Monday, December 3, 2018

Joystick Color Wheel with 3 Op Amps

I love microcontrollers but I've seen one too many 'Raspberry Pi deployed in order to blink a light' projects. Don't they know you can do that without a computer? They might not know.

I was sitting at the hardware table at HackUMass and watching everybody check out Arduinos and Raspberry Pis and ignoring the transistors. So I thought I'd make a few simple circuits for demonstration. First was a simple flex sensor controlling an LED. Then I inverted the behavior--the flex sensor turned off the LED instead of on. Then I did the same sort of thing with the potentiometers from a joystick. It made sense to upgrade to an RGB (red green blue) LED. But there are two potentiometers (and one switch) on the joystick, and three colors in the LEDs. What kind of behavior would be most satisfying?

It was suggested that I implement a classic colorwheel. Three axes, set apart 120 degrees from each other, for red, blue, and green.

Okay, perfect--I can do that with some op amps. There are two axes (potentiometers) on the joystick and each is configured as a voltage divider. We need to make a weighted sum of the X direction and the Y direction outputs of the voltage divider to create the Blue and Green directions. The red is aligned with the Y axis already.

I went through a few plans for the design. I ultimately settled on the LM358N chip using a single-sided supply and a virtual ground. The virtual ground I set to half Vcc with a simple voltage divider, guessing that the joystick rests at half Vcc (might not be completely true).

The Blue direction sits 30 degrees below the X+ direction. X*cos(30) + Y*sin(30) implemented in a summing amplifier--that's the first op-amp. For the Green, I used the same calculation, but flipped the X axis using an inverting amplifier, so that takes two more op amps for the Green axis. There are two op-amps per LM358N, so that's two ICs.

Each axis controls an NPN BJT transistor (I chose the 2N4401 for this). The red axis is the output of the joystick Y axis voltage divider controlling the transistor. It looks like it would go in the wrong direction (-Y) but because the summing amplifiers also invert the value relative to the virtual ground, everything ends up working out.

Finally there are some potentiometers inline with the base resistor to allow calibration of the three color channels. I found I got best results when the lights are all on and balanced for a medium white in the middle default joystick state.

This description wasn't very detailed, but it isn't a tutorial. I'm hoping to create a series of high quality instructional resources in the future, and I'd create a module on this circuit as part of that. It takes a lot of time to create high quality content though and I just don't have time at this moment. So for now, if you want to make one, I'll leave the details of implementation as an exercise to you, the reader.

Finally here's the video. I put a piece of plastic on the RGB led to get the diffuse light, because it had a clear package. That made it much nicer to look at, but the video still suffers from poor dynamic range.

Wednesday, November 28, 2018

Ubuntu won't boot, waiting on dev-mapper-cryptswap

I updated Ubuntu 18.10 cosmic and of course everything was crappy. The trackpad started registered spurious touches of my palm in the upper left corner and made typing very frustrating. And then the machine started hanging with a weird window manager (GNOME?) glitch after I had it in sleep mode all night with the lid shut, so I had to hard reboot in the morning.

This happened twice and then it wouldn’t reboot anymore. It would hang on the purple screen with the word 'ubuntu' and some loading bar dots. I force rebooted (hold shift for GRUB) and selected the option 'Advanced' and chose a recovery mode option of the latest version. Then I could see that we were endlessly waiting on dev-mapper-cryptswap1.device and that is why it will not boot, not even in recovery.

I found several sites suggesting I edit the /etc/fstab file and comment out any lines talking about cryptswap. (to do note for myself for later: figure out if I should make an encrypted swap, or just live without the swap)

OK but if I can’t boot, I can’t access any shell! How do I edit the fstab file??? I found this answer:

But I was missing the context for it. Where do I enter that emergency boot parameter?

I was getting sick of typing google search queries in to my phone, so I went back to Grub and looked around. In the advanced options section, the text suggests you can press ‘e’ to edit the entry. Then I saw something like this:

setparams ‘Ubuntu, with Linux 4.18.0-11-generic’
    gfxmode $linux_gfx_mode
    insmod gzio
    if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
    insmod part_gpt
    insmod ext2
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root [uuid]
      search --no-floppy --fs-uuid --set=root [uuid]
    echo ‘Loading Linux 4.18.0-11-generic …’
    linux /boot/vmlinuz-4.18.0-11-generic root=UUID=[the uuid] ro acpi_rev_override quiet splash $vt_handoff
    echo ‘Loading initial ramdisk …’
    initrd /boot/initrd.img-4.18.0-11-generic 

So that seemed promising. I entered a -b into the /boot/vmlinuz arguments, as such:

linux /boot/vmlinuz-4.18.0-11-generic root=UUID=[the uuid] ro acpi_rev_override quiet splash $vt_handoff -b

Then hit control + x as the instructions suggested to boot. This doesn’t permanently change the options, but rather boots with this modified entry just this once. So I entered emergency mode, hit control + d as the instructions suggested, and I was in.

