.  This is physically possible without assistance, but it's hard.  You'll want to save yourself some time by using some scotch tape and securing the nuts in place before inserting the bolts.

Once this is done, you can slide in the bottom, sides, and top before finally tightening down the bolts.  Don't overtighten the blots, the enclosure is all acrylic and it will crack under extreme stress.

Bottom Installed

Sides Installed

Top Installed and Bolts Tightened

Other Side After Assembly

You can simply leave the top off if you want easy access to the expansion headers.  The Adafruit BeagleBone proto cape will even fit with the top off:

Here are some additional pictures of the enclosure with the USB host and peripheral interfaces plugged in (notice that you can see the status LEDs above the peripheral connection):

Grab an enclosure kit including handy hex wrench from my store and you can grab all the design files and BOM from the Thingiverse entry.

You can grab a sled kit there as well, if you feel like your BeagleBone would like a little more of an open air prototyping environment. You can find the design files for the sled, BOM, and instructions on its Thingiverse entry here.

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Here's a pic of a quick little dongle I threw together that allows you to power up a PC power supply and use it as a bench supply, or in my case, to power a few stepper controllers for my CNC setup. You can also use the dongle to test potentially bad ATX power supplies.

You can find tutorials on using ATX power supplies outside of PCs all over the net but all of them are pretty much a variant of what I have here.  To get the thing up and running, you just need to:

• Connect the green wire on the 20 or 24 pin connector to ground to power on the supply.
• You'll probably want to supply a load to the supply to keep it up and running, usually just a power resistor on the 5V rail, a 10 Ohm 10W resistor is typically used, creating a 2.5W load.  I've also seen people simply plug in an old PC peripheral like a hard drive to the supply to provide this load as well.
• Attach an LED to the output of the supply OK signal to get a nice "everything's fine" light.

Here's what you'll need:

• 30 Ohm 5W Resistor - Mouser P/N: 71-CW5-30-E3 - This puts a nice 0.83W load on the 5V rail.  Don't worry if this warms up a little bit during operation, that's what a resistive load does.  This smaller load seems to work fine for me.
• 20-Pin Power Supply Connector - Mouser P/N: 538-39-29-3206 - Just the socket we need to connect to the ATX supply where a standard motherboard would be connected.
• 330 Ohm 1/4W Resistor - Just a current limiting resistor, not needed if you don't put a power on light on your dongle.
• Green LED - Just a typical LED for your power good light.

Here's a couple close up shots of the dongle on the inside:

Check out the Wikipedia entry for ATX and scroll down to the power supply section for a detailed pinout of the ATX connector.  Note that I'm using a 20-pin connector and the connector pin numbers on the Wikipedia article are for a 24-pin.  I'm using the pin numbers from the article, so realize they could be different based on which connector's pinout you're looking up, so pay attention to the actual signal names!

Basically I've connected pin 16 (power on) to pin 15 (ground), put a 30 Ohm resistor across pin 21 (+5V) and 19 (ground), making sure that the long exposed lead is connected to ground, and connected the positive lead of my indicator LED through a 330 Ohm resistor (hidden in the heat shrink) to pin 8 (power good) and the negative lead to pin 7 (ground).  After testing, I wrapped the whole thing in a little electrical tape and installed it.  I'm using the switch on the back of the power supply as my power switch, but if you want to have a more convenient switch wired to your dongle, just replace the jumper between ground an power on with a standard normally open switch.

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I wish they would have recorded the last session we did rather than the first, because it seemed rather polished in comparison by the third time we had presented (and I need all the help looking like I know what I'm doing I can get)!

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All you need to do to build your controller is remove the controller of a standard servo and replace the potentiometer with two equal value resistors (typically 2.2k, but higher values should be OK) and hook up the brushed motor you want to control in place of the servo's DC motor.  You want to connect the two resistors in the center by twisting their leads together connect the outside leads of the resistors to the outside potentiometer terminals and the twisted center to the center terminal. Here's a pic of a controller (specifically the TGY-S4505B controller, see some more pics of the guts of this servo in my previous post) after it's been removed from the servo and the resistors have been installed:

I used heat shrink on the two outside leads of the resistor to keep the connections insulated.  Next I put heat shrink over the two resistors:

And tape everything down.  Now I'm ready to test it out on Luis' partially completed turtle shell racer using a standard RC Tx/Rx set by connecting up the DC motor I want to power where the servo's DC motor was previously connected:

If everything works you should not see the DC motor turn (or turn very slowly until you adjust the trim) when applying power to the RC Tx/Rx set but the brushed DC motor should respond as expected when you use the throttle on your controller.  If you've modified a servo for continuous rotation this should be very familiar to you because that's essentially what you've done, you're just not using the mechanics of the servo.

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