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Project Overview
The goal of this project was to design and build a working delay effects pedal for guitar. Guitar effects pedals are extremely common among electric guitarists as they allow the sound of the guitar to be modified before going into the amplifier. There are a variety of effect types, including distortion (adds gain to the guitar signal), modulation (modifies the guitar signal using time-based waveforms), reverb (simulates ambient room noises), and delay (introduces echos into the guitar signal).
These effects can be introduced to the guitar signal in a variety of ways, including using dedicated Digital Signal Processors or even through software on a laptop. However, perhaps the most common way the audio signal is affected is through the use of guitar effects pedals. These are often analog circuits, although many recent models are switching to digital platforms. The output from the guitar is plugged into the pedal, and then the output from the pedal is plugged into the input on the next item in the signal chain (whether that be an amplifier or more pedals). When the footswitch on a pedal is engaged, the audio signal will be routed through the pedal's circuit and the effect will be applied to the pedal's output. When the footswitch is disengaged, the guitar signal coming out of the pedal will be the same as it was coming into the pedal.
My plan for this project was to create a fully-functioning delay pedal, which introduces an echo effect to the guitar signal.
Part I: PT2399 Digital Delay IC
For this project, I knew I wanted to work with a digital delay circuit rather than an analog one. This is largely because an analog delay circuit is rather difficult to implement in a cost-effective manner. After having done a great deal of research on different delay circuit types, I found that the most common type of delay circuit by far was using the PT2399 chip. This 16-pin CMOS delay processor is very cheap, easy to implement, and most importantly produces quality results. I will not get into all the specifics of this chip here, but in short, this chip takes in an audio signal and uses an internal clock to introduce an echo effect. This internal clock speed can be modified using an external resistor, which changes the amount of time between echos. I found this ElectroSmash article incredibly helpful when researching how to implement this chip in my pedal.
Part II: Block Diagram
After deciding I wanted to use a PT2399 chip for this circuit, I started building a block diagram of how I wanted the circuit to work (see photo). Because I have some experience building other effect pedals, I had a rough idea of how I wanted this pedal to work. The guitar signal would be buffered at the input, effectively boosting the signal (without distortion) so that the PT2399 could cleanly process the echos. Both the output of this buffer and the output of the PT2399 would be mixed together with a circuit that allows the user to adjust the balance between the dry and wet signals. After being mixed, the signal would then go through an output buffer to ensure a consistent output gain from the pedal.
Part III: Schematic Design
Once the block diagram was set, I began working on a schematic. However, after some rough sketches, I stumbled upon the Rebote 2.5 Delay Circuit from Tonepad.com. As it turned out, this was almost exactly the type of circuit I was hoping to use. This simplified things for me, as it meant I didn't have to do very much testing with resistor and capacitor values - because this is a new type of circuit for me, it certainly would have required a great deal of trial-and-error when it came to specific values of the components.
Screenshot of a PDF of the Rebote 2.5 Pedal (available from tonepad.com)
After settling on a plan for the schematic, I began working in AutoDesk's EAGLE software to develop a schematic and PCB layout for the circuit. With some prior experience in EAGLE thanks to Ben H. at The Hive makerspace, I felt comfortable jumping into the software and putting together a schematic. The thing that slowed me down the most was finding and using the correct packages for this project. Because I need the physical components to perfectly match the PCB layout in the software, I needed to ensure that I was selecting the right components while designing the schematic in the software.
Schematic Design in EAGLE
Thankfully, the vendor I purchase parts from (Tayda Electronics) provides datasheets for each component. After settling on which components I would use and placing an order through Tayda, I only had to design a couple components in the software. Certain components like the PT2399 chip or the TL072 dual op-amp chip were not available in EAGLE's component libraries, but most resistors and capacitors were already available with EAGLE. Because some components (such as the footswitch, potentiometers, and power jack) would not be directly on the PCB, I had to include solder pads on the circuit to ensure that I could run wires from each of these components to the circuit on the PCB.
Library Designer/Manager in EAGLE
Part IV: PCB Design
Perhaps the most difficult part of this project was designing the PCB. I wanted to house the pedal in a Hammond 125-B aluminum enclosure (a standard size for guitar effect pedals), which constrained my PCB size to 60x60mm. After creating a board outline in EAGLE, I spent many hours trying to develop a layout that I felt was effective. One key thing I had to keep in mind was keeping the traces on the circuit board as far apart as possible to minimize any short circuits. The Hive Wiki page offers a number of helpful tips and guidelines to follow when designing circuit boards, and I tried to stick to these as best I could. I found the 'autorouter' feature in EAGLE extremely useful, as it automatically created all the traces I needed.
