sausage fingers (5) – designing the PCB

As I may have mentioned I’m planning to cover a lot of the absolute basics of effects pedal design (some super simple theory, components, breadboarding, schematics and PCB design), so I am hoping to cover those before doing some proper circuit analysis. I’ll be doing a full circuit analysis of sausage fingers in the next couple of months. If you have any questions in the mean time, feel free to contact me. I’d be more than happy to chat!

Anyway, onto designing the PCB!

The first thing to think about is the controls on the outside of the pedal and where they fit into the circuit. Remember when you design a PCB you’re looking at the underside. You’re putting the components on the bottom of the board. The pots and switches will be mounted on the top of the board. So when you’re looking at the PCB you’re looking at a mirror image of how the pedal will look from the outside. Remember that.

Before we start, one thing we do know is where the ‘in’ and ‘out’ are going to be. As standard I use the order – in, +9V, ground, out – for my foot switches and so I match the PCB to make wiring the two easy.

fullsizeoutput_191

Next I drew out a super simple version of the circuit based on the ICs alone (TL072, TL074, LM324 and CD4013) showing what connects to what. Like so.

fullsizeoutput_18f

There are a couple of reasons for doing it this way. Firstly, those ICs are the biggest components so it’s best to get them in roughly the right position first, then you can manoeuvre the smaller components around them. Secondly I know that the CD4013 can introduce noise in the circuit. This was something I hadn’t thought about when I first started to breadboard. After doing some research I found it was good practice to keep ICs roughly 1/2″ (13mm) apart when designing PCBs.

The next thing to consider is where the controls (pots and switches) feature in this simplified map. Which looked like this.

fullsizeoutput_190

As you can see, I hadn’t given the controls daft names at this point. The next step was to balance out where I wanted the controls with where the ICs would sit.

fullsizeoutput_192

Remember you’re looking at the PCB (left) from the bottom, and you’re looking at the pedal (right) from the top.

This seemed like a good place to start, most of the controls were close to where they needed to be. The furthest away was the direct level, but then it has to go somewhere. Another consideration for the layout of the controls is what relates to what. The mids pot only affects the oct 1 signal, so I didn’t want that too close to the direct level pot. The gain affects both direct and oct 1 signals. I think it affects the direct level more, it makes the gain direct level stand out over the octave, so it seemed like a good idea to keep those together.  The bias pot only works when you flip the ‘synth’ switch, so plopping them in the middle together seemed logical too.

The next consideration is the size of the PCB. This is limited to the width of the enclosure and how far down you are willing to push the foot switch and the ‘in’ and ‘out’ jacks. This isn’t a small circuit so I knew I’d be using a 1590BB enclosure. I measured the width inside and decided I could make the PCB 82mm wide. Bear in mind the sides of the enclosure taper, so don’t play it too close. After measuring the length of the enclosure and doing some thinking I decided I could make the PCB 67mm long easily. You need to strike a balance between keeping the components close enough together to keep tracks short, and having enough space between components for all those tracks to fit comfortably.

The rest is all just experimenting, shifting things about and resizing the PCB if needed. Anyway here’s how the first run prototypes came out.

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There’s a couple of tracks I think I can make shorter in the next batch. With a PCB of this size you’re going to spend a couple of hours moving things and getting things closer together/ neater. Your brain will probably be a bit fried by the time you’re ready to send them off for fabrication. The most important thing with the first run is that everything works. You can get on with perfecting and making it neater later. Don’t let the perfect be the enemy of the good as they say, if that is what they say. I’m sure they say something like that, if ‘they’ even exist at all. Maybe it’s just voices.

Next time, we’ll get this fella boxed up and find out how it sounds in an enclosure.

 

 

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