Tuesday, September 27, 2011

74xx Competition! Part 1

As some of you may know, Dangerous Prototypes is putting on this big competition out for circuits that use discrete logic chips like 7400's or 4000's. In case you haven't seen the prize list, I suggest you cruise on over the dangerousprototypes.com and check it out.

         A while ago (in sixth grade) I built a rocket launch controller exclusively out of 4000 series chips and  555 timer. It was all on a breadboard and pretty messy. Now, in ninth grade, I've decided to get it back together, but this time on a PCB. But wait, there's more! I'm etching it myself, so it has to one-sided. Yes, there will be some jumpers, and the 555 will be replaced by an ATTiny for controllability and simplicity, but the displays and such will all be discrete. The first part I'm etching is the ATTiny and 4017 circuit. It will drive a bargraph display. Later, the divide-by-ten output of the 4017 will drive a 4026 [EDIT: 4029] (BCD count-up/count-down), which will drive a 4511(BCD to seven-segment display driver), and another 4017, the tenth output of which will trigger a relay and thus the rocket. In case that doesn't make sense, here's a super-simple diagram:

Sorry for the poorly drawn lines...


I drew up the first part of the circuit (from the clock to the bargraph) on Fritzing, my favorite PCB editor.  I exported it to a PDF and printed in on a rather ancient laser printer. Now, if you're going to replicate this, here's some stuff to know. In this etch, I used some 16 mil traces, much smaller than I've used before. I've had good luck with 24, 32, and 48 mil traces, but I've never tried 16. So it might not work. Also, my printer has an option that controls how much toner goes on the paper. I found it in the print menu, under Preferences, Advanced. I set everything to "TonerSaver Off" and print quality Dark. You want as much toner as you can get. For paper, I used typical Staples Photopaper. Other people swear their lives on magazine paper, but I haven't tried this. This is what my printout looked like:

There are two printer discrepancies in the printout,
a litte north-east of the center.  
Above it is the prepared board. It's copper-clad board that has been roughed up with a Scotchbrite pad and some mild abrasive called Barkeeper's Friend. It's basically fine sand. A plastic abrasive, like a Scotchbrite sponge thing, is better than a piece of sandpaper or steel wool because it doesn't leave as much residue. The idea is that if the board is rougher, the toner will stick better. After that, I cleaned it with some acetone to get any fingerprints or debris off. Acetone is good because it evaporates right off the board, leaving it nice and clean.

When that's done, it's time for the actual transfer. This is the most exciting/tedious part, besides the actual etch. This part basically consists of pressing a hot clothes iron over the paper layered on top of the board so that the toner adheres to the board. Later, during the etching process, the toner acts as a resist so the parts of the copper covered with toner don't get dissolved. The first step is to heat up the clothes iron (I say this because there is also a soldering iron in my house) and align the printout with the board. Set the iron to its hottest setting. This is "Linen and Cotton" for me. Also, you obvious want the toner side of the paper matched up with the copper side of the board. You will also want a bowl of water on hand. Once the iron is heated up, lay the board with paper on top on a flat surface (for me a wooden board) and push down really, really hard for about thirty seconds. This gets most of the toner to stick so it should stay in place.

In the basement lab, ready for transfer!

Pressing really, really hard.
After that, you just want to move the iron around in different areas of the board. I sort of go back and for while twisting it a bit. This ensures that all toner gets exposed to heat. Some irons (like mine) have steam holes in them. If you don't move the iron around, the toner under these steam holes doesn't stick to the board. I usually do this for maybe five to ten minutes depending on board size. In my experience, going too long never hurts the board. When you're satisfied with the ironing, you can pick the board up with either pliers or your fingers (safety warning) and drop it into the bowl of water. It may sizzle.

Nicely transfered board, sitting in the water.
The idea is that the water will dissolve the paper somewhat so you can rub it off with a finger. In both pictures, you can see the line where the paper is getting saturated. When it is mostly saturated, you can begin removing the paper.

Saturation moving in. 

Paper lifting off.

I gently pulled some of the paper of, revealing
the copper and toner
A nicely transfered (and shiny) board. 
What left to do? Etching and drilling. However, that's going to wait for another post that should be coming soon.

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