Using the "industrial" sharpies, fine and ultrafine point, I made two attempts at an AVR RFID card with integrated PCB antenna, with mixed results.
I designed all but the antenna in Eagle and exported to EPS, then imported into Make-the-Cut. The spiral antenna I created using Inkscape spiral tool, then cut-and-paste into Make-the-cut, and just rotate and move so the traces connect.
I draw a rectangular bounding box around the circuit and put into a separate layer, which I print first onto a piece of paper taped on the cutting mat. This shows me exactly where to place the blank circuit board.
Next I tape the circuit board to the paper using masking tape at the very edges only. I snipped off one of the grey friction rollers so it doesn't roll back and forth over the design and ruin it while the ink is still wet.
I remove the Expression keyboard to make room for the pen, and wrap a long thin strip of duct tape around the pen close to the tip to make it big enough for the tool holder to grip. I "cut" (draw) from Make-the-Cut at Extreme speed, low pressure, and etch in the usual way.
The experiment was very successful in that I was able to get pretty reliable traces in two sizes, roughly 0.5 and 0.8 mm, plenty for a sporting shot at surface mount stuff.
Unfortunately, while a magnet-wire coil produces upwards of 10v peak-to-peak, I can't seem to get better than 850mV or so across the pcb coil leads, regardless of my choice of tuning capacitor, so for now running the Tiny85 is still out of the question.
The two antennas have about 25 and 45 turns respectively, but so far I've been totally unable to get them tuned to produce enough voltage to run the AVR.
I've been probing the tag coil across the capacitor leads while the tag is being stimulated at 125khz with my homemade reader.
The second antenna is pushing the resolution limits in the spiral because the "jaggies" from the machine reduce clearance between lines. I had to do a little cleanup work with a razor blade, separating adjacent lines that had shorted in a very few places.
I spent a few hours reading about 125k antenna formulas and designs to try and better understand the relationship between desired frequency, loops of wire, wire gauge, and the value of the tuning capacitor.
I worked through some of the formulas in this datasheet, plugging in 100 turns, 66mm, 30 gauge to see what it would come up with for inductance, ideal # of turns, and capacitor value, to see if they would match the 1nf & 100 turn values specified in scanlime's blog entry. I wrote a simple java program to calculate the values for me after not finding what I thought I was looking for on various online antenna calculators (they seemed to be all too complicated).
I found this interesting note along the way which I had not seen elsewhere: "... For copper wire, the loss is approximated by the DC resistance of the coil, if the wire radius is greater than cm. At 125 kHz, the critical radius is 0.019 cm. This is equivalent to #26 gauge wire. Therefore, for minimal loss, wire gauge numbers of greater than #26 should be avoided if coil Q is to be maximized....".
So, #26 gauge wire next time!
Unfortunately my program isn't putting out believeable numbers yet, so I have more work to do.
In the meantime, I also discovered this other datasheet for a neat little 0.32 cent RFID chip (the Philips Hi Tag-S) that has a good description of the details of the encoding methods used starting in Section 7.3 (including our manchester, for future reference)
And here's another good discussion of the antenna design and coupling (different frequencies, same concepts and formulas).
I played with the open source 4NEC2 Antenna Modeler & Analyzer for a while trying the helical generator, but it kept blowing up at simulation time with my RFID design, complaining that the antenna was connected to ground at the end (duh, this is what the tutorial shows, I don't get it!).
Last night at the Hackerspace I tried a new way (to me) of doing PCBs on the Cricut. So far it looks like the most promising yet.
This time instead of trying to scratch off an etch resist, I'm directly drawing it on using a plain old mini Sharpie pen like a plotter would. I understand the Stadtler Lumicolor pen is also recommended.
I just now discovered this link that shows exactly what I need to try next as far as the pen goes: PCB Plotting
-Then, in Eagle, run File->CAM Processor.
-Select Output Device EPS
-Click File and select your output file path.
-Don't worry about the offset and page size.
-Select the "Pads", "Bottom" or "Top", and "Vias" (it will complain if "Vias" is not selected).
-Click "Mirror" if you are doing the bottom layer.
