Force-directed graph drawing algorithms are a class of algorithms for drawing graphs in an aesthetically-pleasing way. Their purpose is to position the nodes of a graph in two-dimensional or three-dimensional space so that all the edges are of more or less equal length and there are as few crossing edges as possible, by assigning forces among the set of edges and the set of nodes, based on their relative positions, and then using these forces either to simulate the motion of the edges and nodes or to minimize their energy.
Force-directed graphs are a common way to display things like mind-maps. It’s a good way of spreading out the whole collection, while grouping related items together, and minimizing the path length between the closely related items. In my mind, this has a lot of similarities with how PCBs are laid out.
Well, there’s only one way to prove that theory.
Using KiCad PCB parsing code I wrote for another project, I was quickly able to grab the nets and footprints out of a KiCad project. Displaying the nets and allowing specific ones to be turned off was a feature I identified as critical early on, because the ground or power nets would overwhelm any of the others, rendering the important nets useless.
Truthfully, a significant part of this was wrangling TKInter to build the Python GUI that I wanted. It was a success, but I’ve never used it before, and I am not a fantastic UI designer.
Under the hood, the system essentially treats each of the nets as a spring, and applies a simplified version of Hooke’s Law to each connection. Each of the centre point of the footprints acts as a charged particle, and a simplified version of Coulomb’s Law acts upon it to repulse all of the other footprints. These algorithms are a pretty typical way to do this, by essentially setting up a physics simulation. One tweak on this strategy that is unusual, is that the nets don’t act on the footprint origin. They act on the pad itself, which allows a torque to impart a rotation on the footprint.
I’ve gotten this project as far as “fun tech demo”, and it’ll likely fall dormant for a while in this state. At some point, I will build an appropriate PCB using this technology, because I love making unusual designs that are electrically fine.
It’s a Craftex B1979C, which seems to be nearly identical to a Craftex CX704. Similar to many such mini lathes, actually, with varying swing lengths.
The previous owner explained, a little bit sheepishly, that he tried to cut steel a little too hard, and burned out the AC motor. He replaced it with a hobby BLDC motor and ESC, designed for things like quadcopters.
I played around with that system a little bit, and I didn’t like it.
Annoyingly, he bought a $50 motor and a $50 motor controller, both sensorless versions. For these kinds of motors, you need sensors to start up at low speeds with torque on the motor. That would have cost $60 for the motor and $60 for the motor controller – Not much extra for him, but annoying for me to have to shell out the full amount for both, and then have the old ones as somewhat useless spares.
Anyway, instead of going the same route, I chose to get a more sophisticated controller so that I could precisely control parameters, like speed, or acceleration. The options are essentially vESC or ODrive, unless I want to custom build something myself (I do not).
The ODrive, at least on paper, looked a little cooler. Some good communication strategies, lots of control modes, and generic enough to work well for this somewhat strange application. Right after I’d picked mine up, they discontinued support of the V3.6 and went to a closed source model, which rubs me the wrong way. The new version is also more expensive and has a single channel instead of two channels. One of the big issues with the legacy release is that there is a bug in their UART driver – If you send enough malformed packets, like, say, because you have a data line right next to a spinning motor, then the controller eventually stops responding to all UART data. That can be an issue when the packet you want to send is “stop now, immediately”. That definitely hastened my building of a separate e-stop box.
Fortunately, after I wrote a CANBus driver and used that, it’s been reasonably solid. That was just more of a time investment than I wanted to make.
I built a front panel for it as well. It’s a couple simple PCBs holding the switches, OLED, buttons, and a dev board. The advantage of not including everything on a complicated PCB is that I can pop out the dev board to flash it on my bench, before returning it to the lathe. The dev board is an ESP32, which is driving a little OLED screen that shows the target and measured speed, current, forward and backwards directions of the motor controller. The list of features I could add is nigh-infinite, but this is good enough for now. Oh, and I can do software updates over the internet! There is no way this can end badly.
It’s weird how sometimes three sentences can encompass four months’ worth of occasional project time.
The change gears the lathe came with are steel, which is unusual for a model like this. It came with a set of gears, from A-D: 20/80/20/80. However, this combination isn’t listed on the table anywhere.
The formula is ((A/B)×(C/D))×lead_screw_pitch/initial_ratio
where you have a 3mm lead screw, and it has a 2:1 ratio. So for example, 0.4mm pitch is ((20/50)×(40/60))×3/2 = 0.4.
That means the gears I have are about 0.09mm pitch, which is good as a powered feed, but not intended for any kind of threading.
