Finalizing the Pedal Design

Now is the time to prepare my pedal to be housed in an actual enclosure. In order to do this, I need to solder it and to move it from temporary breadboard into its final soldered form. But before that, there are some important parts missing. I still need to make some decisions. Am I going to use battery power only or I’ll use power adapter as well. What about the stomp switch? I want it to have an indicator when the effect is engaged. How about the box? What about my options for soldering? Heck, will everything fit in what I’ve got?

Choosing Components

The reason I have to do it now is that the whole design is not quite complete yet. Schematic symbols have to be transformed into actual physical components. Importance of this step might be unclear at first, but making a mistake here can lead to a lot of frustration later on. I’ve been there before.

Things I need to look at are:

• Enclosure – housing the pedal, this will never be shown on any schematic but it is needed, clearly
• Protoboard or PCB – again, not shown on schematic but needed to transfer pedal from breadboard to something permanent
• DC Jack and/or Battery – for powering the pedal. Depending on choices here I might need to change the main schematic
• Input & Output Jacks – depending on how am I going to power the pedal, I might need to change schematic slightly
• Potentiometer – it is on the schematic, but several choices here. Decision won’t affect schematic but will affect how the pedal is assembled. Requires (well, it’s nice to have) a knob which is not shown on schematic
• Stomp Switch – not shown on the original schematic. It is required unless I want the effect to be always on. Also, if stomp switch is engaged, I want a led indicator to show me it’s on. This will require schematic change.

So without further ado:

Enclosure

The first step is to choose an enclosure I suppose. Without it, it’s not a pedal really, is it? There are options here but the main thing is the dimensions of the box. The box has to be sturdy, that goes without saying, since I’ll be stomping on it, but after that, the main concern is dimensions.

There are some standard sizes/boxes used for DIY as far as I could gather, and at any rate, I’ll be using those that I have around. Those are not the only options but I’ll limit myself to those for now.

On this photo there are 4 different enclosures next to a Boss DS-1 pedal for size comparison. There are different options for size and colour. The one labeled 1590B on the photo is one of my recent builds, I painted it black, it was unpainted originally. 1590B2CB is only different in height to 1590B, but those extra 7mm sure make a massive difference when it comes to final assembly.

I’ll see if I can use the smallest one (nano box, I think I’ll call it that). It would be great if I could fit everything in.

Protoboard/PCB

I’ll need to solder all components. For that, I’ll either need to use a PCB or a protoboard (prototyping board like a perfboard or a stripboard) and they come in all sorts of varieties.

Going PCB route is great time saver and makes it way easier to solder everything. I could purchase a PCB that someone already designed and made. I could design my own PCB using an EDA tool, something like KiCad. Designing it myself is great, but then I need to make it myself (that never really worked for me … etching it myself is a recipe for self inflicted pain) or order it from a PCB fabricator. That is just too expensive and too big of an effort for a once off build (it is super interesting though … that kind of sounded weird just now).

That leaves me with a protoboard route which makes most sense for this once off build at the moment. On the photo I have a few options. There are couple that are quite similar in layout to a breadboard. They have traces interconnecting several pads. I’ll chose … whatever fits the nano box.

What a bummer, I can’t really fit them into my desired enclosure. Nothing to worry, I’ll just cut them to size with a Dremel. But this just shows that this exercise is not for nothing. Even with cutting the board down I might not be able to fit everything. Time will tell, eh?

Powering the Pedal

The original schematic had battery in it. The original LPB effect worked off battery only. But I know that most pedals can use both battery and can be powered with external DC adapter. How does that work?

How it works is that, while the pedal is plugged in to external power, the battery is disconnected. When the pedal is plugged out, the battery is connected and powers the pedal. But, if the guitar is not plugged in, I need battery to be disconnected still so it’s not drained while not in use.

If I go with battery only, no need to change anything on the schematic. Actually, I would still like the battery to be disconnected if the guitar cable is unplugged from the pedal. If I go with external power only, or with both battery and external power – I need to change schematic, so how these options look like?

