2019-09-09

My first experiments with the EleksMaker A3 Pro laser engraver


It all started while following some links...

I was having a look at a review for a new single board computer (I don't remember which one) with a link to the Banggood site, and in the suggestions I saw some inexpensive laser modules. Clicking on them by curiosity revealed this appealing laser engraver that made me wonder whether I'd really need it... It turns out that I've been imagining for a very long time that I could possibly make PCBs either by directly printing on presensitized copper boards, or by painting a board in black and then burning the paint. At this price (around 200 EUR then, it went down further since), I didn't risk much so I decided to order it.

Receipt and assembly

I received it in a quite small parcel, the kit is made of lots of small pieces and long, thin bars. There's no ground plate in it so you have to plan putting something to protect your table. The assembly instructions are a bit scarce, they're made of series of photographs with almost no text in EleksMaker's wiki. But actually the photos are well detailed and almost everything is pretty straightforward. My friend Benoit's kids were impatient to see it so I thought it could make a good week-end activity to let them assemble it, thus I waited for the next week-end and verified I had everything available. On the next week-end, they came and I opened a browser on the wiki with the instructions and let them do the work. They're 8 and 11, don't read English, and it took them two hours to assemble everything with no help (actually yes, they asked me to tighten the screws of the controller board as they were afraid to risk to damage it). I was impressed by the result, they even managed to properly tighten the wheels on the bars so that the motors could move smoothly without shaking. It's really nice when you see that such a device can be assemble by kids, it means the instructions are pretty clear and universal!



There were some extra parts at the end, that reminded me a re-assembly of something I'd have disassembled... Initially that worried me a bit and I realized that it was on purpose, in case you break or lose one. Such extra screws and nuts are welcome for later improvements. And as a small plastic box is provided to hold them you don't risk losing them.

The part on the cables assembly is not obvious, it never appears on the photos. We tried different approaches. Leaving all the cables floating around is not convenient because they tend to catch anything around. Passing them all in the provided plastic roll makes too rigid an assembly. I finally figured that passing the Y motors' cables inside the lateral frames and attaching the laser and X motor inside the plastic roll was almost the best solution.



Similarly, the controller board isn't well placed: on some photos it appears vertical, which is the best solution for the cables. But the screws that hold the PCB prevent from centering it and you have to push it much lower, where it prevents the device from touching the table. We placed it horizontally and it used to push the cables below the laser beam, not a good idea either. Finally I moved it vertically again after removing two of the 4 screws that hold the controller board. They're not critical and it still holds well in place this way.

Build quality

I was really impressed by the build quality, especially for the price. I had expected something a bit flexible or shaky but no, it's extremely rigid, everything holds well in place and moves smoothly. There are no tension areas and the axis move perfectly parallel to the frame.

I don't like the cable passing method at all. The one on the sides is far from being perfect, with the cable falling under its weight and occasionally getting stuck between the plastic block and the frame. It doesn't happen often but when it does it ruins what you were doing. And the plastic spiral to hold the X motor and the laser cable together isn't completely smooth due to being rolled around the cables, and tends to occasionally force either against the plastic part of the controller, or against the X motor's fixation. I found that making it arrive by the right of the motor and using the laser's cable to hold it in place is what gives the best results. But it can happen, I'd say in 2-3% of the operations, that you hear your motors "jump" and miss a step because this spiral got trapped somewhere. This really is not anything to worry about at all, everyone will find their own solution to this.

A support is missing, and you have to put something riskless under the engraver. I initially used a glass plate on top of a thermal protection for ovens. Then I made a support using 2 aluminum corners to serve as a support for the glass, thinking it could also serve as a ruler, but it immediately became much more complicated to align stuff on this because we'd lose some degrees of freedom:


Then I thought that some marble could be nice as it would cast the light once the material is cut without risking heating, but finally went for a thin 40cm*40cm concrete slab apparently made for walk paths in gardens. And it was very cheap (4.5 EUR).

A few security rules


If you consider that you'll be careful and will never damage the table under the machine so you don't need a protection, you're wrong. The laser is turned on during resets or certain operations. It can also hang turned on due to a software bug. You definitely want something which can dissipate 2.5 watts of power in a 50th of square millimeter. That's roughly 125000 times the light density that reaches the ground in summer under the mid-day sun (at 1000W/m²). By the way, protection glasses are MANDATORY.

They are mandatory for two reasons. First because you never know if you'll get a reflection. Second because the light point is so small and concentrated that you don't feel blinded by staring at it, but it destroys your eyes. I found that working on the device one evening despite wearing the glasses was straining for my eyes. So you really want to be careful. And take care to work a very bright room, this will further protect you by having your pupil almost closed, compared to being wide open when working at night. The glasses provided by default do not offer a very good protection to be honest, but you can actually watch a piece being made with them if you don't stare too long. Red ones are easily found on the net and protect much better. However they protect so well that you don't see much around and you develop a higher sensitivity (your pupil extends). So in the end I found a better comfort with the green ones. It's possible to stack them on top of each other for an increased protection but then you won't see anything else.

I found that illuminating the piece being etched with a strong concentrated beam of powerful light helps a lot while observing. You even get a clearer image.

And if you're working on something too bright or with reflective parts inside, don't look at the beam, use a camera instead. You can easily replace a dead camera, you cannot easily replace a dead eye.

Very first tests

The difficulty starts right here. Laser engravers, like 3D printers, use a language known as G-Code to describe the moves. I've never used G-Code before, and have zero experience with the required software. The vendor's site suggests to use their EleksCAM software, but I couldn't find it on their site, it looks like you have to create an account on the forums to find the download link, and I'm definitely not going to create one other account on yet-another-site just to download a piece software, which apparently is only for Windows, which I don't have.

Digging on the net quickly drove me to a number of important software parts:
- GRBL G-Code parser and CNC controller, which is the software running inside the controller on the board. It is important to know this because there are some variants betwen language implementations and it's better to make sure other software are compatible. The device is shipped with version 0.9 of the software.
- UGS (Universal G-Code Sender), which is a Java-based application made to send some G-Code to a device over a serial port
- J Tech Photonics' plugin for Inkscape, allowing to convert any vector drawing to G-Code

I also found quite a useful test file called "test_pcb" on this page, producing the test pattern appearing below (I hope it's OK to show the image here):



It brings the benefit of allowing me to first test the hardware and low-level stuff before having everything ready.

I found UGS "classic" to work "well enough". It's a bit heavy and you hear your laptop fan spin when it works, and it spews hundreds of Java backtraces into your terminal while it operates, but it has the merit of being cross-platform and apparently not crashing. Also it gives you access to the G-Code console where you can send directly commands to GRBL, so it's convenient to use to report some settings, move the head or turn the laser off/on.


I found that the "reset zero" operation didn't work, but this was a limitation of my old firmware (0.9) which was later addressed. So I just had to first go to zero position, then move the axis by hand where I wanted. I figured moving the axis by hands is not a good idea because the controllers support a mechanism called micro-stepping, and when you move the motors by hand, you simply "reset" all possible micro-step that the motor might have engaged. Also it's possible to get the left and right motors slightly shifted, thus having the X axis not perfectly parallel to the frame. It's better to use the arrows or X/Y buttons to move the head, then power the laser off, click "goto zero", and power it on again.

Then I wanted to give the J Tech Photonics plugin a try. I downloaded some test SVG images from the net. These are not always easy to find, most links are squatted by vendors who want to charge you for a small image, or by links to this crappy Pinterest site which suddenly sends you a modal pop-up forcing you to either create an account or click "Back". Despite this I easily found various ones including an HAProxy logo, an ant, a ruler, an Eiffel tower Kirigami (to make card board unfold in 3D), and a lot of freely downloadable and well categorized images here at onlinelabels.


Selecting everything and clicking "Extensions/Generate Laser Gcode/J Tech" proposed me a number of options to generate some usable G-Code. I experimented with various speeds, numbers of passes etc. The "travel speed" is the maximum speed at which the axis may move when the laser is off, and "laser speed" is the maximum speed at which the axis may move when the laser is on.

The travel speed depends on the hardware build quality and motors power and precision. Traveling too fast results in imprecise movements. I found that I shouldn't go over 2400mm/min to keep accurate moves. That's 40cm/s so it means one second to go from full left to full right. That's fast enough for almost anything.

I observed that the laser speed heavily depends on the material used : moving too fast doesn't even mark because the material doesn't have the time to heat enough to vaporize or melt. Moving too slow can easily make the material catch fire. In this case it's better to pass faster with a lower power and run multiple passes. I found that speeds around 400mm/min were perfect to cut paper and thin cardboard in a single pass without ever catching fire. Cork and light woods tend to catch fire almost instantly.

