Holiday Ornament Design Challenge

Over the Holiday break, I challenged my family to design some ornaments for our tree. So far, only Mason (17 year old son) rose to the challenge, and I think he did very well indeed.

He did the designs in OpenSCAD, and at first hardcoded all the measurements until I encouraged him to make the designs fully parametric. This piece was designed to accept a light (from the string lights on the tree) in the bottom. Printed in Natural PLA with about 15% fill, giving it a kindof stripey, plaid texture.

This was printed without support on the Ultimaker 3. The very inside top is a little stringy, but hardly noticeable, and very easy to clean up.

The second design was really the first one he did, and isn’t fully parametric. You’ll have to scale it in your slicer, but it turned out pretty nice.

I printed it in white, thinking it might be fun to use a sharpie to color in the facets. This was printed with the PVA support.
I also printed it in transparent ColorFabb XT. I had a problem with the wipe tower, so the top was a bit messed up with mixed PVA and XT, which broke off, but a light fit into it, and I think it looks nice.

I encourage you to sit down and design with your family this holiday. You can get the sources for these at: Feel free to do some Pull requests to add your own designs!

For younger, or less mathematically inclined designers, I recommend, or I’m sure there are cool Ipad apps out there. Let me know if you have a favorite!

Building Access Security Research

I’ve been researching a lot of contactless payment, and authentication stuff for work, and thought I’d share some of the most interesting links. This post will focus on building access.

Building access

It seems like many building access keycard systems are pretty weak in terms of security. Essentially, many of them present an ID code that is checked against a database. If you can copy that code you can clone the card (replay attack). Also most of them use something called Wiegand signalling as their output which is just a protocol to decode, so if you can tap in, you can sniff or inject stuff pretty easily. There are more secure systems out there that use a cryptographic exchange, but the insecure systems are in abundance! blog — This is a company that sells a more secure building access system, so they’ve done a lot of competitive research about vulnerabilities of common building access systems. — Security consulting firm. hacked a long range reader to steal ID’s. Essentially used an arduino to listen in on the Wiegand output.

Jonathan Westhues — EE and software guy did a lot of reverse engineering of some badge signals, later created a whole platform for reading and spoofing badges.

Lasercast Activate!

I recently got my thumb reconstructed (due to Osteo arthritis). And my new cast felt like it could use an upgrade. Laser! I figure it will be a good addition to my mad scientist costume for Halloween. Chinese laser pointer from ebay or aliexpress (don’t remember, I have a drawer full of laser modules). Adafruit powerboost 500 basic (If I’d had the full one, it could charge too, but this is what I had lying around).  I designed a very basic platform in OpenSCAD, and stuck the board down with mounting tape. The enable switch is soldered directly to the board (I cut off one of the terminals.)

Both the laser platform and the activation button are mounted with Velcro (the cast is coming off…) the button is superglued to the disk that holds the velcro dot.

If you want to make it, it’s very likely that your laser module will be sized differently so edit the openSCAD file to  fit.

3D printed circle the same size as a velcro dot

First Video, and a few Quadcopter lessons learned

I had a lot of fun flying the 3DR Solo in the last couple of weeks, but after a while you just want to DO something. Until I develop my “copilot” add on board to activate some cool add ons and sensors, photography is the obvious thing, and my bundle even came with the stabilization gimbal. I was planning on buying a “fauxpro” from MCM electronics but found out the gimbal is really “locked” in to Genuine GoPro ™. I could probably hack it, but the biggest advantage of using a real GoPro is the expansion connector on the back allows you to start and stop video, take pics etc. from the remote. So I gritted my teeth and plunked down $250 at on a GoPro Hero4 bundle that included an extra battery and SD card.

Here’s my first flight video:

I’m very pleased with the video (shot at 1080p and 60 fps) though obviously, my control and shot planning skills need some work.

Shortly after this, I crashed and trashed 4 propellers (ouch$). I was running it on “cable cam” where you fly to different positions, record them and then you can run the quadcopter between those positions like it was on a cable. When I ran it to the end of the “Cable” it ended up being about 5 feet higher than when it started, got tangled in some wires, and crashed upside down. It sat there and ground the propellers to dust, as I didn’t know how to turn off the motors until it detected the crash (maybe a minute later).