If I go with battery only, the only change is in the input jack. In the original it was mono input, but now it is a stereo jack. Guitar cable has a mono plug, so when I plug it in, sleeve and ring will short on the stereo jack. Since sleeve is connected to the ground, when the plug is in, battery’s negative pole is connected to the ground, so the battery is connected and it is powering the pedal. Note though, the battery will be powering the pedal no matter if the effect is on or off, the only thing that matters is that the cable is plugged in.

External Power

If I go with using only wall adapter for powering the pedal, I need a DC socket (J2 on the diagram). I no longer need the battery, but I need a power supply decoupling capacitor (C3 on the diagram). It’s a relatively large electrolytic capacitor which is used to absorb noise coming from the DC supply and anything else that is connected to it (say, if I plug in multiple pedals to the same supply). It also works other way around, prevents noise generated by the pedal from going out. It also prevents voltage drops if pedal has a spike in power usage acting as an ancillary power source (not that this pedal draws a lot of power, but still worth mentioning).

A word of warning is in order I think. Most pedals use negative tip polarity – tip (centre) is negative and sleeve is positive. Outside pedal adapters I did not come across other adapters using this arrangement. I’m sure there are others out there, I just thought it was worth mentioning. Because of this, most of the pedals that use external power have additional reverse polarity protection in case a wrong power supply is plugged in. The simplest ones are with a single diode. I chose to totally ignore this and rely on myself to use correct adapter.

And the third option is using both external and battery power. That one is a combination of the previous two. DC socket (J2) pin number 3 in the diagram is shorted with pin number 2 when external power is not plugged in. When external power plug is connected, it breaks this connection effectively disconnecting battery.

If I choose to use one of the external power options there’s a number of options again.

The metal DC socket on the left hand side of the photo is great in terms of size, and would be great if it was not metal. As I noted before, tip of the socket is with negative polarity, the sleeve is positive polarity. When DC plug is inserted it will short the sleeve with the enclosure (aluminium is conductive) and the enclosure will be at 9V potential which is not good at all.

That leaves us with either the top one which is panel mounted, or the bottom two that are PCB mounted (one with PCB terminals and one with soldering terminals), but without something to hold PCB firmly in place it’s not going to be possible to use the latter.

By the looks of it, it’s going to be very hard to fit battery into that small box, I might need to go with external power supply only.

Jacks

At this stage, input and output jacks are going to be one stereo and one mono. Even if I choose external power only option, I can still use one stereo and one mono and will just leave ring connection of the stereo jack disconnected.

There are several options here as well (I probably had more different jacks somewhere in my drawers). PCB option is not a runner really for the small box. Input and output jacks would have to go on separate PCBs or I wouldn’t be able to fit them (There are jacks with snap on mounting cap that I could use, but I don’t have them right now).

Choosing between open frame and enclosed body, I’ll go with enclosed one. I reckon, this is going to be real tight and there’ll be a few hookup wires around so might be easier to handle. They can barely fit the nano box though.

Potentiometer

There is a surprising number of options to choose from when selecting a potentiometer. Besides being of correct resistance and taper, I want it to fit the box, of course. But I also want to be able to screw it onto the box, and I want it with the shaft of correct length and size for a knob I have. I don’t really mind if it has solder lugs or PCB pins.

I’ll use one with bushing – a threaded collar for fixing it to the box using a hex nut, split shaft and solder lugs (the top left one on the photo). Truth be told, that’s the only one I have with correct resistance and taper.

When looking for the suitable potentiometer, good quality one with correct resistance and taper is non-negotiable. Everything else is then how to fit it into the box. Length of bushing, length of the shaft (possibly even the material of the shaft), type of the shaft. There’s actually quite a few more bits to look after in the datasheet, and it can be slightly daunting. Unfortunately, most of the time datasheet is a required reading since lots of retailers show generic pot photo so you can’t really tell with certainty what are you getting. Of course, once settled for one type the best is to use it all the time 🙂

Stomp Switch

And the final part is the actual switch to turn the effect on and off. Most DIY pedals use either 3PDT or DPDT foot switch.