The plugin is a bit confusing to use at first because it draws the paths onto your artwork and displays some coordinates on top, that you have to manually select and delete afterwards:


A few times I managed to render the G-Code not for my object but for the previously produced path, so that can be a bit awkward when you don't know but is not a problem once you got used to deleting these.

Sometimes the path doesn't appear just because it's at a totally different location. In this case just press Ctrl-A to select everything an you'll generally see it caught by the selection. Apparently it's how most Inkscape plugins work. It's not necessarily bad but you need to get prepared to this and think about saving first. Another hint is that the plugin will not produce any valid content if you open a file that is not part of a layer, so you need to see a later in the layers dialog. In this case, just select everything (Ctrl-A), copy (Ctrl-C), create a new document (Ctrl-N) and paste at the same position (Ctrl-Alt-V). Also don't use the plugin with multiple Inkscape documents open, or you never know which one will be rendered (it's not necessarily the one you're seeing and I still have to figure how it choses which one it renders). It's particularly troubling when working with multiple versions of the same artwork and you can't seem to observe  the effect of your changes. And since testing G-Code tends to be destructive in that you have to burn some material to compare the results, you generally prefer to stay on the safe side and limit uncertainty.

It's also important to note that Inkscape is still in very active development. My version was 0.92.1, I had too many issues, upgraded it to 0.92.4 and while it's better, it still rarely works more than 5-10 minutes without crashing if I'm not careful. I found so many situations causing it to crash that if I wanted to start to file bug reports I wouldn't even know where to start nor how to describe what I was doing :-( At least it's kind enough to save your work when it crashes. What I figured is that you must never ever undo an operation, you have 50% chances that it crashes (that was my first attempt to get rid of the extra paths lefts by the plugin), and each time you open an bitmap image (png, pdf, ...) you're taking risks as well (I suspect I'll end up being able to describe a reproducer for this one which is extremely frequent, I don't remember having successfully opened more than 2 images in the same session):



There are issues when importing PDFs, where units displayed on the ruler do not match the ones displayed or that use use, and even if you change them, each time you save, they're switched back to "px". I found that selecting and copying the artwork into a new document and saving it as SVG would often fix these issues. The interface is not intuitive at all for a beginner since many disallowed operations simply do not give you any feedback nor hint, so when you fear you did something wrong you press Ctrl-Z and either lose some work, or see it crash. Once you get more used to it you tend to be extremely careful to avoid such operations, you save more often and it crashes less often.

Despite being extremely annoying for these issues, it's still worth making the effort of trying to use it without getting discouraged, because even if it does have a long learning curve, it's powerful and heavily documented on the net (even though most tutorials are painfully long videos instead of just saying "press Ctrl+Shift while doing this or that").

And for PCB ?

Then I tried to play with a PCB I made under Eagle. That's where I understood I was going too far. One first needs to understand that Inkscape works with "paths". Objects, pixels or anything are totally irrelevant, what matters is the vectors you're making with your pen on the paper in some sort. When you import an image, you can't do anything useful with it by default. And importing a PDF only makes an object from which you cannot derive a path (other than the rectangle surrounding the image). There are examples on the net explaining how to detect edges on such an image to make a path but these required a menu entry that I didn't have under "Path", which is "Trace Bitmap" (not to be confused with "Trace Pixel Art" which is something different).

I later figured that Inkscape had to be built with "libpotrace" installed to have this, so I ultimately rebuilt it from latest sources after installing this library (both of which went amazingly well by the way). Anyway there's quite some value in rebuilding Inkscape, as bugs are reported and fixed almost every day so by staying up to date you avoid a lot of them).

Thus I attempted to export the bottom layer of my PCB in Eagle as a PDF, then to convert this PDF to SVG using "pdf2svg", then import it into Inkscape, to realize that it had lost all the tracks' width and they were replaced by thin lines! It was not Inkscape's fault as ImageMagick would display the same lines. Then I tried again by exporting the layer as a PNG, using "potrace" to convert it to SVG, open it in Inkscape and convert it to a path. This actually worked pretty well, but I had to figure the scale myself and resize the shape appropriately. I later figured that superposing the unusable PDF version and the SVG that I was resizing would significantly help to provide the scale.

I could produce the G-Code needed to draw the tracks' contours, send it using UGS to the engraver over a mate black-painted PCB, cleaned it under water and watch the result:



There were two defects where some tracks ought to have been straight but one side followed an arc. I suspect it's a combination of the path detection and the plugin because on the screen this issue doesn't exist. At first glance this wouldn't prevent the circuit from working.

One very cool aspect of working with paint is that if you don't like the result, you can clean it and try again. I was fine with this one so I sent it to the ferrochloric acid bath to etch it and test the result.


Now looking back to this I'd say it wasn't terrible, because I did so much better since. But actually I was impressed. At first glance, the cut around the tracks was 0.12-0.13 mm wide. Some copper tracks less than 0.1 mm wide were left intact. I didn't select a specific speed and roughly tried to properly focus the laser. The rounds in the holes were a pain to drill, I had to use a cutter to remove the copper first. And the circuit actually worked! I never had this level of precision before. Just to provide a scale, the 8 rectangular pads in the center are an SOIC8 microcontroller, the pitch is 1.27 mm. And on the left there's a mini-USB connector which is half that, it's 0.635 mm, with around 0.315 mm between two pads. The copper left in the middle between two USB pads is around 0.08 mm and the space is around 0.12 mm. Some issues definitely need to be addressed but the initial goal was already met!

Going further

I tried other G-Code senders, such as bCNC which is very complete and even allows you to edit and fix the G-Code, but relies on multiple views from which you have to navigate a lot. There's also "UGS Platform" which is the next version of UGS. It's supposed to support a feature called "jogging", consisting in pointing to a location on the artwork and move the head there, which is important for centering, for example when making double-side PCBs. But for me the menu entry is always gray and I cannot click it.

I found that all of them (UGS classic, UGS platform and bCNC) would occasionally hang during a complex drawing, leaving the laser on. I think I figured what happens in this case. The software are actually not frozen, I can send a reset to the controller. It is that the byte stream is too fast and they likely fills GRBL's buffer, causing some characters to be lost, and the firmware is then waiting for the end of a command (likely an LF) that never comes. Maybe their flow control is not perfect, or they don't well respect the output status from GRBL. I later found SourceRabbit G-Code Sender which does not suffer from this issue at all. But its visualization window actually never displays anything so you cannot easily control what's being done. Its progress bar is more accurate though.

While trying to work around transfer issues between Eagle and Inkscape to maintain scaling and avoid issues with improperly converted tracks, I finally found an excellent plugin for Eagle, which is called "pcb-gcode". It's seems to be maintained by a small community though it's difficult to find it for downloads, resulting in many copies floating around. I'm using version 3.6.2.4 from here, and a copy of it was moved to github here, sadly as a single import, thus losing all of its history (no updates since then).

This plugin does something I really wanted, which is to make multiple passes with a growing clearance around the tracks. It produces the G-Code output by itself, for each side, and can even make the stencils needed for the solder cream! Now to be clear, this is the proper way to go to make PCBs using Eagle and the EleksMaker laser machine. The results are purely astonishing and there is no need anymore to switch between multiple formats nor fight with conversion issues under Inkscape. Here's just an idea of what can be achieved with this awesome plugin:


To give an idea of scale, the small horizontal lines in the middle go to a small chip using 0.65 mm pitch, and are spaced by 0.3 mm. Since I didn't use much clearance, it managed to leave some unused copper of around 0.05mm between certain tracks! With proper tuning it must be possible to etch tracks as thin as 0.1mm. I didn't know that it was possible at all with this chemical process.

I also discovered by accident that the device by default uses power level values between 0 an 1000, but the plugins were emitting values between 0 and 255 by default. Thus the machine was operating at only 25% of its power, explaining the hissing sound of the PWM output I was hearing during operation. When pushed at full throttle, the beam is even scarier:



I first made a script to convert the values (G-Code is easily editable), but that's not terribly convenient. After upgrading it to a new firmware version I figured it was possible to change the high and low values in the firmware so I changed them to use the 0-255 range and always use it at full scale. I'm not certain it was possible in 0.9 however. Changing the laser power in the various tools to write 1000 instead of 255 was often enough. I haven't made a PCB at full power yet, so I don't know if it's better for the paint or not. Given that some PCBs can take ages due to the multiple clearance passes, it could make sense to etch them faster at higher power. More experiments and reports on this in another article.

Filling areas and writing text

Writing text is also one fun thing to do with a laser engraver. You can write your name on your mailbox using materials that don't fade away under the sun for example. My first attempt was made by cutting letters in duct tape and sticking them onto a plastic plate. This looked great and is still on my mailbox. But I wondered if it was possible to make plain areas instead using patterns fills.

Making a filled shape in Inkscape is not easy. You need to create a pattern made of many lines, extend/replicate it to the size you need, then superpose it with the shape you want to have and play with intersection/difference between the paths to only keep your stock of lines limited to the path of the original object. But this is actually possible. This is what I've used for the tests below (paint on aluminum, then etched using ferrochloric acid):


It's difficult and not very precise, as it's very easy to end up with irregular lines or lines that are not spaced using an integral multiple of the motors' resolution possibly leading to Moiré effect. I started to think that it might be possible to draw following horizontal only lines in "pixel mode", by just modulating the laser while moving. This is actually what 305engineering's Raster 2 Laser plugin does. And it's pretty straightforward to use. It can even adjust the laser's power depending on the luminosity of the pixels to print grayscales. With GRBL firmware in version 0.9, each change of laser power results in a short stop-start sequence causing a small burn. With version 1.1 there is a new "laser mode" which allows the motors not to stop between laser power changes, resulting in a fast and very clean result. This is actually very convenient to print photos, but more or that in another article. Here are a few examples achieved using this plugin:

Funny faces, gradients, Escher drawing, and backs on cardboard or even a toilet paper roll (those having known the Internet in the 90s will surely recognize something which was famous by then). Dense cardboard like is used for food packing is very good at rendering gradients. The lightest ones like toilet paper roll burn on the surface and do not render gradients:


Cat and dog on wood, village on cork, all of them ignite very quickly and are very hard to get gradients :


Symbols on plastic (more on plastic in another article):



Even on crude steak (a proof that you definitely must not put your hand under the beam):

Making wood and acrylic enclosures

One nice purpose I didn't think about and which I've already abused is to make plastic enclosures for various devices. There is an Inkscape extension called "Lasercut Box" which prompts you for a few parameters (dimensions, how much does the laser beam eat, speed and strength) and which produces G-Code that you just have to send to cut acrylic or wood. I initially tried with wood and didn't get good results due to the limited power, even after 12 passes it didn't go through 3 mm plywood. But a new attempt at full power proved it was possible in 6 or so passes (even if the result is not clean). The wood tends to burn under the laser so it's a bit tricky to figure the good speed and power, some experimentation is needed, but it definitely is possible at least:


For plastic, I bought an acrylic transparent red sheet and tried it. The result is marvelous. I can cut 3 mm in one single pass at a very low speed (20mm/min). It's important to perform some tests, like the pattern below I made at various speeds for different squares. I could finally make an enclosure for my NanoPi-Fire3 board!


Conclusion

This machine is really amazing and opens new perspective for working on hard materials, especially for someone not very manual like me. I think I found the tools I needed to exploit it fully (please check the download links in the text above):
  • G-Code senders: SourceRabbit for raster (very likely to hang with others), UniversalGcodeSender (UGS) for PCBs and regular use, bCNC for stuff that requires manual adjustments.
  • for PCBs, I design them in Eagle 7.7 (the last version supporting to work offline), convert them to G-Code using pcb-gcode from within eagle. Since the plugin gives a preview, it's never needed to switch to another application to control the result.
  • for drawing and cutting, I'm using Inkscape using J Tech Photonics' extension which is quite simple to use, supports multiple passes and is really stable.
  • for images and text requiring filled areas, I'm using 305engineering's raster2gcode plugin and I'm using my device in laser mode with firmware version 1.1 (more on that later).
  • for laser cut tabbed boxes, I'm using the Lasercut Box Inkscape extension and found that it was best to configure the Kerf parameter to 0.15mm when cutting acrylic.
That's all for this review, more articles will come later on more specific subject.

2019-06-01

Making a cloud chamber to see radioactivity

Background



Several years ago I was intrigued by a strange table at the Palais de la découverte in Paris, which was showing randomly moving lines supposed t be traces of particles falling on it. By then I found no explanation about it.

Later I learned that this is called a cloud chamber, which is quite well explained here.  These devices are not very hard to build and I happened to have all the required parts, so I felt urged to build mine.

Principle

The principle consists in evaporating alcohol in a closed but transparent jar, above a very cold metal plate. This plate being between -20C and -40C, it condensates a small part of the alcohol vapor showing a thin layer of tiny droplets which move quite fast. By lighting them horizontally it's possible to see them moving. When a charged particle travels through the cloud, it ionizes some of the droplets, increasing their density and showing a brighter track for a second or two.