Lesson Learned: Don’t expect GPS to get you exactly to a spot (it’s only good +- 3 meters at best) and don’t fly in a confined airspace unless you are really good and manually controlling it!

Other Lesson Learned: RTFM, except there was no FM! Well, online there is, and a quick web search revealed holding down a and b buttons together will cause an emergency motor stop.

3DR Solo of my very own!

If you’ve followed this blog, you know I’ve played with toy quadcopters and built one from parts. I’ve also built an FPV racing drone that I’ve only successfully flown twice.

For a long time I’ve lusted after a GPS drone capable of autopilot, especially after a demo by a neighborhood friend of his DJI Phantom 3. I think the DJI drones are pretty cool, but I hate that they are not open, and being both an Open Hardware guy, and never satisfied with factory settings, I really wanted open source.

3D Robotics, founded by former Wired editor Chris Anderson has been making open source autopilot drones for quite a while now, but they’ve been quite pricey, and don’t include a camera. I just couldn’t justify it.

My friend Michael Castor at clued me in to a sale at Bestbuy and I scooped up a Solo, extra battery, gimbal, extra propellers, and a backpack for $399 (plus tax). SCORE!

Note, last time I checked, the price just went up to $599, but you can still get the Solo for $399

Free 2 day shipping said it would be here Thursday, but the Solo came on Wednesday, and the rest on Thursday.

solo box

It was well packed, comes with battery, two extra propellers, transmitter,  and chargers for the transmitter and the battery. It comes with an eggcrate material carrying case that would probably do for a while, but I hate to think what would happen when it rains.

The backpack, which came later (and I may cover in a future post) is terrific."carrying case"

DJI, by the way also gives you “carrying case” packing, but in their case close cell foam, which would probably hold up longer.

Solo unboxing

Setting it up was pretty easy, download the app to my phone, power everything up. There’s a required firmware update before flying, and while it crashed my phone a couple times (I am suspicious because I have CyanogenMod) it took only about 2 minutes.

I don’t have a camera yet (I’m waiting for a new model of “fauxpro” from mcm electronics.) but It was amazing that I actually managed to work all week without flying. A quick stop at the FAA site to register, print a label for my Solo, then I did get out on Saturday, and I’m hooked!

Kevin flying his new Solo

Photo by Will Caldico0tt

Auto take off and landing are the bomb! The orbit mode was pretty easy to use, once I figured out how to set the center on my tiny phone screen. I ordered an acer tablet from ebay (about $70) and hope that will be better.

3DR recommends initial flights in a wide open area, and I concur. While it’s really easy to fly, it’s also hard to judge depth at distance when it’s flying near trees etc. The next day, I did manage to crash it, breaking 2 propellers, and chipping a third. The Solo was fine though, and shut itself down with the remote talking to me “Crash detected”.

Hacking potential

I wouldn’t have even spent $400 (and will likely spend even more) if it wasn’t easy to add my own hardware and software mods. 3DR makes this really easy, with a well thought out Dev kit python API, and well documented hardware expansion. Check it all out at:

What makes this really exciting, is the Solo (in spite of the name) actually has Two processors, a pixhawk flight controller, and a linux based computer. You can actually ssh to the drone, and store scripts for execution during flight.

I’m excited that you can even use OpenCV on the video stream from the camera.

Arduino Robot Class preview

I’m teaching an Arduino Robot class June 29 from 6-8pm at YouDoitElectronics in Needham, Ma. The cost is $99 and you get to take home the robot you build. I’ll show you how to use an Arduino to control DC motors, and read sensors to react to the environment. The robot we’re building will have a sonar sensor for distance, and two line detectors for following a line.
To register email your name phone number and number of participants to Please include Arduino Robot Workshop in the subject line. You will receive a call back within 1-2 business days. Fee is required at time of registration prior to the start of the workshop. Once registration and payment are complete a reservation confirmation number will secure your spot.

DIY Rayguns!


I’ve been going to Arisia (one of Boston’s Science Fiction Conventions) for several years. I always have a great time, and it always amazes me the amount of volunteer effort that goes in to providing amazing programming for everyone. This year I wanted to give something back so I came up with the idea of people building rayguns.

(NOTE: My extra kits are available in my etsy shop:

Regular LED strip is super cheap when you order it from China. It normally runs off 12V, but a 9V battery lights it up just fine. I created a simple laser cut frame to put some LED strips, and then some cool laser cut acrylic disks to give it that groovy 50’s raygun look.

