Build Details I – Input Circuit
The input circuit consists of one RC network for each of the 16 channels we are adding and one 74LVT16245B IC to get data from those 16 channels. Installing all the components for 16 RCR networks is tedious, but simple work. Installing the ’245 is tricky and requires a steady hand. But you’ll have lots of soldering practice with the RCR networks to get you warmed up for the ’245!
The RC network consists of 3x 1MEG resistors, plus a 510 Ohm and a 10pF cap. We will call these R1-R3, R4, and C1 respectively. They will be named differently on the board of course, these names are just for illustration purposes. First, let’s look at the schematic view of the RC network.
Fig 3 – Schematic View of the Input Circuit
We cheated and made the schematic look exactly like the layout, hope you don’t mind. The operation of the input circuit is as follows: The resistor divider from Vtrig to GND sets up a 2/3 Vtrig and a 1/3 Vtrig at it’s two center nodes. For a floating pin, the ’245 sees 2/3 Vtrig which should be out of the way if ZP did their math. If the pin is pulled high or low, the trigger divider acts to also divide down the pin voltage in order to keep the input to the ’245 in the 3.3v range. Great stuff, very clever work. The 10pF is a fast path to ram a nice sharp edge into the ’245 when the pin changes state, because sending that signal meandering through a 1MEG will really soften it up. At this point, you should easily recognize the layout view as seen over and over on the PCB in Section 1.
Fig 4 – Layout View of the Input Circuit
In order to solder all these components, we suggest you get an assembly line going and do all the 1MEG, then all the 510 Ohm, and then all the caps. It’s much easier and less error prone than having several little piles of SMT components and trying to grab one of each.
The biggest pain in soldering SMT’s is the situation where you have solder on both pads and want to melt it all at once – it doesn’t work. One side always cools and you’re stuck tapping back and forth with the iron. You’ll end up overheating the PCB and peeling traces, which really sucks and can ruin your board. Your strategy should be to get in, get the part mounted well, and get out quickly.
We suggest soldering one side of each component at a time, and even cleaning off the other pad using solder wick if it already had solder on it or accidentally gets some solder on it. That way, you don’t have to deal with the case of trying to heat two pads at once. We’ll break our soldering technique down into three steps that fit with our assembly line analogy.
Step 1 – Put a small blob of solder onto each of the right hand pads. Or the left hand pads if if you’re a lefty. You’re going to be holding the iron in your good hand and tweezering in components with your other hand. In order to avoid having to cross over, simply blob the side you’d naturally have the iron in. You want a small blob – not a pinprick, and not a huge overflowing mass. Our pic here is actually showing a little LESS solder than we usually get, but it illustrates the point.
Step 2 – Tacking the components down. For each of the components, use the iron to melt the solder on that first pad. Then, bring in the component with the tweezers and push it into the solder blob slightly so the solder wicks up onto it’s contact. You should be able to position the device with the tweezers at this time so it looks nice and is flat against the board. Remove the iron while still holding the device in place and after a few seconds the solder will cool and you can let go. Fix any ugliness or misalignment by grabbing again with the tweezers and touching up the solder with the iron. Don’t let go with the tweezers until the solder is cool, or the device may “tombstone”, standing up on end due to the thermal contraction of the solder.
Step 3 – Solder the other side. You can wait until all components are tacked down before doing this, or you can do it one at a time. The only restriction is that you finish soldering one component before attempting to solder a neighbor with adjoining pads. In the example here, this means finishing R1 before doing R2. R2 before R3. This is because heating R2′s left hand may conduct enough heat to melt the right side pad of R1. Or you may accidentally touch R1 in the process. If R1 is only held on one side and you melt that side, you may knock it out of alignment or tombstone it.
Fig 7 – Completing the Component
Lather, rinse and repeat. You’ll be soldering 160 terminals so you don’t have to put a lot of thought into it, jut get into a groove and consider it SMT soldering practice. When we look back at our own work, it seems pretty bad! But the first rule of electical engineering is:
You don’t talk about electrical engineering! Well not to girls at least. They’ll run away.
The 2nd rule of electrical engineering is:
It doesn’t have to be pretty – it has to be conductive.
And on that note, we inspect a pic of 16 finished channels on Port C. You can see the difference between the nice, reflowed, original devices on the left and our shoddy hack on the right. Also take note of the tinning on the ’245 pads. That’s the first-pass of the next soldering technique you’ll learn.
Fig 8 – It doesn’t have to be pretty…
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