Most of the other examples of this family of lathes use plastic change gears. Obviously using cast or machined nylon, but with a little bit of babying, I bet 3D printed gears would be Good Enough.
As near as I can figure, the change gears dimensions are module 1, with a 20 degree pressure angle. And a thickness of 8mm.
To practice threading, I copied a bolt that I had on hand, which was 1/2″-13 TPI, using the table to get 40/65/60/30.
Looks neat! But my tool geometry is bad and the angles are all wrong, so I reset, bought some more tooling, and moved on to something actually useful.
3D printed change gears work great! I’ve printed off a whole set, and I’m stoked. Arbitrary threads are unlocked.
I wanted to turn a gear shift knob, which has a thread of M12*1.25mm.
Using the formula above, the gears I need are 50/40/40/60.
The outer diameter of the stock is about 11.85mm.
This process ended up being incredibly drama free. Twenty minutes after starting, I had a very-slightly too-loose thread, but it snugged right up when I tightened on the knob.
I haven’t properly built anything with my hands in a while, so it’s time.
I mocked up a few lamp ideas, and this one seemed fun. Really, really rough mock-ups.
That was neat looking, so I fleshed out the hinge section, which I expected would be the hardest part to figure out and build.
First, I 3D printed the hinge, just to get a feel for if my sizes felt right – I am bad at judging size in CAD, things often turn out way larger or way smaller than I expect, and therefore really difficult to build.
These looked fine, though.
I picked up a ton of brass from a hobby shop. Can you guess what the main focus of the hobby shop is?
For the hinge pieces, I didn’t have the right shapes or size of brass stock available, so I needed to cut a bunch of smaller pieces and then braze them together with silver solder. And then took them to the belt grinder.
Pretty close!
And then tried to add one more piece, dropped it while the solder was molten, and had them splatter apart on the ground.
Okay one more time.
It’s always better the second time, anyway. I used an M1.5 brass screw as the hinge pin.
For the stand, I 3D printed some saw guides so that I could get the tube angles perfect. The stand wouldn’t end up with the right geometry unless the cut angles were just right.
Then, after brazing:
It matches up with my CAD perfectly, I’m stoked.
Some blu-tak was used to hold the hinge pieces and light backing together to get a sense of how it was all going to go together.
The light panel itself was a circle of aluminum that I spraypainted black. Then I riveted the hinge on and laid out my LED tape.
Wiring.
This could have been done much neater. Next time.
Encircling the LED panel, I bent some flat bar aluminum, riveted it, glued it, and painted it. Then glued on a plastic diffusion circle. This was all done quickly, so I didn’t take many pictures.
For the base, some nice dark wood would be ideal, but that would have required a fair amount of material acquisition and hunting down tools I wasn’t up for, for this project. So I took my base, and split it up into easily printable chunks.
I was going to attach them together, sand, patch, sand, and paint them to turn them back into one object, but I kinda liked the jigsaw effect, so I left it. PETG sands really well, and I treated it with a light oil coating to keep it from taking on fingerprints.
The pieces are just bolted together, and mostly hollow to allow mounting of the electronics.
For the controller, I have a surplus prototype lighting controller of dubious origin, and wrote some firmware to handle fading and mixing the two LED channels. The 24V power supply comes from AliExpress.
Similarly, for the knobs, I went for an easy and quick solution using 3D printing for the ends to retain a little bit of the brass tube onto the potentiometers. The left knob controls the light intensity, and the right knob controls the light’s colour temperature. This lamp can fade in between a really cool white and a really warm light.
Running the wires through the brass tube was a fun adventure. What I ended up doing was fill the tube up with oil, run a single wire up through the bottom, then solder it onto the other three wires. After that, I could use the first wire to pull the whole mess back through the tube. Then I flushed the tube with alcohol to clean it all out.
And, final wiring, and it works! It looks great. I can see all the small defects, but that’s okay. I’ll do better on the next one.
Voltage glitching, also called fault injection, is the process of dumping the energy on a microcontroller’s power rail very briefly – Just enough to cause a glitch, where it skips an instruction, rather than causing a brown-out-induced reset.
I first learned about the technique in connection with the Chip Whisperer, which is an FPGA-based board with all the bells and whistles. It’s quite expensive, and like any other situation where you add an FPGA, very complicated. Naturally, that has elevated it to a level above what mere mortals can perform in the limited free time we have.
After knowing about this, and having it in the back of my mind for several years, I finally have a good use-case that can justify spending some time on this. I have a pile of generic STM8-based devices, and while I have written some of my own alternative firmware, I’d love to be able to dump the original flash to revert them back to factory code.
The function of the device doesn’t matter, that’s not the point of this exercise.
Some quick searching for options leads to this recent write-up, which is sparse on details, but serves as excellent inspiration that this task is very doable.
A few architecture notes:
The STM8 has a Read Out Protection bit in the configuration area of its flash memory. When the programmer attempts to read out the flash memory, the bootloader first checks this bit, and if it’s cleared, it starts reading out the flash to the programmer. If it’s set, it just reads out zeroes. Write capability is never blocked – That is, you can still write a zero to that ROP, and then the microcontroller will be “unlocked”, but it does clear the program memory, too.
One of the pins on STM8s is called VCAP, and it’s attached to the internal voltage regulator. The CPU runs on this voltage rail, not the actual voltage that is provided to the IC’s power pins. The pin is intended to be connected to a decoupling capacitor, and that provides an perfect spot to inject my glitches. Most microcontrollers also have something called Brown-Out Resets: When the input voltage rails sags too low, the peripheral triggers, and resets the microcontroller. Obviously, this is something we want to avoid, and using the VCAP pin should help with that.
In terms of glitching, there are two important considerations:
The glitch must start at the same time or during the cycle in which the CPU is trying to read the ROP bit, and the glitch must not last long enough to trigger the BOR or to make any other important instructions fail. It’s not easy to know these exact timings, so any reasonable values must be tried, essentially brute forcing the process.
Now, the logical way to do this would be to use an external microcontroller to wait for the programmer to reset the system, wait a set period of time, and then trigger the output transistor to glitch the voltage rail. That’s boring! You know what else can do that? That’s right, a pair of 555s.
Here are two 555s set up as monostable pulse generators. The input is the RST line on the STM8 programmer. The first 555 then sets the delay. The second 555 sets the length of the output pulse. Both of these timings are controller by a different potentiometer. These then go to a MOSFET that dumps the energy stored in the internal voltage regulator cap.
After building it with larger-than-designed time values and testing it to prove that the waveform looks as expected, we solve the next hurdle:
To figure out decent ranges for the potentiometers, my STM8 board runs at 8MHz, which means that each clock cycle takes 125ns. The STM8 requires at least 2 clock cycles for each instructions (one for retrieval and one for execution), and more for instructions with multiple bytes or arguments. So, ballparking, we need a pulse that’s anywhere from 0.2us to 1.2us or so.
One problem with a typical 555 is that it can only generate pulses as small as 10us. Fortunately, I have a pair of high speed versions up my sleeve, the LMC555. It has a 10ns minimum pulse instead, which is very zippy. They’re SMD only, so they get popped onto a breakout board to fit the breadboard, and replaced. Some other components got tweaked too, as I played around more.
Now on to the programmer. I’m using a standard STLink V2, which speaks the SWIM protocol that allows programming and reading of the STM8’s flash.
With a little bit of bit of Python and stm8flash, we get this:
import subprocess
# .\stm8flash.exe -c stlinkv2 -p stm8s105?4 -r out.bin -b 1
out = b'\x00'
while out == b'\x00':
subprocess.run(['stm8flash.exe', '-c', 'stlinkv2', '-p', 'stm8s105?4', '-r', 'out.bin', '-b', '1'])
f=open("out.bin","rb")
out = f.read(1)
print(out)
subprocess.run(['stm8flash.exe', '-c', 'stlinkv2', '-p', 'stm8s105?4', '-r', 'out.bin'])
In PC applications, writing text to console is a surprisingly slow process, so a low-effort tweak I made to make the loop run faster is to remove the flash utility’s console logging, just by removing some lines here.
So, all set up, potentiometer on the left controls the delay, pot on the right controls pulse length.
And, bam.
Firmware dumping with a 555.
Well, not that fast. It took about 45 minutes of fiddling with the knobs until all its secrets were unlocked. I’d sweep the right knob the whole way, then tweak the left knob very slightly, then sweep the right knob again. It only really worked because the knobs only had to be within the right range for a very brief period of time. It only had to work once.
Would this have been easier with a microcontroller? Oh yes, of course. But that’s not nearly as interesting.
Here is a brief overview of how light and colour work, in the context of LED lighting.
We’ll mostly be discussing the CIE 1931 colour space, with reference to the chromaticity diagram, shown below.
This is the 1931 version. Newer versions that look slightly different have come out, but the general intent is the same, and they are all used for different calculations. The 1931 version is “good enough” and is universally the one that is referred to when a colour is described in xy coordinates.
The points along the edge are pure visible wavelengths (displayed in nanometres). Anything inside the diagram is some mixture of the wavelengths.