I bought most of the parts myself, but special thanks goes to my friend Jeremy Green at Charles River Maker for donating Laser time for cutting the kits. He’s your goto guy for design, prototyping and digital to physical services with laser cutting and lots of 3D printers!


A switch makes the trigger, and there’s a fin with holes to add decorations.


First cut a strip of LEDs. Mine consisted of groupe of 3 led sections.. Cut a strip of 5 sections, two sections on each side and one straddling the middle:


next glue on the handle “dimensional” bits. These are mainly to make a recess for the battery, and maybe wrap the handle. Super glue works pretty quick, but you could probably use wood glue or whatever is appropriate for your frame:


Glue the trigger button on, (kindof low, as the disks are pretty close. Actually I moved it down abit after I put the disks on (later step)


Line up the battery clip and shorten the leads to connect one end (I chose the red, positive wire) to the positive contact on the LED strip. A small wire went from the negative contact to the switch and the other lead of the switch to the negative lead of the battery clip.


Test, viola!

Next line up the disks where they fit. back them off and put a drop of superglue where each disks goes in order.



The source files for laser cutting are at:

And now a few pics from the workshop at Arisia!







Parametric Littlebits Mounting system

TL;DR version: I got to borrow the littlebits synth kit, and created a parametric version of the mounting board. The source files are available on github:

The Newton Free Library applied for and got a cool LSTA ( Federal Library Services and Technology Act) Grant to do a bunch of STEM related programs, and I have the honor of doing a bunch of coding and robotics workshops there over the next few months.

They got a bunch of cool new equipment to run these programs including Finch Robots, Arduinos (Sparkfun Inventors Kits), a KIBO robot, and … wait for it… a bunch of Littlebits sets. NFL Assitant Director Jill Grabowski was kind enough to lend me the coolest littlebits set: The Korg Synth kit.



If you aren’t familiar, they are modular electronics blocks that snap together with magnets. Unlike earlier “electronics construction sets” these are pretty foolproof, well thought out modules instead of just individual components. As Founder Ayah Bdeir says ,


Unfortunately the thing that makes Littlebits easy is also it’s greatest weakness: The magnetic connection system.

Especially when making something you have to manipulate (like a musical instrument), they tend to come apart, you lose power, etc.

Now some of the other (more expensive) sets come with mounting plates (that you can also buy separately. I didn’t have one though, but I do have a laser cutter and a 3D printer. How hard could it be? Well, the devil is in the details.

First I looked to see if it was already done. Thingiverse (I know boo….) has a few designs, including some by littlebits themselves, mostly specific component mounts.

Rex Brodie posted this cool clip that fastens adjacent modules together.

It works pretty well (very solid) but it was difficult to put the two bits in without pulling and pushing and putting a bit of strain on the boards. (Especially scary when the ‘bits are on loan).

I tried just measuring and cutting holes with my laser, or a simple model with Open SCAD but I just couldn’t get the spacing quite right.

Thingiverse user Kris Kitchen had posted this design:

which I printed and actually worked quite well. Only problem was, it was an STL (Surface Tessellation Layer) file, and thus not easily modifiable. I wanted wider, arbitrary shapes, etc!

I could however pull Kris’s design into OpenSCAD, and do a trick to measure the “holes” and spacing.

This little bit of code:


cuts off the bottom so the holes will show and projects the outline in 2D.


I then exported this as a DXF file, and used inkscape to make measurements.

If I haven’t lost you by now, the part you are waiting for, the measurements (which I couldn’t find anywhere after much googling!)

  • Holes: 6 mm diameter
  • Spacing (edge to edge or center to center): 6.63mm
  • spacing is also 10.5% of hole
  • 1.294mm knockout line

The percentage bit is important (I calculated it, 6.63/6) if you are designing in inkscape because that is how you do a grid of evenly spaced objects.

I made a couple of attempts at a laser cut version, but it was very brittle, and I didn’t take into account the laser’s kerf (width of cut)

The 3D printed version came out pretty well, but the holes were a bit too small, so I enlarged them by .2mm (.1mm on the radius). and it worked perfectly!

Now we can make mounting boards in any shape!

The source files are available on github: