Archive for May, 2009

(Mostly) Assembling My CupCake Motherboard

Monday, May 25th, 2009

The MakerBot CupCake‘s motherboard (RepRap generation 3 motherboard) is Arduino-compatible, connects to all the other boards in the CupCake / RepRap, and has lots of spare connectors to boot.

Ah, spare connectors to prototype, I mean; the spare connectors aren’t needed in order to get the board to boot. Ha ha.

Mostly-assembled CupCake motherboard

I got mine mostly assembled last week. Like every other piece of electronics in my CupCake kit, my motherboard had missing parts — I got five extra 4.7KΩ resistors instead of the five 1.8KΩ resistors. And unfortunately, that was another part for which I couldn’t find replacements at hand — although I did end up sorting and filing a bunch more SMT components while searching.

After waiting a week for the missing resistors as the solder paste was drying out and getting crusty, I gave up and baked the thing without them. I can hand-solder the 1.8KΩ resistors onto the board when they arrive. And because they’re voltage dividers for 5V → 3.3V level conversion to the mini-SD card and pull-ups for the I2C bus — neither of which is needed for basic operation — I can even run the darn thing without them. I just need to get them on there eventually.

Crusty solder paste bakes just as nicely as gooey solder paste, by the way.

Unlabelled R1 on MakerBot Cupcake motherboard

I did spend a while searching for the placement of R1 on the PC board. I eventually found it through a combination of the process of elimination and checking the schematic. As far as I can tell, the name label didn’t make it anywhere onto the silkscreen — can you spot it?

Simple LED Brightness Tester

Monday, May 25th, 2009

Back in March, I was contacted by Amanda McClellan of Coordinate It wedding planning. She’s interested in LED lighting for paper party lanterns; and although she has an engineering degree, electronics is not specifically her forte. She wondered whether I help her figure out how to drive the LEDs, and, well, you know how I feel about LEDs. :-) So of course I was interested.

The challenge, of course, is that LEDs need a constant-current drive; and it’s not easy to drive a string of LEDs at a constant current (hence constant brightness) using a simple current-limiting resistor. I’ve looking at constant-current LED driver ICs, and am close to putting together a simple but useful LED driver circuit, with intention to both open-source the design and offer the hardware for sale. More on that soon.

LED Tester

Meanwhile, as I’ve researched driver chips, Amanda and I have been talking about how many LEDs (and indirectly how many mA) it takes to light a paper lantern. She wants the lanterns for decoration, not illumination — Last weekend I assembled and shipped her a simple LED tester to allow her to try some LEDs inside lanterns and see how many LEDs and how much current she’s going to want.

My idea was to make something that could:

  • run off a 9V battery (enough to power a series string of two white LEDs at ~3.4V each)
  • allow her to vary the LED brightness for testing
  • allow her to measure the LED current without having to stick an ammeter in the circuit

LED tester schematic

Regarding that last point — ammeters have to be connected in series with the circuit under test, so the LEDs would go dark when the meter wasn’t connected. Instead, I used a 1.0Ω current-sensing resistor in series with the LED string. Across the 1Ω resistor one can measure 1mV for each 1mA of LED current; and removing the voltmeter from the circuit has no effect on the LED operation.

R1 (20Ω) is there to save things if the potentiometer is turned to its minimum value of (theoretically) 0Ω. With two 3.4V LEDs in series, the combined LED voltage drop is 6.8V and the voltage remaining across the various resistors is 2.2V (or .4V with the NiMH “9V” battery I use at home). 2.2V / 21Ω ≈ 105mA, which is way too high for a normal LED and puts almost a quarter watt across R1. Not great; but it’s a last resort instead of a dead short, not a desired operating mode.

The Board

PC board with iron-on 'silkscreen' layer

I used the laser printer and household iron to do toner transfer for etch resist, and also to transfer the “silkscreen” on the top side. I get best results on the silkscreen side peeling off the paper while everything is still hot, and I was particularly pleased with the transfer quality here. Not perfect, but it’s pretty legible for 70-mil text.

