7414 Success

September 19, 2010 8:28:44 AM CDT

Here's a version of the previous board that does what I want it to.

I added 1 megohm pull-down resistors to the inverter inputs, so now the LEDs stay off until specifically told otherwise. While I was at it, I threw a debouncing cap across the power rails.


It took a bit of rerouting for the traces, but frankly, I like this layout better.

The new input layout stretched my capabilities a little by running two traces under an 0805 package. It's one of those things that looks fine on paper, but you don't know if it will actually work until you get in and start soldering. Sure, it's easy to get good placement and isolation with reflow paste, pick-and-place robotics, and all the other fancies, but I'm working with a soldering iron and tweezers.

It worked though, so that's one more item I can add to my bag of tricks.

Looking at the photos, I admit that I could have done a better job of assembly. Some of the components are sitting at wonky angles. I need to reheat all the joints, press the parts down, and make sure they're seated properly. A good scrub with ammonia to get rid of the excess flux wouldn't go amiss, either.

As an aside, regular household ammonia makes a perfectly good flux cleaner. The traditional cleaner is a mix of alcohol and acetone, but I've had mixed results with that. Sure, it dissolves the rosin, but the flux I work with seems to contain some waxes as well. Those don't dissolve in the solvents, so they stay behind as a white powder.

Rosin is an organic acid, so it reacts with ammonia (a base) to form a soluble salt. I don't know the exact chemistry, and probably wouldn't be able to pronounce the name of the compound if I did (rosin is mostly abietic acid, so.. ammonium abiecitate?) but I do know that it washes off of a circuit board easily and doesn't leave any white gunk behind. It's also cheap, easy to obtain, and non-flammable.

Getting back to the circuit, the board is just under an inch square (shown with a reference bottlecap in the last picture), and the total cost for parts is probably about 50 cents. I can make up a half dozen in my spare time, throw them in one of my gear boxes, and have them ready the next time I feel like building something.

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