Either DPDT or 3PDT switch has a simple task – to bypass effect by connecting Input directly to Output and when toggled, to connect Input to the input of the effect, and output of the effect to the Output (essentially making guitar signal bypass the effect, or go through the effect). But I also want them to turn on a LED indicator when the effect is on. LED indicator is simpler to wire-up with 3PDT switch, since it has 3 switches (it can be viewed that way), so two are used for input and output, the third one can be used for LED.

DPDT has to be used with some clever circuitry to be able to turn LED on and off. Nothing too complex, but still more complex than 3PDT.

There are several options how to wire-up the switch, I normally choose this one:

The diagrams above show wiring of the switch, then how the wiring works out when the switch is in bypass mode, and then when it is ON. At the far right, it just shows that it really is like 3 different switches that have common position control.

LED

I also need to choose a LED. They come in all sorts of colours and some different shapes. I won’t use the square one (I’m sure I had a clever plan with it when I ordered it years ago). They can also be multi colour (the one with 3 leads is 2 colour LED) – I don’t need multi colour for this.

Then there’s size, 3mm or 5mm (these are common sizes). I think I’ll use 3mm one, that’s the amber coloured small one. I could use the bigger, 5mm one. The one I choose will determine the size of the hole to drill.

The main thing is to look at the datasheet to check forward voltage and current so I can choose correct current limiting resistor. The amber one (datasheet says Orange Red … OK) has typical forward voltage (Vf) of 2V and 20mA typical forward current (If). Considering that the whole effect uses less than 2mA, this will be a major consumption contributor. This might be an issue if I use battery to power the pedal.

Actually, when it comes to LED I have to look at diagrams of Luminous Intensity (how bright it is) vs Forward current, which tells me in this case that at 5mA If the brightness will be at 50%, but then Vf at that current is less than 1.8V. Lets say 5mA If, 1.8V Vf, that means I need 9V-1.8V = 6.2V voltage drop on the resistor, that makes it: 6.2V/5mA = 1.24 KOhms, say 1K. Again, if I’m unhappy with the brightness I can always lower the resistor value and get it to be brighter. I only need to be careful not to exceed maximum current value for the LED. Also, if using battery, there will be some trade-off needed.

If I was ordering a new LED I’d probably think about this more, in this case I’ll just use what I have.

Checking One Last Time

A quick check if everything will fit into my box.

It looks like I can’t fit battery there, even with the slimmer DPDT switch. There are screws that are used to attach the lid to the box on each 4 corners of the box and everything needs to clear them. This means the battery simply won’t fit there. Also, there will be wires, protoboard, potentiometer, LED, it’s just too crowded.

If I go without battery and use 3PDT switch, I think I can make this work. This is just to give me an idea if components are going to fit, I normally spend some more time here to ensure 100% that everything will fit. I take datasheets and look at the drawings in them, they have exact measures there. Then I check out if everything will fit, especially focusing on the components I’m not sure if they’ll fit or not.

An example of how everything might fit in a bigger box. This is from another build I did recently. I drilled the holes in and then placed components. It looks tight, right? Despite of a much bigger box. It turned out to be way tighter than I wanted it to be in the end when I ran wires through the box.

Final Schematic

The final schematic looks like this:

It looks a bit more complex now that everything is on the diagram. Even though I’ll only use external power, I still kept that battery symbol in. In practice I will just omit the battery snap when soldering and that will be it.

Final Bill of Material

Finally, the list of components I’ll need:

I did not touch upon choosing resistors and capacitors here. I’m using metal film resistors that have 1% resistance tolerance and 1/4W power rating. Power rating for resistors is important so that they don’t go up in smoke if it the rating is exceeded. This is unlikely to happen at low currents used here but it still needs to be considered.

I’m going to use metal film polyester (PET) capacitors for coupling capacitors (C1 & C2). Polypropylene (PP) caps are supposedly better (less signal distortion) but I don’t think I can tell the difference.

Electrolytic capacitor is just whatever I got. In this case, since it is power bypass cap it does not affect sound. In reality I would want some better quality one, it’ll just last longer and its performance won’t deteriorate with time as much.

That’s it. There are many things to choose from, some pitfalls to avoid along the way, but now I’m ready for the next phase – making the circuit permanent – soldering.