Computer monitors don’t have the same kind of visible range as your eyes, so the colour space diagram above is just a representation, not the actual colours that you would be able to see if it were displayed side-by-side with the actual light wavelength.
There’s a curve along the centre, called the black body curve, that represents “white” light. White can be many different things – Typical incandescent bulbs are around 2500K, towards the right (warm) side of the black body curve. Daylight is very cold, with lots of blue components, towards the left. As long as it’s along that curve, it can be considered “white”. The vertical-ish lines indicate where that particular temperature extends, if a given data point isn’t exactly on the line. Duv, or “delta u,v” is a measurement describing the point’s distance from the line.
In terms of white LEDs, there will typically be a “warm white”, also called “tungsten”, and a “cool white”, called “daylight”. As shown by the black body curve, if you have both LEDs, you can’t interpolate between the two of them and expect the light output to follow the curve. It will be a direct line between the two. There are a couple solutions to this, and an easy cheat is to explore the strategy that one of the industry leaders is utilising.
The Philips Hue tunable white smartbulbs are excellent. I’ve taken apart a handful of smartbulbs, and the Hue has the largest quantity of LEDs I’ve seen(read: brightest), as well as using a few tricks to get better light quality than any other system I’ve seen.
The daylight LEDs, labeled “CW”, and tungsten LEDs, in this case labeled “FW”, (we’ll get back to that) are fairly standard. In the centre is where it gets interesting, with the LEDs labeled “L”. This is lime green.
Here is a closeup of those LEDs plotted on an XY graph, with the black body curve, and an interpolation line between them:
With both of the tungsten and daylight LEDs located on the black body curve, fading in between the two will drag the light significantly away from what we perceive as “white”, essentially making the light look magenta. The lime LEDs, towards the top of the graph, are perfectly situated to pull the light output back onto the curve when the bulb is at an intermediate temperature. Some further investigation into the meaning of the LED designators on the PCB, “FW”, reveals that some LED manufacturers with that particular temperature of LED call it “flame white”. It’s substantially warmer than most bicolour white LED tapes, at around 2200K. Mint LEDs are also sometimes used for the same purpose as lime LEDs. Mint has similar spectral components to lime, but with a little more blue components, moving their xy coordinates a little more towards the bottom-left.
Here they are next to a high quality warm/cool (bicolour) LED tape:
From left to right: my daylight, Philips daylight, my tungsten, and Philips flame. Daylights are fairly similar, within binning and measurement tolerances. My tungsten LED is 2600K, showing how far the Philips flame LED is into the red spectrum. Another strategy for getting temperature that warm is to add an amber LED.
Tungsten, flame, and amber, left to right respectively:
Tungsten LED can be combined with amber to get a warmer (lower) temperature. Amber has an additional purpose: Standard RGB gamuts don’t have a whole lot of range in between the red and the green colours, which can be filled out with the added amber channel. More on this in a bit.
“Gamut” is the displayable area made possible by mixing the available colours. Displayable points drawn with each primary colour, and the available gamut is any point within the area.
Here’s a typical RGB LED tape.
And their spectral distributions:
This green has a slightly wider spectrum than the other primaries, so it doesn’t fit as nicely on the edge of the xy graph. It’s made up of mostly 520 nm and 540 nm components.
The MacAdam ellipse is a graphical representation of what observers see as “one colour”. That is, anything in the ellipses shown appear to be the same colour to a regular person. This particular image is only the results from one participant in the study, so it shouldn’t be taken as definitive, but it does show trends that can provide actionable insights. The shapes are also exaggerated by about ten times for ease of clarity.
The ellipses tend to flow in the direction of the blue-red line, for example, showing only a few different perceptible shades in between those two primary colours. Red to green however, particularly in the red to yellow region, are extremely granular. This is where amber comes in.
The amber gives additional granularity in the existing gamut region where our eyes are very sensitive, and in the space where the gap between the RGB peaks is largest.
Emerald green, at 560nm, is another LED that would help fill out the spectrum gaps, providing more range towards the green part of the red-green line. Emerald and amber doesn’t alter the overall gamut significantly, however. There are areas without coverage on all sides of our gamut triangle, but the area to the left of the blue-green line is the most significant. This could be supplemented by InGaN cyan LEDs, at 505nm. That will be the topic of future experiments.
For reference, here is a graph of all of the colours measured and discussed.
That follow up post (already written almost two years after the original work) promised project pictures that were never delivered, so here, I went spelunking in some nearly decade-old backups for them.
Recently, I came across the ESP32-Cam, which basically hits all of the cost/hardware requirements I was aiming for, nearly a decade ago.
It uses an Espressif ESP32, which I’m very familiar/comfortable with, and is about $6. The camera sensor is 2MP, which is small for images, but fine for video.
There are a lot of timelapse projects out there using this board, but none of them really have the features I’m looking for. Espressif even has an official “webcam” codebase that others have packaged up into cool projects. But here are my requirements:
A web interface to control capture / timing parameters
Serving up images over the web interface to check that those parameters are good
Saving images to SD card
Nonvolatile storage of the parameters, along with the current snapshot number, in order to gracefully recover from power failure
SD card images available to download over the web interface
Over-the-air updating
Reasonably well-laid out codebase that’s easy to hack on
Essentially, I want to be able to plug this into a hard-to-reach closet and not have to ever physically touch it again. Lots of projects have a subset of these features, but nothing has hit it all, that I have seen.
One of the other ideas I had to improve upon the typical timelapse design was to get the ESP32 periodically grabbing the real-world time from NTP servers. It’s set to run every half an hour, which is probably too often to make a difference that overcome the latency from hitting the NTP server. Regardless, it should keep average timelapse interval perfect over a long period of time.
So I hacked something together. Originally I used PlatformIO with the Arduino framework, because it’s quicker to get a project off the ground than the official Espressif ESP-IDF. The branch is still there, but I abandoned it after running into issues with partitioning, OTA updating, and the additional PSRAM hardware all working together. The main branch uses the ESP-IDF now.
It’s essentially a webserver that controls the camera. You can open up a page and modify settings, including timelapse intervals:
You can also get existing parameters:
And snap takes a one-off photo to check your setup without touching anything. That made this super easy to tweak with one hand, while holding a laptop with the other.
When a sequence is done, it’s easiest to grab the SD card and plug it into a computer to copy over the files. For longer sequences, maybe to check progress, it is totally possible to grab the images over the web interface. Just slow.
To stitch together all the images, initially I used this command:
ffmpeg is a little inscrutable, so the parameters that require tweaking warrant an explanation. Order matters, too – Some of the parameters affect the input, some affect the output, depending on position.
-r:v is the input frame rate, relative to the base (25 FPS) framerate. So a 250 frame video with -r:v 25/1 would be 10 seconds, while setting it to -r:v 50/1 would result in a 5 second video.
The -vf transpose option rotates the video. The ESP32-Cam’s camera is mounted sideways, but that’s a minor irritation.
To mount the device, tripods are the obvious choice. I created a quick, minimalist enclosure that allows access to the GPIO pins, SD card, reset button, and fits a 1/4″-20 nut that is compatible with most tripods. A Gorillapod, in my case.
For the video posted at the top of the page, here is the setup:
It was done with a picture taken every 40 seconds, 8 second exposure, low gain and brightness.
The -t option sets the end-time, there was a lot of “content plant” at the end that wasn’t moving much.
Future goals might include shoving the whole mess into a closet, along with timed control of the lights and a watering pump, and leaving it for a month.
This is a fairly old project that I just wanted to document to current-state and provide a conclusion. See here for the old logs.
Some of the original design goals for this project were ease of design, and manufacturabilty. Ideally, the design would be clean and turnkey enough for me to be able to at least sell the populated PCB as part of a kit. I also didn’t want to spend a lot of time doing anything cutting edge enough to turn into a time consuming side project, as some of my designs are wont to do.
So here is where I started.
The board is 7cm in diameter with 0805 side-mount LEDs all around the perimeter, green on one side, blue on the other. The LEDs were too easy to see individually (right), so I used hot glue to create a diffusion layer (left).
In the centre, I decided to go the easy route and make the board battery powered. The cutout can hold three LR44 batteries in series.
After some hemming and hawing about how to mount them exactly, I found an acrylic tube with a 12mm ID (just barely larger than the batteries), cut a slope into one side so that it can be inserted into the slot and then rotated to lock in. On the PCB, there is battery spring on one side, and some copper braid on the other to hold the batteries tight and make electrical contact. This solution works better than it has any right to, it is fantastic but blows my “manufacturable” requirement right out of the water.
For driving my LEDs – twenty per semicircle – I used a TLC5947. It’s a constant current, 24 channel PWM LED driver, controlled over SPI(a requirement for speed). There are two of them, and they seemed to work well, with one caveat that I’ll get into in a bit.
There is an interesting power section that I never ended up populating. The goal with it was to have a switch mode power supply attempting to boost the battery voltage to 4 or 4.5v from whatever voltage the batteries sag to. The idea being that with fresh batteries, it wouldn’t be doing much of anything, but as the battery runs down (internal resistance gets higher), the power stage attempts to pull more and more current to keep the output rail steady. The circuit ended up working fine with all of that jumpered over and direct from the battery, but it was an option in case that hadn’t turned out.
I also had a clever system on there to ensure proper polarity to my circuit regardless of how the batteries were hooked up, but again, didn’t matter enough for this version to use.
The TLC5947 has an SPI output pin to allow chaining multiple chips together, but naturally, it would halve the refresh rate I’d be able to get. I ran a trace between the two chips, just in case, but I was also able to avoid using that. The dual effective SPI ports I managed to get working(written about in a previous log) saved me from that.
Early tests were promising – I quickly got a recognisable image of the world showing.
Introducing blue turned it into an unrecognizable mess, though. All further testing stuck to green, for now. The problem is that my hot glue light diffusion is too good, and allows light to be directed back through the sphere. Some sort of baffle on the inner side of the LEDs is needed.
An additional problem that I hinted at in the last logs is that my vertical resolution isn’t really high enough. The world map is recognisable, if you know what you’re looking for, but it isn’t fantastic.
And the final problem, the real deal killer: the TLC5947 blanks its output before displaying the next value. That is, a channel displaying 255 (full on) when a new value is sent to it, even if the new value is the same. It will go 255->0->255. That’s why you can see vertical gaps in all the images I’ve posted. This is mentioned nowhere in the documentation, and is just poor design. So I future version will have to rethink this portion, too.
For the mechanicals, due to an unfortunate move, the old system got a little bit damaged and I 3D printed a replacement. This one also uses a dirext drive brushless DC motor instead of the old brushed motor and DC motor. Again, just for fewer components and faster to put together.
It worked well, other than some harmonics causing a lot of vibrations on some narrow bands of frequency. Future versions would also have a proper magnet holder, my hand is not a permanent solution.
I’m on another clock kick. More of a wristwatch this time.
The dream is to have a couple metal bars floating in oil, and when a button is pressed, they float to the appropriate hour and minute positions.
For an initial version, I’m paring this way down to just a single indicator hand.
On the mechanical side, I start with some preliminary sketches.
There were many more, with very little polish in any of them. They’re just to get the mechanicals down and direct the overall thrust of the design.
Watch crystals are quite inexpensive from China, so I picked up a selection of sizes, and while waiting for shipping, I modeled and 3D printed some tests for the three most likely candidates. Shown here are cases I designed for sizes 30mm, 34mm, and 38mm faceplates, but you really can’t tell that they’re different sizes in pictures. The 38mm one seemed to fit best on my wrist, so that’s what I went with.
Rolling my sketches into the CAD, I came up with this:
The main body is just a shell with internal threads, and inside there’s a stack-up consisting of:
1.2mm crystal face
37mm OD x 1.5mm o-ring
Double sided o-ring retainer
Another o-ring
15x3x1mm neodymium magnet
Faceplate PCB
Control PCB
Coin cell battery
Threaded cap
Dimensionally, it’s a very tight system. It doesn’t really fit with the battery, so stuff will have to be moved around. The o-rings are there to seal in the oil, which is kinda fun.
Jumping to the electrical side, I need some electromagnets.
A guy named Carl Bugeja has designed coils drawn out in PCB traces to make a PCB motor. He shows off the design a lot here.
Included, naturally, are a set of gerbers which were worth ordering to play around with.
I tossed a magnet on there and was able make it jump around the coils using a small bench power supply. The resistance at each phase was about 12 ohm, the same value that Carl Bugeja measured. Each phase does go through three entire coils, however, so assume 4 ohms per coil.
The coil pattern is illustrated quite well with Carl’s coloured gerber export:
Application of the right hand rule shows that as current goes in a clockwise spiral direction (curl your right-hand fingers in that direction), your thumb will point in the direction of the magnetic field, out of the screen. Interestingly, the ordering of the coils on the layers isn’t from top to bottom. Carl ordered it from layer 1(top)->layer 3->layer 2->layer 4(bottom). Probably to (very very slightly) increase the length of the current path and therefore the resistance of the coil.
To drive a coil with any polarity we like, we need a standard H-bridge circuit. A future version might do some interesting stuff with that, so the fine control that an H-bridge provides is kinda cool. Generally an H-bridge consists of a couple matched pairs of transistors of opposite substrate. Poking around, there’s a single chip MOSFET solution that looks suitable, the VT6M1.
So here’s a quick PCB layout of how that would work:
But of course, just dropping that into a board is boring! We’re going to make a pretty PCB, we might as well go all the way! I’ve spoken before about PCBModE. Using my tool to convert the above PCB into a compatible format, I pull it into PCBModE, cleaned it up, added some electromagnetic spirals, and a little bit of flair. There are solder pads to hook this PCB up to another PCB, containing all the brains.
Figuring out a compact way to drive all those connections is somewhat challenging. With six coils around the perimeter and one in the centre, a “drive high” and “drive low” connection for each side, there are 28 signals, plus power and ground. All of those need to be transferred out to a “controller” PCB, so I added those green teardrop solder pads. The idea being that pins are to be soldered onto them, and then pushed into through-holes on the controller PCB.
For the controller, after all of the through-holes that connect to the above-mentioned solder pads are placed, space is very tight! Investigation of various shift registers and their sizing yields no encouraging results. NXP has a cool line of miniature 74HC series logic, but even those don’t fit in my given space.
Fortunately, there is a nice STM32 part I was able to squeeze in.
Despite actually being designed in ECAD software, I think it turned out quite beautiful in its own way.
Some first revision woes:
In retrospect, this was a terrible way to design the electronics. By putting the MOSFETs on the faceplate PCB, I maximised connections needing to cross to the other PCB, and minimised space on the controller by filling it up with through-holes. By crossing with the coil connections directly, I would have needed only two wires per coil, instead of the six that the H-bridge is requiring (plus grounds).
An unregulated coin cell is just about the worst possible power supply for a miniature, portable, long-running device that can’t be opened easily, and leaks oil when it does get opened.
Additionally, I was rushed to get PCBs out and forgot to add a button to activate the watch face.
Next version, with more time, will include improvements for all of the above, as well as a proper external RTC, and LiPo battery circuitry.
Jumping back into the mechanical side:
Everything was 3D printed to test fit. Surprisingly enough, threads worked pretty well. There was a ton of stringing in the internal threads, though.
Initially, I was using a knife to clean those up, but that got old quick. Additionally, oozing from the print of the outer threads made it a little tough to mate for the first few cycles. So I printed a tap. You can see the taper on the leading threads, and the notch to clear out the chips.
That worked great! Passing it through the system a couple times made the threads work easily.
Here all the non-electrical components:
Here is the faceplate PCB:
And, assembled:
Testing the coils directly with a benchtop power supply once again made the magnets jump around.
And then adding the H bridge transistors and testing with a low current signal to control the direction of the magnetic field.
Status: super cool. Listen to that click!
For prototyping, everything is 3D printed. The goal would to mill it all out of aluminum and then anodise it dark blue.
The o-ring holder needs to be lathed or otherwise made out of a solid material, too. 3D printed parts are going to have a bad time if I try to use them to seal an oil filled cavity with porous plastic.
I got the endorphin hit I needed out of this for now, though – Some interesting concepts have been demonstrated, and perhaps I’ll come back to it another day to add polish.
This is a follow up from Infin1D, Rev 0.5, where I threw together a proof of concept in a few hours. This is the one where I spent some actual time on it and built a base that, while still quite rough, can be incrementally improved to arrive at a final product.
This miiight have been the original inspiration I saw, but the video link is unfortunately dead.
Right off the bat, I should mention that PCBWay kindly sponsored the 4 layer PCB for this. In addition to using them for most of my personal stuff, I get more complicated professional boards assembled by them as well. The quality, speed, price, and communication are all better than what I’ve gotten domestically. So I’m genuinely happy to plug them, they rock.
On to it:
I mentioned previously that I’d like to use a scanner sensor instead of the easy line sensor module I had before. That previous module was 128 pixels long, greyscale, and $20, but also well documented, ubiquitous, and easy to use (mechanically and programmatically).
A linear CCD sensor out of a scanner, on the other hand, is almost the complete opposite. It’s huge (107,000 pixels long), poorly documented, electrically hard to control, full RGB, requires external optics, and needs to be mounted to a circuit board. It is also only really available via Taobao, but it is about $7 in singles. Or at least, the one I chose is.
I started by scavenging a bunch of scanners and seeing what was inside. Most of them happened to be the Epson Perfection 1250. I guess it was just an insanely popular scanner.
Searching “Epson Perfection 1250 Service Manual” yields excellent results, so this particular RGB linear CCD is a 22-pin package called ILX555K.
The main processor chip on the PCB is the National (now TI) LM9833, which is a little ridiculous. It contains an analog CCD reader, DRAM storage, high current stepper motor outputs, a USB interface, and even extra helper functions like paper load detect. Basically a single-chip scanner brain.
Initially, I didn’t even have to do any proper hardware hacking (sad day), because there are Linux drivers to talk directly to the chip. It uses something called SANE, an easy Linux scanner interface. SANE is actually a really nice codebase to hack on. Well laid out, clear, and simple.
