Last month I described my Arduino LED clock project; this month I thought I'd give you an update on my progress. Using the shift-register idea I described in that post, I've designed the circuitry to control the two digits representing minutes; this circuit includes two shift registers, 30 resistors, and 30 LEDs. (All told, a particularly simple circuit, really.) Here's the schematic:

As I came closer to completing the circuit design, it occurred to me that trying to solder all of these components on a simple prototyping circuit board would be onerous at best. (If you've ever done this kind of thing before, you know what I'm talking about!) Typically, prototyping boards have a matrix of pads for placing components, but no embedded traces connecting components to each other (or, if they have pre-printed traces, they are usually not in the most useful places). So I began researching designing a printed circuit board specifically for this circuit.

There's a plethora of freeware available to download for this purpose; many packages come with schematic drafting software as well, and are able to link the PCB design to the schematic, which greatly assists with ensuring traces connect the correct pads. The package I chose was ExpressPCBand ExpressSCH; I like the interface and the company provides low-cost PCB board printing, if you're willing to wait a little while for them. (For the record, I'm not endorsing their product or their company per se--it works for me, but it may not work for you.)

After a lot of experimenting (and chatting with a couple of electrical engineer friends), I came up with this two-sided PCB design:

(top layer)

(bottom layer)

Then I got restless. I didn't want to wait the month for ExpressPCB to print and ship my board. I wanted it right now! (I mean, the name of the company is "express," after all.) I remembered that a few months ago I had stumbled on a website with some crazy scheme for home-printing PC-boards, using some complex combination of laser printers, acids, and duct tape or something. So I did some additional researching and discovered that printing a PC-board at home is much simpler than I imagined, and significantly cheaper than having a professional shop do it, especially if I was only doing one-off boards.

There are dozens of sites which describe the method (here's one). Essentially, you create an etch-resist mask of the printed circuit design by printing it (using a laser printer) on glossy paper, then transferring the laser toner to a blank board using a hot iron. Once the mask is transferred, you drop the board into an acid solution (available at Radio Shack), which eats away any copper not covered by the laser-toner mask. After some cleaning with acetone to remove the toner from the finished/etched board, you've got a shiny new printed circuit board!

Of course, not one for doing anything easily, I had to go and design a two-sided board; this adds the complication of having to ensure that both masks are aligned when they are applied. By lining up the masks face to face and taping them together along one side then sandwiching the board between them, I was able to solve this problem pretty easily.

Or so I thought. Turns out, when I made my first board, I managed to flip one of the images in the wrong direction, so they no longer lined up properly. (I checked all the edges, but never bothered to check the pads themselves, and didn't discover the mistake until cleaning off the toner.)

Here's the first (non-functional) board:

(top layer)

(bottom layer)

Next time (at the end of November), photos of the finished board, with components!