Flexprint cable in micromodels

[Fred]

Hi,

after good and bad luck with magnet wire (=enameled wire) and very small litz wire (multistranded insulated wire) in micromodels, I successfully tried flexprint cable (i.e. the stuff found interconnecting commercial PCBs etc.). Some of these cables are excesssively temperature sensitive, simply melting away when you attempt to solder to them, others can be soldered quite nicely.

Scratch off the insulation (which is usually quite tough) on one side to expose the actual trace and directly solder to that trace. In my current model, I also made use of the connecting end: I liberated a matching connector from a computer PCB by heating the area around the connnector with a heat gun and then smacking the PCB to make the components fall off. The flexprint wire in this picture has 0.9 mm pitch. I use the connecting end to attach my programmer (and bus analyzer) so that I can reprogram and test the 3 microcontrollers (will be 5 when the model is complete) in what is going to be a 1/87 scale wheel loader (BTW: the frame is not soldered but held together with M1 and M0.8 screws). The image shows a US cent and a Euro cent for comparison. The small "pig tail" in the top part of the picture is a nylon thread which makes it easier to unplug the rx module.



I believe this is a very worthwhile technique to explore.

Regards,

Fred

[kalkurap]

Looks like a viable alternative to the magnet wires. I used the 30AWG wire wrapping wire that the radio shack sells in my first truck. These aren't stranded, hence not all that flexible and break off if you aren't careful. I am looking for an alternative for my second truck, and might try these.

The model that you are building looks interesting. Would you mind sharing some more info and photos? What are the microcontrollers suppose to do? Also, the motor look realy tiny. Would like to know more about it as well.

Thanks.

[Fred]

Hi kalkurap,

since you brought up single-stranded enameled wire: The microcontrollers in my project are pretty tiny (Attiny2313-20MU: 4x4x0.75 mm MLF package). They were designed to be soldered to PCBoards. But I found out than 0.05 or 0.10 mm wire can be directly soldered to the pads. For some reasons, 2 twisted strands of 0.05 mm wire seem to work best. Leaded solder is too soft, there is not really enough "beef" for the wire to hold. I was much more successful with silver solder (to be more specific, SnAgCu0.7, 217°C melting point).

So far, there a 2 motors attached to the frame, one for steering (the one you see in the picture above), one as the main "engine". Both are stepper motors driven by one microcontroller each. The third controller handles IR reception in conjunction with an NJL21Vxxx IR demodulator.

Top view showing the main motor on the left:



Here is an early picture of the main motor & control circuit (before I discovered that twisted wire is much preferable). Without shaft, the motor is 8 mm long and has a diameter of 8 mm. The second chip in the pic is a dual H-bridge. When everything works, I embed each "module" in methamethacrylate glue. The motor's connector block is "huge"; I remove it routinely; but that implies direct soldering to the fairly short and thin ends of the motor windings.



The motor is glued to a milled brass motor support with 4 holes (tapped to hold M0.8 screws). More pics to follow within the next couple of weeks.

I'd wish for a bit more activity in the micromodel section of this forum...

Regards,

Fred

[kalkurap]

Thanks for the tips on soldering, I am going to try these very soon.

This build is looking a lot more interesting since you added details about the micro controllers and the related circuits

I don't know much about the AVR controllers, so I went ahead and looked at their website. Those MLF packages with their 0.5mm pitch look extremely small. I am used to PICs, and the smallest that I can get is SOIC package and looks large compared to these! Are you writing all the programs by yourself? Looks like you are not using a dedicated PCB to put all the circuits together, that should help keep everything really small.

Those motors look really cool, are they bipolar? How many phases do they have and how many H bridges per motor? Looks like one can build really small fully proportional steering mechanism with these since no servo is required, that is really awesome.

The chassis looks really nice with all the screws etc. Sorry, a lot of question, hope you don't mind. But this build is getting really interesting. Please keep us informed.

Thanks.

[Fred]

Hi kalkurap,

for soldering, it can be helpful to use silver solder (higher melting point) and leaded solder (lower melting point) on different ends of the same part. E.g. when connecting wires to 0402 LEDs, I attach the first wire with silver solder and the second one with leaded solder. Otherwise, the soldering iron (I use a 6 W type for these jobs) may radiate just enough heat that any small movement into the vicinity of the first connection will melt the solder there.

With a dedicated PCB, one might in fact save some space. My current approach of having "modules" (receiver, steering unit, drive unit) has some advantages as well. Here is a picture of the receiver module sitting on a US cent:



I do all my own software development (in assembly language for the most part; rarely in C).

The motors are of the bipolar variety, requiring a dual H-bridge each. They move 18° per step and are not microsteppable.

Here is a slightly outdated pic of the frame components, some of which I have modified by now.



More to follow.

Best regards,

Fred

[kalkurap]

Fred,
Nice trick there to use the fishing line to handle the modules. Just a quick question, are you using the methacrylate glue to keep the wire from coming undone? Or is there any other reason such as to make the functional block look like a "block"?

Thanks.

[Fred]

Originally Posted by kalkurap

...Just a quick question, are you using the methacrylate glue to keep the wire from coming undone? Or is there any other reason such as to make the functional block look like a "block"?...

Hi kalkurap,

the fishing line "trick" did not work as well as intended, because I accidently touched the line with a soldering iron... Maybe I can still connect both ends by heating them.

I began using methacrylate glue when I was exclusively using leaded solder, which simply is not strong enough to withstand any shear. In some occasions, 2 chips are at right angles to each other (4th picture up) and the glue turns chips + wiring into one solid unit which can be easily handled. The rx module is pretty sturdy.

