FISHERS (project pt.4)

 -DESIGNING CIRCUITS:

Designing the circuit boards is by far my favorite part of any project. I am still using an old version of KiCad 5 that I have really had no reason to upgrade for the last few years. I have created my own custom footprint libraries for all of the parts I use, and the last time I upgraded KiCad it was a disaster, so I'm happy to stick with the old version. I'm sure I am missing out on new features, but as is the case of all of my other old-working software versions like CorelDrwaX5, sketchup2017, Mach3, Arduino1.8.5, and even Windows10, as long as it still works, there is no reason to upgrade!..

When designing circuit boards like this, I like to try to fit as much of the circuitry around the interface hardware as possible. Sometimes I am able to fit everything onto one circuit board with minimal need for wiring. Ideally there would be no need for wiring, and everything could simply be clipped together with interconnects. Unfortunately I am not that good yet, but I am pretty good a getting the wiring down to an absolute minimum. Despite my best efforts with this circuit, I was unable to fit everything onto the hardware boards, and had to include a fourth mainboard for all of the big through-hole chips from the original keyboard. That was kind of expected, but it will probably require a lot more wiring than I wanted to do.

Thousands of parts!

Designing circuits with surface mount parts has huge advantages over through-hole components. For one thing, they take up almost no space in storage. They also usually cost much less than through hole, depending on the part. Despite their advantages, some considerations should be taken when using surface mount parts, especially if you are using through-hole components to breadboard your prototype before converting it to circuit board. Some components have different ratings, and tolerances, and some components are just not transferrable, or if they are, the surface mount equivalent is not as cost effective as the through-hole version. 

TO-92 vs. SOT-23

This is most noticeable in capacitors and sometimes transistors. I use MMBT3904 SMD transistors for 99% of all of the NPN transistors in my designs, but there are times where the T.H. version is necessary, such as with the curious case of the PT2399 CV input buffer. For whatever reason, that circuit does not work well with the MMBT3904. It probably has something to do with the reverse breakdown voltage–which is not usually standardized–or possibly the HFE, which is a standard 100 on the MMBT3904, and varies from 200-500 in the 2N3904's I have. All I know is that the 2N3904 works, so that is good enough for me.

P5.0mm vs. 0805

Surface mount capacitors are small and inexpensive, but not always ideal for audio signal paths. Ideally, the audio signal path would only use SMD poly, ELC, or tantalum capacitors, but those usually cost much more than their MLCC equivalents, and sometimes cost more than their through-hole equivalents. I am not too concerned with audio fidelity in this case, since the audio signals I am dealing with are pretty simple and mostly noisy anyway, so I generally tend see what I can get away with using X7R MLCC capacitors as much as I can. In the case of the PT2399 circuit, I designed some of the filtering capacitors with dual footprints so that I could use through-hole parts if I needed to, and in this case, it does make a noticeable difference. The same goes for the VCF capacitors. It only needs two, so they don't take up too much space. Using too many through-hole capacitors would shrink the work area down pretty quickly, so it is a fair trade off in using surface mount capacitors as much as possible. You just have to be thoughtful about it and have a plan-B in places where you think you might need to change the part during debugging.

Dual footprints for capacitors

Some chips can be touchy, too. I have noticed that some latch chips that I use on my bread board are sometimes more forgiving on their timing inputs than their surface mount versions, and I have had to improvise filtered buffers to get them to work. Now I always check that every chip I plan on using in the final design actually works in the breadboard version by soldering one to an SMD adapter board, and plugging it in. It is also good to put in extra footprints on empty spaces on the circuit board in case you do need to add a filtered buffer or inverter somewhere. Adding extra footprints costs nothing!

Sequencer PCB

Voice PCB

Designing complex circuitry can be stressful, but also extremely satisfying and addictive. The goal is to get the circuit board as close to perfect as possible, the first time–which is completely impossible–but I have gotten pretty close. It's important for me to remember that there will probably be lots glaring mistakes that I did not see even after checking over my work a hundred times before sending it out to be manufactured. Keeping that in mind, I am able to take steps to design circuit boards so that they can be changed or fixed easily. Adding test pads in sensitive areas can be a huge help too, when you know an area of the circuit could be problematic. Clearly labeling everything in the silkscreen layer is also worth the extra effort. Labeling trim-pots clearly, and even leaving yourself notes on the board will save your future-self a lot of work trying to remember how things are supposed to work.

Main PCB

I typically only use 0805, 1206, SOT-23, SOIC, and 3x3mm trim-pots, so that I am able to easily solder and de-solder the components by hand(hot air wand) if I need to change anything. Also, since I use a hotplate to solder all of the SMD parts to the boards, it is easier to see when there are flaws in the reflow when using medium sized parts, and I am able to quickly fix any problems while the solder is still hot. I use cheap lead-free low-temp solder paste, so using anything smaller than TSSOP is probably better left to the factory. 

Joystick PCB

I have yet to try ordering assembled circuit boards, as opposed to populating them myself by hand. I am sure it is worth the added price, but so far I have not had the need to have more than 5 large circuit boards to be replicated. Maybe if I ever start selling things regularly, I will try it out, but for now I will just continue to do it the hard way.

Part 5 coming soon...