That file gets modified with the found USB vendor/product/etc info.
And you’re done!
Running scanimage will run the scanner, including homing and making the carriage move, but it doesn’t provide the granularity we need. The source for that provides detail around line 1862, probably calling feed() or reset() / initialise cmd. Sending the initialise byte for a long scanner homing sequence is still pretty high level, so it must be handled within the IC.
That was a fun detour, but ultimately not what I wanted out of the system.
So I threw together a really quick PCB, and I made many mistakes. But that’s okay, that’s what revision 1 is for.
And it works! My initial firmware can display images and I also wrote a bitmap generator to save to the SD card.
A rudimentary interface:
Obviously lots of bodges had to be made. Mostly because of the many voltage rails – the sensor has 5V signaling, while the CCD itself is charged with 12V, the microcontroller is 3.3V, and there’s the whole 5V / battery voltage mess. So this board is more bodge wires and hot glue than PCB at this point.
So how well does it work?
Well, here’s another difference between the original module I used and this sensor: The sensor doesn’t have an integrated lens.
Due to ubiquity, I picked up an M42 camera lens mount for a couple bucks and found a $20 lens on Craigslist. The M42 lens is an old 35mm standard, which is coincidentally just the size of my sensor. Used to be very popular too, so good deals can be had.
However, I have not yet designed a chassis, so anything the image captures will be a blurry mess.
One more issue is that I’m still tweaking the hardware around the analog output. The sensor datasheet recommends feeding its output through the base of a BJT transistor as a sort of transimpedance amplifier. It’s kind of a strange way of doing that, so I’m playing around with proper op amp methods to do that consistently and repeatably. That requires a fair amount more tweaking though, as the datasheet has no information on the nature of the output that is expected. That’s just a matter of putting more time into the investigation, but in the meantime, I have poor dynamic range.
So here’s the first test image that came out of it:
The scan is the red channel only, from left to right. Starting with something dark covering the sensor, and at a very obvious point in time, I removed it and put an object centre of the sensor. I would consider that a success, albeit not as pretty as I’d hoped.
Here’s the object:
In addition to the many above changes, some software updates I’ll write down the line are to integrate better the two obvious viewing modes: waterfall, and histogram. The histogram mode is useful for getting focus right, and then switching over to waterfall mode (as shown above) to see images.
Histograms, similar to the feature that is often available in DSLRs, could show the sensor input intensity as a vertical line on a graph, with pixel position on the horizontal. The steeper the histogram, the sharper the focus.
I’ve used a lot of solder paste, usually from one of those big syringes.
When depositing little tiny blobs of solder on a few hundred little tiny pads, two problems emerge:
That big syringe has a big plunger that really starts to hurt the heel of your hand after a lot of usage, especially when the paste is still a little cold from storage. And,
when it’s so challenging, accuracy of deposition goes down.
Yes, both of these problems sound really minor, but they’re actual problems! And solvable!
There are a few different strategies for paste deposition. One of them, which I had a mild infatuation with for a while but never tried personally, uses an air pump connected to a syringe. It drives out the paste with air pressure. Apparently, though, the compression of the air leads to inconsistent and ultimately disappointing results.
Another strategy is to make it electric and attach motor, drivers, and a control system on the back of the syringe. The problem I’ve had with this is that it makes the whole assembly heavy, and therefore difficult to accurately control. The syringe-mounted button also seems like it might cause a little bit of unwanted movement when it’s being pressed.
So, like all quick projects, I let it sit for years while I pondered the issue.
After deciding on a strategy that would allow me to put the least amount of effort into this as humanly possible, I bought the following:
DC motor with gearbox and threaded rod:
Brass heat-set inserts:
1mL syringes:
Copper tubing:
And a pile of electronics I already had kicking around.
I soldered an insert onto the brass tube, and then with the motor, I had a linear actuator.
Pile of electronics:
Neat.
I also glued the rubber gasket/tip to the copper tube.
The solder paste I had available is a couple years old, and really chunky/nasty. Solder paste has a shelf life, and paying attention to that is important.
Put it all together anyway:
Little bit dribbly at first:
But not bad for a first test:
Obviously not perfect, though. The mechanism stopped being able to force the crusty paste through the needle, and broke the (admittedly flimsy) 3D printed bracket when the motor tried to exit its situation.
Using the flimsy bracket as a mechanical fuse might not even be a bad idea to keep around for future versions. The real problem was the paste, it was toast.
This will definitely be tweaked a lot. I’ll add pullback to the software to prevent the nozzle dripping, and maybe some other features. But it’s a start!