First attempt

I first tried to stack several Peltier elements in order to increase the temperature gradient between the hot and cold side. This is the most difficult thing in fact. Indeed a typical Peltier module will consume around twice the heat it extracts, thus it will emit 3 times the power it extracts from the cold side. If you stack modules with less than 3 times the Qc rating (extracted power), you will not manage to cool the cold side enough. At 3 times, the colder one is almost useless as the hot one will suck just the extracted power. I found that multiplying the power by roughly 4 is what allows to provide the best gradients but it is hard to achieve.

The second problem is that with two layers you can roughly lower the temperature 40C below ambiant temperature. At 20C ambient, it just gives -20C. So I really wanted to stack 3 layers, but this means producing 9 to 16 watts for 1 watt extacted. However on the cold side we don't produce heat in the cloud chamber design. So we can afford to have little extaction power on the cold plate if the insulation is good enough.

I first tried by stacking 3 cheap Peltier modules (12706, 6amps under 12V) and adjusting their voltages. I installed a small anodized aluminum plate on top of last one and an old server fan on the hot side. I managed to reach -37C, to see a small cloud but couldn't notice anything in it.

Looking back at various deisgn notes, I found that a high voltage gradient is usually needed (positive at the top, negative at the bottom).  Some report that it also works (though less efficiently) without the high voltage source. Others indicate that the alcohol needs to be heated and that the top of the chamber must be sealed. And I didn't have any radioactive source (at least nothing I was certain was radioactive).

Thus I decided to order new parts.

Second attempt

For the second attempt I decided to order higher power Peltier modules. I got a 12715 (15 amps) and a 12710 (10 amps) to experiment a little bit. I also ordered a small set of tiny uranium-doped pearls and a fire alarm module using an americium 241 source, to have something to test my device.

I decided to use a larger cold plate. I found a transparent box which had been used to present a toy car, which had the good taste of being black at the bottom, slightly longter than two Peltier modules and larger than one, and with very clear sides. I installed a metal grid at the top, and used thick wires as supports, passing through the plastic base to connect to the high voltage source. The bottom plate is now about 8cm x 4cm which covers two Peltier.

I've cut thick copper plates (2mm) to arrange two modules per layer. It turns out that copper is too soft and very hard to keep flat once cut.  And pressing on the sides is often enough to bend it, so I ended with a few bent copper plates which were not making a good contact. Then I figured that my fan didn't have a good contact over 8cm. I spent hours filing it to try to make it flatter, and eventually it worked. Thermal paste is also a problem. The silvery ones are not sticky at all and can let the plates go off very easily. The white compound made for transistors is way better but doesn't conduct as much. In the end I found that mixing the two gave a very good result, it becomes a gray sticky paste with high thermal conductivity and enough viscosity to keep the modules in place.

I don't count how many times I had to disassemble and reassemble everything, using large setups is a huge pain as you cannot even afford 0.1mm difference between two extremities.

Then I found that it was not possible to go below -13C with this setup, whatever voltages I used on the various layers. In fact the 12715+12710 modules at the bottom (hot side) were producing too much heat for the server fan to cool down and I noticed that the top of the fan with burning hot while the air out of it was just warm.

So I decided to replace the heatsink + fan assembly for an old 1U copper heatsink made for a power-hungry Pentium4, and started to stack as many fans as needed to blow into it strongly enough. In order to keep the top in place, I'll use thick steel plates on the sides.


These ones were assembled using threaded rods which also serve as feet to keep the assembly away from the table :


So I tried 1, then 2, then 3 fans. They are 12V/0.25A each (3W), but I power them under 19V (~7.5W). Normally fans do not blow fast when you stack them like this, but here it's different, the heatsink's fins are extremely thin and the air pressure between the fan and the heatsink is high, so a lot of power is needed to further increase this pressure. With a single fan top of the heatsink was at around 44C. With two it was at 40 and with three it's around 37. So that's 7 degrees won :


For the Peltier modules, after lors of experimentation, I ended up with this design :
  • at the top (cold plate) : a single 12706 module, powered under 4V through a DC-DC converter.
  • just below it, two 12706 modules in series, powered under 12V. This results in roughly 6V/3A per module, so this layer is about 3 times stronger than the first one.
  • and below, the third layer is made of the 12715 and the 12710 modules in parallel, totaling 25A under 12V, to suck the power produced by upper layers. Note, for a better design I should have opted for two 12715.