LED tester PC board

Here’s the populated board, sized the same as the battery it runs from. In retrospect, I should have moved the potentiometer further down or the LED connector further up, but I was intending to use a right-angle connector that would lay flat on the board. Couldn’t find any in my parts bin when I went to assemble it.

The test points at the bottom are wire loops sized to friction-fit my voltmeter probes. They’ll also work well for clips or gator wires.

In Use

LED tester

Here it is in action with a couple of amber LEDs plugged in. They don’t look bright enough to me — but letting Amanda determine that is the whole point of this exercise. She’s the expert on event planning; and it’s her assessment that matters, not mine.

Paper lantern with LED lighting

This is two warm white LEDs inside a paper lantern. Under normal room lighting, you can tell there’s light inside, but it doesn’t look dramatically lighted and diffused as I think one would want it to.

Paper lantern with LED lighting in the dark

In evening darkness, the lighting is more noticeable. And in spite of having set white balance carefully before taking the shot, my old camera changes the warm yellowish white light into cold blueish white light. Ach, what can a fellow do. Looks good in person, though — if not necessarily bright enough (to me).

Wired IV

Friday, May 22nd, 2009

The video from this year’s Technology: Art and Sound by Design final show is now posted:

Wired IV from MRC Video Services on Vimeo.

And I’m very impressed with our students’ write-ups of their projects on the class blog, including pictures, video, circuit diagrams, and Arduino and Pd code.

Go students!!!

Soldering My Last Two MakerBot CupCake Stepper Boards

Monday, May 18th, 2009

Over the weekend I received the three missing electrolytics, and yesterday I built the other two stepper driver boards.

MakerBot CupCake stepper driver board with flowed, half-melted solder paste

Just one noteworthy item — I meticulously lined solder paste on every IC pad, and then during the “soak” period of reflow (also known as “the time it takes my 500W heater to creep the plate up to 185°C”) the paste all slumped together anyway. (Forgive the poor focus in the picture; my camera’s not very good and that’s the best I could get.)

When the solder paste reflowed, surface tension pulled it all into nice little fillets anyway.

Lesson: Don’t bother tracing every IC pad with solder paste; just run a bead perpendicular to the leads and trust surface tension.

Also, I had one solder bridge during reflow. I took a small screwdriver and poked it between the leads, breaking the surface tension and the bridge. Nice trick, and a quicker (and cleaner!) fix than anything you can do after the solder cools.

Assembling the MakerBot CupCake Extruder

Monday, May 18th, 2009

I’ve been plugging away at the CupCake “Plastruder” in the evenings, and I now have the mechanical assembly mostly finished, pending the arrival of some custom parts.

MakerBot Plastruder plastic extruder

I’m delighted by the clear plexiglas design. And having the mechanical assembly — particularly the extruder — put together really highlights how small this thing is. That’s not a bad thing — for a given build capacity, the smaller the machine is, the better.


Soldering the MakerBot CupCake Extruder Controller

Friday, May 8th, 2009

Over the last two evenings, I got my CupCake’s extruder controller assembled.

RepRap extruder controller

I have just a few notes below following up on solder paste, hotplate soldering, and missing and unlabeled parts.


Assembling the MakerBot CupCake Stepper Controller (or My First Reflow Solder)

Tuesday, May 5th, 2009

Having got my reflow soldering hotplate assembled Sunday, last night I sat down to build the first stepper controller for my CupCake rapid prototyper. Besides being the first of the CupCake’s stepper controllers, this is also the first thing I’ve ever reflow-soldered, EVAR (although not the first SMT I’ve soldered, as I’ve done that by hand before).

RepRap (MakerBot CupCake) stepper driver on hotplate, assembled and ready to reflow solder paste

There are lots of solder paste and solder reflow tutorials online, and that this ain’t. This is just my observations about parts of the process I hadn’t previously picked up from reading.