Back to the instructions from the answer from before:
In case this leaves the root file system read-only, you can run mount -o remount,rw / once in the shell.
I didn’t give this a try without doing that; I just assumed it was necessary, and that had I skipped that I would have found that I was looking at my file system in read-only mode.

So back to
The solution is to remove or comment out the "cryptswap" entries from /etc/fstab and /etc/crypttab. This can be done easily by editing the above mentioned files as commenting out the lines that say cryptswap by placing a "#" in front of the matching lines.
I did that, saved the file in nano (I forgot how to use nano, but the bottom of the screen suggested some commands, so I did the one for exit and then it asked me to type Y to save before exit). Then I restarted the computer, I believe with the command shutdown, and then pressing the power button afterwards to reboot.

Then I figured before I got back to work, I could make a short blog post out of it. Here you go.


Tuesday, June 5, 2018

CRT and Magnets Exhibit

This Cathode Ray Tube + Magnets exhibit started off with a couple of CRT monitors that were gathering dust in storage. I was asked to consider putting them out in the main space of the makerspace. I decided I would only allow this if they did something. I set out to decide on what that something would be. It turned into a fun, easy, accessible to all ages exhibit that we now turn on for all the tours we lead through this makerspace. It is a great way to quickly and cheaply construct a meaningful interactive science exhibit to add to your collection.

Here's a document I put together to explain what's happening. I taped this to one of the TV antenna (the antenna is not used) so it stays front and center to the exhibit and people are encouraged to actually read it. Here's a lower quality image of the same document so you can see it embedded in the post.

Here's the bill of materials:

  • CRT television. You may have to turn to eBay or Craigslist. These may only get harder to acquire with time.
  • If the CRT has VHF/UHF inputs, you'll need a box like this one to convert the signal to composite video:
  • For the camera, the backup camera is cheap and outputs a composite video signal over RCA connectors.
  • If you'd like to place the camera elsewhere, you can get a cheap 2.4GHz transmitter like this one
  • A powerful neodymium magnet. You want something strong enough to have an effect and a good size to be easy to handle. Something like 0.5 inch diameter and 0.5 inch height seems like a good size to me, but I'd suggest just seeing what's available and trying it. You can always stack up multiple smaller magnets.
Assembly is just a question of plugging everything in to power and getting the signal in to the TV. Note that the backup camera requires a little work to plug in; it is designed to be hooked up to a 12V car battery connection point. You will need a 12VDC wall adapter if it does not come with one. You will need to make sure you have the red/positive connector and black/negative connector going to the right places. You will probably need to solder at least one connection, or use another strategy to connect the wires.

The magnet should be protected with something soft to avoid it hitting against metal and breaking or pinching fingers. I used two furniture feet and some masking tape. I also suggest putting it on a string so it doesn't wander off.

Cable management was the longest part of the project. I zip-tied all the extra length of cabling in back of the assembly. A single switch controls the extension cord to which the entire unit is plugged in. Note the CRTs make a high pitched noise that some people don't like to hear all day, so I don't leave it on all the time, since the main room of the makerspace is also a meeting and study space.

The cameras are taped to the top of the CRT and pointing at brightly colored pieces of paper. This is important because the magnet effect is not nearly as visible on black & white images.

An additional, optional modification I made to the back up cameras was the removal of the infrared lights that it contains in order to provide better visibility at night. The camera was very warm and when the IR lights activated, which sometimes happened if the exhibit experienced low light conditions, the image was washed out. I opened the unit and de-soldered the IR LEDs. My original plan involved putting gaffer's tape over the IR LEDs, but that seemed to make the camera heat up even more. There is a sensor inside the unit and in bright lighting the infrared LEDs are not activated, so this is not necessarily something that needs to be addressed for the exhibit to function.

It is also important to note that the backup camera is designed to mirror images. Note the sign in the image above is printed as a mirror image in order to show up correctly on the display. The backup distance overlay is another artifact of the choice to use a backup camera; I find it is fun and adds to the color distortion effect since it is displayed in bright colors.