-Click "Process Job," this will write the output file.
-Install a copy of Ghostscript & GSView, and run "ps2pdf [options] input.[e]ps output.pdf" to convert the EPS file from Eagle to a vector PDF.
-Fire up Make-the-Cut, and do "File->Import->Vector PDF File", leave "Import Strokes and Fills" selected, select your PDF file and click "Open".
-Select the imported image and click "Ctrl-B" to Break the circuit up into its pieces.
-Deselect all, then click on each of the four border lines and delete.
-Select all, and click "Ctrl-J' to Join the circuit back up into a single piece.
-Position the circuit on the cutting pad as needed.
-Load up your Sharpie in the tool holder and print a test piece on paper to verify positioning.
-Load up your copper in the machine. If you're running anything thicker than 0.01 you may need to raise the pen in the holder. I use double-stick tape or at least a fresh spritz of spray-tack.
-Print your design. I don't know how many coats are necessary, but I am doing two coats, one after the other. Don't use Multicut for this!!! It does each line multiple times immediately instead of doing the whole pattern completely and repeating it: This causes the pen to dissolve the previous coat and move it around a little.
-(optional) Put the board into the toaster oven just briefly to make sure the ink is fully dry.
I tested a few different etch-resists over the weekend with good luck, but the process itself still needs improvement before the boards will be usable.
The three resists I tried were Johnson "One-Step No Buff" floor wax, Krylon purple spray paint, and black Lacquer spray paint.
I made three test pieces of PCB material, one using each coating, then scratched each one manually with a nail, and etched. The wax was applied by pouring a thin stream over the top of the metal and propping it up at an angle on a paper towel to let the excess run off.
After etching, the floor wax was a clear failure, the coating was just too thin and failed over about half the surface. The purple Krylon seems like it worked well, as did the black Lacquer. Clear paint looks awesome but its hard to tell when you've sprayed enough on.
Also, one thicker coat seems to work better than two thin coats like you might normally use.
I still have a couple of problems to solve before the process will be more useful however: I need a wider scratch mark, and there's an annoying jitter in the tool during the first quarter or third inch of motion on the X-axis.
After making the test pieces, I scratched a full-size test piece using the matte black Lacquer.
Edit: Unfortunately, the etched scratches still do not completely separate the areas of copper, resulting in 100% shortage across the board surface.
Clearly I need to find a better resist and tool usage combination. The fine parallel lines still resolve clearly, but are also not fully clean. They are also not close enough to merge when etched. I think a softer resist might be desirable, so now I'm thinking .. what about melting a very thin layer of candle wax onto the surface with a hair dryer and then scratch wax but not metal with something pointy but not sharp, like a tooth pick?
Also, much hotter etchant may work better as well (this most recent board was done at room temperature and appears to be incompletely etched before the resist started to fade. Previously I used a double ziplock bag with a few TSP of etchant in a hot water bath with good results.
One of the first things it seems like every interested hacker asks me about the Cricut is, "how can we get it to do PCBs?" Well, I've finally done it!
I created a single-sided circuit design in Eagle, using 50 mil traces, then exported just the pads and traces as a monochrome PNG file.
Next, I sprayed a coat of clear spraypaint on some single-sided 0.01" PCB material and let it dry well. (Next time I will use two coats)
Finally, I imported my design into MTC using pixel trace and used my scribing tool to scratch the design onto the PCB (multicut 2, pressure high). This removed the spray paint around the edges of my traces. After it was done, I used a toothbrush to brush the removed-paint-bits off the board.
Finally, I etched with a few tablespoons of ferric chloride in a double-bagged-ziplock immersed in hot water. The whole etching process took less than ten minutes. Aside from some unwanted specks where my one coat of spray paint was a bit light, the result looks perfect!
Obviously I still have to do more work to make this board functional, I'll report back when I have more to show.
EDIT 3/13: I tried drilling the board and soldering up one of the oscillators, and it turned out that there were a couple of shorts in my etched diagonal lines where some "jaggies" were really close together. Also I discovered that the "donuts" need to be larger in diameter to leave more solder pad remaining after drilling the hole, especially the ones for the 555 chip.