Sunday, May 7, 2017

FRC Machine Shop with Sector67

In April Sector67 volunteered to run a machine shop for both FIRST Robotics Seven Rivers Regional and St. Louis Worlds events. It had been a while since I participated in FRC in any form (though I did mentor an FTC team this year, which is another FIRST competition).

Here are some photos. Since I only took photos when there was a break in the work orders coming in to the shop (usually because there was some mandatory attendance event for teams), this photo set gives the impression that we were a lot less busy than we actually were.

Seven Rivers Regional one view of the machine shop

Seven Rivers Regional game field

Teaching lockpicking at the table photo left

St. Louis Worlds Machine shop front side, 3D printer and laser cutter with fume extractor

More stuff in the St. Louis machine shop setup

View of the St. Louis playing fields in the convention center arena

Monday, April 3, 2017

Giant Display and Home for my First Arduino

The extra large (6.5") seven segment display array pictured above has been at my desk for a long time now, but I never posted about it. Remembering how it came together is a fun walk through memory lane.

Central to this project is my very first Arduino. My boyfriend at the time bought it, spent an afternoon on it with a friend, and then decided he didn't want to do anything else with it and asked me if I wanted it. It is an Arduino Diecimila, and this must have been around 2008-2009, because I remember one of my first projects with Arduino was interfacing with an ultrasonic rangefinder and an optical encoder that year. It later become Swervy's brain. Memories of my freshman dorm remind me why students need hackerspaces:

I was a really bad roommate...I shared this room with my unfortunate roommate. The dividing line is supposed to be roughly in between the black backpack and black storage cube.

My 'hackerspace' at the time was a $99 refurbished Ryobi drill+circle saw, a $30 set of Ryobi drill bits,  a Radioshack soldering iron, and misc. things taken out of the dumpster from USC, along with some sensors I got from my FIRST robotics team and the aforementioned Arduino Diecimila.

Sophomore year of college, beginning of the spring semester, Sparkfun has a big promotional event/server load test. They were giving away one hundred $100 free orders (Sparkfun Free Day). I skipped that morning's Calculus lecture and hit refresh on the page until I won my free order. The four 6.5" red seven segment displays were in my cart for that order, along with a few other things including a Simon Says kit that taught me how to do surface mount soldering.

At some point in 2015 I dug up these parts and put it all together. The circuit is just a bunch of BJT transistors and some buttons to set the input. I never figured out what I wanted it to do, so I put it up on the wall and update it as a calendar, manually, whenever I come in to my office. The Arduino doesn't have enough output ports to handle all the digits at the same time so it enables/sets one at a time, repeatedly and very quickly. Very common solution to a common problem, but it was a fun afternoon project anyway. The wiring harness took the most time. I think I glued the whole thing to the wall with no consideration for whether I'd want to move it in the future. I'll have to tear it off the wall eventually, since Sector67 is getting a new building. Chris jokes sometimes that it looks like what the TSA might think is a bomb; this clock slightly predates the recent clock incident. Which also reminds me of this page which left a huge impression on me when I first read it in college and turned me into a fan.

Friday, March 31, 2017

Telegraph Sounder Player

Yesterday I put the finishing touches on my latest project, the telegraph sounder player.

Video link first if you want to see it in action before you read:

The device on the left is the telegraph sounder player, and the device on the right is the telegraph sounder. The telegraph sounder itself is a vintage telegraph module. The player allows for up to four sounders to be plugged in to it, and plays back random snippets from text files loaded up on an SD card. It uses American Morse Code (as opposed to international). I loaded the SD card with excerpts from my favorite novels as well as full novels from Project Gutenberg.

The project was made for the owner of the telegraph sounder, Don. Don used to work as a telegrapher for a railroad company (US land based telegraph operations used American Morse code). He had a first version of the player built for him during the most recent Build Madison event and came back to Sector67 with notes for some features he would like for the next version.

Features I included in this telegraph player include: SD card playback, 4 sounder support, knob for speed of playback, separate knob for inter-sentence interval length, status led and ON/OFF toggle, CNC milled hardwood box with brass corners, battery power (est. 10 hours of playback), battery charging circuit onboard.

Design process in brief:

I had four 5V relays I found in a bin here at Sector, so I chose to support up to 4 sounders and to use a 5V Arduino for the brains. The relays are each switched by a BJT transistor circuit, and when switched they connect the sounder port to the battery pack.

The ports are barrel jack connectors with a 10uF capacitor across the leads and a high power rating. low resistance resistor. The resistor is mostly just in case the leads going to the telegraph are placed so that they are touching each other.

I had been playing around with trying to minimize the currents induced by the sounder turning off (the clack, as opposed to the click). A large ferrite choke close to the telegraph with many windings of the connector cable worked, but was a little unwieldy. The capacitor doesn't entirely solve the problem, but helps make the clack sharper. Too large a value and the clack actually bounces presumably as the capacitor charges on the induced current, and then discharges back into the sounder and re-enables it somewhat. The 'problem' is in any case limited to the Arduino resetting only when connected to a computer during the disconnection of the sounder, so it was possible to ignore. 

The SD card is connected through a bi-directional level converter. The Arduino has limited memory, so the code handles things by grabbing small chunks of the files into four small buffers for the four ports. The Morse codes are stored as 2-byte unsigned int objects, with the exception of a few special 4-byte unsigned long objects for some of the punctuation. The code can be found here: Arduino Code.

Access to the Arduino is through the bottom of the box, which is attached with screws to corners that are glued inside the box.

The project uses two battery cells that each have overcurrent and undervoltage protection built in. The charging circuit is as described here.

You can find more photos of the project and an additional write up here:

And a video here:

Friday, March 17, 2017

Painting Robot Compilation

A while back somebody donated an old robot arm kit to Sector67. I built a small model of it with glue, tape, foamboard, and potentiometers, and hooked it up to an Arduino. The project is very simple, but it has been very popular at science fairs (and has survived many hours of use requiring minimal fixes only). Here's a compilation video.

Friday, March 10, 2017


Another set of magnetic tile polyhedra. This time, three different trapezohedra.

I used this resource to define the kite and dihedral angles, but the hexagonal and heptagonal did not work out (and I believe the octagonal also has issues). Fixing them is on my to do list. I plan to write a script that actually generates them from the antriprisms.

Tuesday, March 7, 2017

Talking Calipers 2

Preface: Voting on a contest I submitted this to is open right now. You can vote once a day through this link.

Before going all in on the research for my minimal-cost minimal-footprint all on one custom PCB version of the Talking Calipers, I went through my bin of parts and made one more with just what I had already available. It uses:

The video demonstrates that the reading is taken when the button is pressed, so the display can potentially change to reflect that the measurement has changed since the reading began being spoken aloud. The button is a limit switch super glued next to the thumb wheel.

It was a useful learning experience to build, and it was good to have it done quickly for demos and such, but in the end I built something very, very similar to the Adafruit Wave shield ( but slightly more expensive.

Feature list (all were tested on the breadboard, though some haven't yet migrated to the somewhat portable prototype):
  • Rechargeable battery on board, recharges via USB
  • Separate amp with lower gain for headphones for hearing safety
  • Headphone detection for automatic headphone/speaker output switching
  • SD card for easy file loading--can be made to work in any language by switching out files
  • Relay mutes headphones to reduce pop
  • Sleep mode that wake up on request for a reading, for longer battery life
Features not yet added:
  • Filter to get rid of a hum caused whenever the SD card is read
  • Low battery indicator (through sound or vibration ideally!) (though thanks to a circuit built in to the battery header, it will turn itself off to protect from under-voltage to the battery)
  • Inches mode protocol is different and currently reads off incorrectly
  • Audio files and code support for reading numbers more naturally (currently reads off each digit separately)
  • Change sleep mode to trigger only if the readings are constant and the button is not pressed for a certain time interval (right now just based on button)
I actually recorded my voice for this project since I couldn't find everything I wanted online. I used to create the files and then used to make sure the files were 44.1kHz sampling rate and 16 bit sample size and then volume adjusted to a max volume that did not clip. (Note: the wav shield played most any wav files I could find fine, but the Teensy audio library was picky about the settings, necessitating use of the sox tool).

I estimate the battery life at about 10 hours of use (assuming it never enters sleep mode). The main power switch does lose the zeroing, but is convenient to have (out of the box, these calipers draw enough current to drain the coin cell in half a year to a year, unless the battery is removed). Auto-entering sleep mode when the user is away helps extend the battery life even more.

Monday, February 20, 2017

ThinkPad X200 Tablet as peripheral device Attempt 1

The goal of this project is to turn a "Wacom Penabled" Thinkpad Laptop into a peripheral screen and input device for my main Windows PC machines.

This documents just the first attempt, as not everything is working yet and I need to move on to other projects for a while.

The starting point: I have a Wacom Penabled ThinkPad x200 Lenovo Machine, pictured below. I read the following articles to figure out how I might go about my goal: article 1article 2

The instructions from the article above were not sufficient for my setup and included some steps I skipped. Below are the steps I took to get as far as I have so far.