The plastic box is then installed on top of this. There needs to have enough clearance below it so that it can press the plate to keep the modules in place. It is important not to screw it too strongly so that it doesn't bend the upper plate :


Note that at this point there is no lighting installed in the box. I'll simply use a handheld torch for now. For testing the temperature initially didn't want to go below -13C again. I figured that some of the hot air from the fan managed to go below the cold plate and to heat it. I then installed some polystyrene foam between the bottom layer and the top layer and it went down to -32C this time. Due to the heavy copper plates it takes about 5 minutes to reach the lowest temperature. I photographed it inside under low light conditions to see better (no alcohol yet, only the moist in air freezes) :


In order to start to see the cloud, I let it cool down, then heated some alcohol in the microwave oven. It's easy and allows it to quickly evaporate, you just have to watch and stop heating when it boils. I'm using 95% ethanol, which works pretty well. Others mention the need for isopropyl alcohol but it is much harder to find and not necessary. I've placed some absorbing paper on the top grid and just poured some hot alcohol on it. I quickly closed before it evaporated and connected the high voltage supply.  First some alcohol started to condensate at the bottom and to dilute with the water ice which had already formed there, and at some point while lighting with a torch, droplets were clearly visible at the bottom, indicating that the cloud had formed. I didn't notice the effect of the high voltage and am stil having doubts about its effectiveness; the cloud has the exact same aspect with and without.

Very quickly I started to see thick tracks spontaneously appear in it! It worked! The straight ones are made of Alpha radiation (42He atoms ejected as the result of the transmutation of Americium 241 into Neptunium 237). The broken lines are made of electrons or positrons (Beta radiation). On the images below, the left one shows the chamber with no visible track and the americium module installed on the right. The middle one shows a track in the middle coming from the americium module. The right photo shows another track at the top (less visible).


I uploaded a video here.

Lessons learned

Peltier modules advertise currents that don't match reality. The cheapest 6A ones (12706) really draw only 2.5A approximately. In fact it depends a lot on the temperature gradient between the two sides. Some of them are unmatched so it's not easy to connect them in series as they will not react similarly. Also, connecting them in series may require to increase their voltage if they are not strong enough. I think instead I'll buy some high-power DC-DC buck regulators and install each stage on a separate voltage so that I can control them individually. I noticed that it's possible to significantly reduce the amount of power once the plate is cold. Typically instead of 12V, the setup continues to work at 8, which is about 2.25 times less power, and makes the fans more efficient. In fact, what matters is to maintain the most optimal power levels at each stage. One must start from the coldest side, see how far it's possible to go without heating the next side too much, then start the second side, then the third one.

I miserably failed at using two modules for the coldest side. I think that the difficulty is that some of them are not as good as others, and that since it is very difficult to make them work at very low temperature, the gradient remains the one of the poorest of the two. If one module requires 2 watts to reach a low temperature and the other one requires 3 watts, with two modules in parallel I'll emit 5 watts which make it way harder to cool down by the next level, while 4 would have been sufficient with a set of identical modules. I don't know if larger modules exist. These ones are 4cm wide, it would be nice to have larger ones, it would simplify the setup. The 15A ones (12715) are theorically more efficient than the 6A ones as they correspond to 2.5 of them placed in parallel, and have a lower resistance. Thus in theory by running them at a lower voltage and under the same current, they should produce less heat (less Joule effect). So I think it's better to use higher power modules and run them on a regulator than using several low-power ones in parallel.

Since not much heat has to be extracted, a single module is sufficient. It is important however to put some polystyrene foam between the plates, especially the top one, to prevent radiated power from the lowest ones to heat it up. I intuitively think that this layer has to be thin enough not to touch the two sides in parallel, thus it becomes a polystyrene+air insulator.

Fingerprints (especially those with thermal grease) tend to mark the anodised aluminum plate a lot. I'll probably have to clean it up several times again as it's harder for the cloud to form on top of these traces. Running the device "dry" (no alcohol) lets air moist freeze on the plate and reveals the traces, so it's easier to spot them.

Future improvements

I need to install a led strip around it to avoid having to hold a torch. I'd also like to experiment a bit with higher voltages to see if they really produce something. I've read some reports that high enough voltages increase the probability to see tracks, though it wasn't obvious here. Anyway it was an awesome experience which was worth all the difficulties I went through while building this device! I'll probably have to choose different Peltier elements.