Just finished the rear lights:



From L to R: rear light [red], reverse gear warning light [white], brake light [red], indicator light [orange]: 0402 LEDs, the white one looks greenish, I tried painting the others with a red indelible marker -- as you can see, this is not the ultimate solution. There is still white primer on the vehicle's rear.

Best regards,

Fred

[kalkurap]

The LEDs really neat. Moreover, they look scale. How are they held together? Did you line them up and glued them together? Or is it just the magnet wire with the solder that is holding them in place?

Do you have more photos showing the body of the model?

Thanks.

[Fred]

Hi,

Originally Posted by kalkurap

...How are they held together? Did you line them up and glued them together?...

Excactly. I cut a trough in a piece of PTFE, lined up the LEDs (belly down), and added a tiny drop of Loctite 401 (dries up crystal clear, no "blooming") on each side. Capillary forces "suck" the glue underneath and between the LEDs. If you are careful soldering, the heat will not undo the CA bond.


Single wire: tinned 0.1mm copper wire; wire pairs: 2 strands of 0.05mm magnet wire, twisted

...Do you have more photos showing the body of the model?...

Not at this point, it's all in pieces. I want to finish the rear of the wheel loader first and turn it into a working "half-model" before I delve into the drive mechanism for the bucket (although I have some good idea how that can be accomplished).

Best regards from Germany,

Fred

[kalkurap]

There pictures are certainly deceiving . I had to solder some 0603 SMD capacitors on to the PCB yesterday, and they were really small. The 0402s are not clearly visible at all. It is amazing how you are able to solder them neatly, without shorting the adjucent LEDs!

Also, where do you get those 0402 white LEDs from? I looked at www.digikey.com and www.mouser.com and they don't have them. They have the red, orange, yellow etc, but not the whites. The whites start only at 0603 package.

And what is the part number of the H-bridge you are using? Both mouser and digikey have the H-bridges made by Zetex, but it looks like they come in a slightly larger package than what you are using (I think they are called SM8, 8 pin version of SOT223).

Thanks!

[Fred]

Hi,

Originally Posted by kalkurap

...The 0402s are not clearly visible at all. It is amazing how you are able to solder them neatly, without shorting the adjucent LEDs!

I used to be able to do this type of work with just a pair of 3.5x mag loupes ... but my vision is no longer that good. Therefore, I prefer a stereo microscope (10x mag, occasionally 30x mag). For taking pics, I simply hold the camera lens close to one of the eyepieces.

Also, where do you get those 0402 white LEDs from?

Here in Germany, a couple of ebay shops offer 0402 LEDs. Getting red, green, white and yellow ones is fairly easy. The orange LEDs are samples I obtained from Kingbright (leftovers from another project).

And what is the part number of the H-bridge you are using?

Allegro A3901

All the best,

Fred

[kalkurap]

Originally Posted by Fred

I'd wish for a bit more activity in the micromodel section of this forum...

I too feel the same, it would have been more interesting. But that doesn't discourage me all that much. I like building circuits and models to go with them. I like scale models as well as scratch building. This hobby combines all of them perfectly. Moreover it interests my wife as well. Guess I can't expect more than this from a hobby

When you mentioned that you are building a wheel loader, I didn't know what it would look like. That is because from where I am, we call the wheel loader a "bulldozer" and I found out this only after doing a web search. Is your model going to have a working lift as well? This one is going to be very interesting to watch.

I went ahead and ordered samples of the H-bridge drivers. Hopefully they approve my request. I am also collecting parts (from old cassette players, TV remotes etc) to build a simple IR system. I don't get to spend a lot of time on hobbies these days but your work is really encouraging. It might push me to a point where I actually start building the IR system

Regards.

[kalkurap]

..and thanks for providing the part number. I wouldn't have found it if I had to search for it myself.

[Fred]

Originally Posted by kalkurap

...When you mentioned that you are building a wheel loader, I didn't know what it would look like. That is because from where I am, we call the wheel loader a "bulldozer" and I found out this only after doing a web search. ...

Hi kalkurap,

there may be differences in terminology. To me, a "bulldozer" is chain driven, a "wheel loader" has wheels. Here is a link to a photograph of the type (full size) I am building as a model:



Eventually, the bucket is supposed to be fully functional.

I agree that building micromodels combines several very exciting interests, such as mechanics and electronics. Many newsgroups are pretty much limited to discussions like "which commercial servo fits into this or that model". That is OK, but I would like to see more people introducing their scratch-built models.

Yesterday, I went a little overboard and added license plate lights.

My skills painting models are non-existent and I failed miserably attempting to add just a first layer of paint. An airbrush has been sitting unused in one of my drawers for a while; and it looks like I will have to acquire that skill pretty soon.

Have fun,

Fred

[kalkurap]

Fred,
How is the build coming up? Are you going to post your progress here? I would like see it!!
I saw that on may of the german forums, people use the type of brass gears that you use. What is your source for the gears and the motors? Do you know whether they ship to US? Here, I haven't found any source for small brass gears so far.
My Allegro Stepper motor driver samples have arrived. They sent me 4 of those, and they are really small. I don't know how I will be able to solder them on to the PCB. If I fail to come up with a way to solder them, I might glue them upside down, and try to connect the pins to the PCB traces with copper wire.
I also made some progress regarding my IR system. I have identified the IR receiver module and the LED that I need to use, and verified that they both work properly. I also made a plastic enclosure that attaches to my radio, in the place where the antenna normally is attached. Now, I need to make a cable to access power and PPM signal from the trasmitter. That way, I can switch between the IR and RF module without any difficulty.

-Kalkura.