I installed Ubuntu (latest desktop version) on the Thinkpad machine. It installed cleanly alongside my existing Windows 7 install without need for any other tools (such as WUBI  mentioned in the articles). The pen worked out of the box on this system.

I verified the port of the pen with the following command (the port is /dev/ttyS4 for me)
    dmesg | grep ttyS 
I installed ser2net:
sudo apt-get install ser2net
And changed the configuration file located at /etc/ser2net.conf and added the following line at the bottom section:
    7000:telnet:600:/dev/ttyS4:38400 remctl NONE 1STOPBIT 8DATABITS -XONXOFF -RTSCTS -LOCAL 
I added my username to the group dialout with this line:
    sudo adduser myusername dialout
At this point, I was not able to see the output of the pen on port /dev/ttyS4 because all attempts to access manually it gave me a resource busy error. I fixed this by commenting out the second to last line in /lib/udev/rules.d/69-wacom.rules. This steps should require a restart of the machine afterwards in order to have effect. The line to comment out reads as follows, with comments:
    # comment out the next line if your distribution does not provide systemd
    # If a /dev/ttySx device with the WACf name is detected, start the
    # wacom-inputattach service with the kernel name as parameter
    # SUBSYSTEM=="tty|pnp", KERNEL=="ttyS[0-9]*", ATTRS{id}=="WACf*", TAG+="systemd", ENV{SYSTEMD_WANTS}+="wacom-inputattach@%k.service"
Now it is also necessary to disable the Wacom pen services from xinput. If not done, streaming the data out with the cat command will make the computer behave erratically, and probably caused the kernel panic I experienced once as well. To list the devices, use command "xinput" which will return a list with ID numbers listed to the right. I found three Wacom entries under Virtual Core Pointer. They were the stylus, eraser, and touch. They had IDs of 12, 13, and 14 respectively. I disabled all three with commands as follows. These should be put in a start up script, as otherwise they must be manually entered with each start up of the system, possibly including log ins.
xinput disable 12
xinput disable 13 
xinput disable 14

At this point the pen won't work on Ubuntu anymore, which shouldn't be a problem because the goal is just to get the data over to the Windows machine anyway.

Now I can finally test the device locally. I entered the following command and saw data (unintelligible gibberish symbols as displayed) come out whenever I touched the stylus to the screen.
sudo cat /dev/ttyS4
Now I opened up a PuTTY session on the Windows PC . I set it up to 'Connection Type: Telnet', 'Port: 7000', and host name as the IP of the Ubuntu machine on the network which I identified using the command 'ifconfig' on the Ubuntu machine. The connected network interface, which for me is called wls1 and indicates my wireless card, states the address on the second line. It looks something like the following:
inet addr:
With the address entered into the 'Hostname (or IP address)' field I hit the "Open" button and saw that whenever I moved the pen to the Ubuntu screen, unintelligible symbols appeared in the window. Excellent. [Note: close the PuTTY winow after this test]

The only remaining steps were to get the Wacom modified tablet drivers to work and install HW VSP. I only made half-successful progress here, so I won't write it up yet in detail. I followed the instructions for the drivers from article 2. I tried this on two Windows machines each running Windows 7.

On my desktop computer, the Wacom drivers (installed per the instructions on  seem to be the issue. I get a complaint about them every time I boot the machine, and the HW VSP says "connected" but all the packets end up just adding to the number in the Queued section, and the VSP fields other than Status all remain blank. On the Windows 7 laptop, I do not get the Wacom drivers complaining at me, and the HW VSP window shows some values for all the VSP fields, as well as incrementing values for Rx and Tx packets in the VSP column. But the pen on the Ubuntu machine still does not move the pen on the PC.

For now I'm shifting gears away from this project, but hopefully I can get it working in the future. The Windows drivers are clearly the problem, and I've tried a lot of things, including other drivers, as well as messing with the registry and the driver files. I might have more success going through a second Ubuntu machine and using Ubuntu tools and drivers here. But the point is in part to get this working for a Windows PC, just because of my software and OS use patterns (I develop on Ubuntu, but use design and entertainment software on Windows).

If you think you have a solution let me know!

Wednesday, February 15, 2017

Simple Educational Robot Arm with Inverse Kinematics

The point of this project is to test a lesson that covers the theory and construction of a simple, two degree of freedom robot arm that moves in a plane and is controlled by specifying the X and Y position of the end of the arm (the 'hand'), rather than by specifying the angles of the two servos directly.