Copycat PID-Controlled Solder Hotplate

Monday, May 4th, 2009

In early February, a correspondent pointed me to Jeff Keyzer’s mightyOhm blog. I immediately ran across his homebrew PID-controlled soldering hotplate and improvements, and immediately knew I had to have one.

I contacted Jeff through his blog and he was great about sharing his knowledge. He’d built his hotplate using the last of some surplus parts he’d picked up at a now-closed store in the Valley and was considering ordering a batch of parts to make a few for all the folks inquiring about them, but hadn’t done so yet. I was eager, decided it’d be quicker to make my own (and three months later, that may actually have been correct), and went off to eBay to find myself some parts. I also bought aluminum and took a practice run at polishing it.

Most of the CupCake PC boards are SMT; and although I’m very comfortable soldering SMT by hand, I really wanted to get my hotplate up and running and use the CupCake boards as a chance to try out reflow soldering. (That’s why I started by assembling the opto endstop boards, which are the only all-through-hole boards in the kit.)

PID-controlled SMT soldering hotplate

So last night I got a working proof-of concept hotplate going, and tonight I can start on the CupCake SMT boards. W00T!

Here’s the tale of how to build a copycat PID-controlled hotplate, with a digression into how lucky I got buying exactly the right PID controller with no idea what I was doing.


Makerbot Cupcake Opto Endstops

Friday, May 1st, 2009

I’ve been massively interested in the RepRap project since I first heard of it a few years ago. RepRap — short for replicating rapid-prototyper — is a CNC machine to extrude hot plastic and build up a model additively, like a robot hot-glue gun. It promises to lead the affordable desktop fabrication revolution, printing at home on a sub-$1000 machine what the aviation lab at work printed for me on a $30 000 machine (which itself is already orders of magnitude less expensive than the ones I saw in use when I worked at Cessna Aircraft).

The RepRap can replicate many of its own parts — so once you get one, it’s a matter of feeding it plastic and a few hours (and motors, and circuit boards, and commonly-available hardware) and you can have another one for your friend. It should be pretty viral once it gets going — but getting going is the problem. Even with various “RepStrap” (RepRap bootstrap) designs, the barrier to entry to build my own from scratch was still a bit too high for me — particularly for the heater/extruder nozzle.

So when I saw that Zach “Hoeken” Smith (a member of the RepRap team who’s designed most or all of the current-generation electronics), Bre Pettis (the “talent” on the first Make Magazine videos I remember seeing), and a couple of guys I don’t recognize (sorry, couple of guys!) had teamed up to found MakerBot and were selling the CupCake CNC kit, a complete set of parts to build a RepRap-compatible machine, I was onboard in a heartbeat. The price still feels a bit steep, but the time was right and I got one of 20 presale kits.

It came last week and what with other obligations, I haven’t even had time until last night to start working on it. So I’ll be doing this a slow step at a time; and the first step was assembling the through-hole optical endstop PC boards.

Complete set of Makerbot CupCake opto endstops

The CupCake calls for six endstops — two for each axis — and they use RJ-45 cables for most of the connections, but three-pin headers for the Y axis due to space considerations.

The boards are nicely made; I like it a lot that they’re no larger than they need to be; and I really enjoy the red.

RepRap opto endstop optointerruptor alignment

I have only a couple of cautions for assembling these. First, as noted in the CupCake electronics assembly instructions, the PC board’s optointerruptor footprint is a little off, and it takes some shoving to get the mounting holes aligned. This does matter because these holes are how the board mounts to the chassis, so take the time to get them as close as possible.

RepRap opto endstop RJ-45 jack modification

Second, use a chisel to shave off that spacer tab on the underside of the RJ-45 jacks. If you don’t, they won’t sit flush, and that bothers me.

RepRap opto endstop RJ-45 jack fit with and without modification

Take my word for it, it’s way easier to do this before you start soldering the pins.