The Hammond 125B Enclosure I used for this project
For the first version of this PCB, I opted to go for a 2-sided circuit board. Having traces on both sides meant that I could solder components to both sides of the board, thus conserving space and allowing me to produce a very compact board. I also used the "polygon" and "ratsnest" features in EAGLE to ensure that all the ground points on the circuit were connected not only with small copper traces, but also with larger copper ground planes - a technique commonly used and recommended to me by Ben H. on the Hike Wiki. After designing the PCB, I exported the file from EAGLE as gerber files so that the Hive's PCB equipment could be used to fabricate the board.
Two-sided board design in EAGLE - the red lines are on the top layer, and the blue lines are on the bottom
Part V: PCB Fabrication
The PCB fabrication process was new to me, so I needed several hours of training to learn how to create circuit boards in the Hive. This was a fun process (albeit tedious), and I learned a great deal. After being trained, I went ahead and fabricated the first version of the PCB. However, after I had finished with the board, I could immediately see a variety of issues. Many traces overlapped with the ground planes, which would almost certainly cause short circuits. Lots of other traces ran too close to others, and several traces also ran too close to pads where I would be soldering components. Because I would be hand-soldering the components, I knew I needed a higher tolerance for error on the board too account for the less precise nature of hand-soldering. I also realized that several of the drilled holes for the components were too small, and so I had to go update the EAGLE libraries to increase the diameters of these holes so I could mount the components on the board. One final issue I realized was that although a 2-sided board could make the board more compact, it would also be very difficult to hand-solder the leads of some components to both sides of the board.
Images of the first board. Notice the white annotations where I marked spots that I felt were problematic
So, after taking these errors into account, I went back into EAGLE and developed a much better, one-sided version of the board. This time I was much more careful with the autorouter and ended up doing a lot of the routing by hand to ensure tolerances were met. Similarly, I cranked up the tolerances on the ground plane so as to avoid any possible shorts on the board. I also adjusted the drill sizes so that all the components could easily slot into the board for soldering. I realized that a one-sided version of the board would actually be roughly the same size as my original two-sided PCB, so this all turned out to be a fairly straightforward (albeit tedious) process.
One-sided board design in EAGLE - notice how this time the traces are blue and only on the bottom layer
Part VI: Assembly
After fabricating the updated board and inspecting it for possible mistakes (there were none this time), I moved into my favorite part of the process: soldering the components to the board and assembling the unit. I had previously prototyped the circuit on a breadboard and slightly modified some component values to dial in the exact sound I wanted, so I had all my components ready to go. However, I quickly realized I had forgotten to order a couple extra parts (including the aluminum enclosure) to finish the project. So, I placed those parts on order and continued on with assembling just the circuit elements of the pedal. After soldering all the components to the board, I had to also solder wires to the off-board components (power jack, audio jacks, and potentiometers) and attach those to the proper pad locations on the board. Instead of soldering the ICs directly to the circuit board and running the risk of damaging them, I instead ordered sockets for the PT2399 and TL072 chips and clipped them into place.
The start of the breadboarding process and the start of the soldering process
Part VII: Testing
After the circuit was complete, I hooked it up to my guitar and tested it. Unsurprisingly, it did not work on the first try, but rather produced a very loud buzzing noise. After some fiddling, I realized I had made a few errors in the assembly process: first, I had not grounded the voltage regulator for the PT2399 chip. This meant that there was no reference for the regulator, and it could not effectively take 9V at the pedal's input to supplying 5V for the PT2399. I soldered the ground pin in place, and while doing this also noticed that I had not grounded the audio output jack, which explained the loud buzzing. After these two mistakes were corrected, the delay circuit worked perfectly, and I was able to produce beautiful echos from my guitar's signal.
The soldering process and the completed circuit board
Conclusion
Overall, this was a very fun project to work on. I already have experience building guitar pedals, but had not yet attempted a circuit like this one. Before this project, I also had no experience with EAGLE or PCB fabrication, and I think that learning these skills was an invaluable part of this project - perhaps more important than the final product itself. Because of the busy-ness of the end of this semester and because of shipping delays with the enclosure and a couple extra parts, I was unable to get this circuit housed. My plan is to install it in a nice blue enclosure with knobs to adjust the effect and a footswitch to enable the circuit. I also want to get the enclosure laser etched in the Hive makerspace with labels for the knobs and perhaps a logo on the enclosure. I plan on working on these things in my free time over Christmas Break after the semester ends.
Demonstration of the pedal circuit. Note that while the final circuit worked successfully, this video is from the breadboarding step
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