I ran most of this lesson last year at a summer camp, but due to limited time/high variance in math background of the students, we weren't able to reach the point I wanted to reach. (Really what happened is that I volunteered to have another class join my class for three hours for an activity, which meant a lot of my prep time ended up being soldering leads on more potentiometers...)

This activity seemed promising so I went through it again, at a less hectic pace this time, to try and make a list of the tricky 'gotchas' that would be useful to provide as hints if the goal was to have most of the students complete the project in the class time allotted.

Students who have completed a class covering trigonometry should have sufficient mathematical background to cover the math, which is described nicely on this page:

A partial list of the tricky parts students may get stuck on (beginners tend to try to build the whole thing at once; a few compound problems can make it very difficult to debug):
  • Constructing the arm so it is sturdy enough to last until demonstration time.
  • Including math.h and finding documentation for C trig functions
  • Keeping track of radians vs degrees
  • Measuring link lengths accurately and setting up the physical arm to match the model (the arm should physically be perfectly straight  line if both servos are at their max range
  • Mapping the analog inputs (reading from the potentiometers) to a reasonable range of the X and Y axis, in accordance with the units chosen for specifying the link length in the code
  • Dealing with floating point error--floating point operations are not the optimal solution here but the goal here is to avoid adding more complex C programming concepts onto an already dense lesson. See the code for more detail. A good summary of the issues can be found here:
  • Using a separate power supply or filtering the Arduino power supply. The arm can otherwise get stuck in a cycle where servo moves, drawing a lot of power momentarily, which causes the analog inputs to jitter (and the onboard LEDs to flicker), which causes a new reading which causes the servo to move... Most students will use the separate power supply if it is provided, but it is also common for students to forget and tie everything to a single rail.
Here's an illustration of an issue that will happen if the student mounts the second link so that it cannot fold inwards. The green area on the left shows where the end of the second link (the 'hand') can reach. The shape is awkward to navigate. The right hand side shows the area reached by a better choice of angle limits for the second link. If it can fold inward, similar to how a human forearm/elbow/upper arm works, then it is easier to use. (This image assumes links are equal length).

I did mount the second servo flipped downwards, to reduce overall height of the assembly. That was a mistake in hindsight, because it means the arm can collide with itself.

Here's a video of the test arm I built, demonstrating that it more or less works correctly.

Finally here's the sketch. It isn't clean or commented...but I've decided it is better to share these things than to never get around to it at all.

Arduino Sketch for this project

Tuesday, February 14, 2017

Talking Calipers 1

At a recent Sector67 monthly meeting, a member who teaches at a school for the blind and visually impaired mentioned a need for talking calipers. They are hard to find for sale and they typically cost in the realm of $250, though if you have a link to something significantly cheaper please let me know. My overall goal is to design a cheap little add-on that slots on to standard calipers that have a serial output already. This post is about the first step: a quick talking calipers proof of concept. Here's a picture of the working setup. See bottom of post for a video of the setup in action.

Here's what's in the photo. Everything came from either my collection of prototyping stuff, or from the collection of a fellow Sector member who took interest in this project.

If you add up the total cost of the items listed above, it comes out to about $ already cheaper than the $250 price point. (I'm aiming to get the total cost down a lot further than that though).

Here's how it all works together. The calipers are bumped up from their usual 1.5 volt voltage from the coin cell, to a 1.9 volt supply from the breadboard power supply. This makes it so that the logic level converter works with the signal from the calipers (because the minimum signal it will support is 1.8 volts, so 1.5 is too low). The signal from the calipers is easy to access; there's a port on the calipers accessible by a removable plastic cover. On the other end of the logic level converter we have the Arduino at 5 volts.

The wave shield comes with it's own example code. The calipers communication protocol has already been reversed engineering by a number of folks online; I got my code snippet from

The main challenge here is that the Wave shield uses all the external interrupt pins on the Arduino. I needed to use a different kind of interrupt. After a brief read through the forums and the datasheet for the Atmega328P, I decided to use the analog comparator. Comparing against some code snippets made it a fairly easy job to set all the registers I wanted to the values I needed. I used the internal bandgap reference as the positive input to the comparator and my calipers CLOCK signal as the negative input. Then I set the interrupt to trigger on rising output edges. [Noting that the positive input to the comparator, Atmega pin AIN0, refers to Arduino pin D6, and the negative input to the comparator, Atmega pin AIN7, refers to Arduino pin D7.]

The code for this project is available here.

Here's the video:

Next up I'll tackle building a prototype of a cheap and portable version.