The Shakey Legs is the Swamp Witch's modulated digital delay. An LFO controls the delay time, and this ever-changing delay time causes mild-to-wild pitch modulation, arpeggiation and all sorts of warbly, wibbly, wobbly timey-wimey goodness. The following is a run-down of how I construct one of these time-benders, and some of the challenges I've faced along the way.
The pedal is comprised of five component-boards: the delay itself, the LFO, a wet/dry blend, a post-effect boost to compensate for lost volume in the delay signal, and the ever-present 3PDT footswitch board.
Board Construction
Batch Builds
I found one key to efficient building to be batch-building boards. I received two orders for the Shakey Legs, and assumed I'd be getting a third in the near future, so I built three batches of boards for the pedals. The last picture on the right shows an older delay board, that has since been upgraded to include Anti-Latch protection for pedal startup, as well as an onboard trimmer for controlling oscillation of the delay.
LFO
An LFO, or Low Frequency Oscillator, is usually the heart of any rate-modulated control. As the control oscillates, it changes the target value -- the volume in a tremolo, the depth and sparkle of a chorus, or the laser-frequency of a phaser. Some LFOs are as simple as a few transistors to make a square wave; others venture into more complicated and fine-tuned wave control. I use the StompLFO chip from Electric Druid. It is an extremely versatile LFO, delivering 8 waveforms, a frequency range of 10 - 250Hz, and even tap-tempo support. The LFO controls a vactrol, which ultimately controls the delay-time of the Shakey Legs.
Vactrol
One of the most popular vactrols (resistive opto-isolators, or LED/LDR combos) was the VTL51C. It is long obsolete, and very difficult to find in the original form, so I'll have to roll my own!
By combining a light source (usually an LED) with a reactive element (here, a Light Dependent Resistor), I can achieve the same functionality, with slightly different response. The Shakey Legs LFO uses a much smaller than normal LDR, so I have to test each one before use. Sanding the top of the LED gives a flat surface that the LDR can adhere to, with super glue! Then they are sealed with heat-shrink to protect from any stray light, and installed into the LFO board.
Anti-Latch Protection The first iteration of this pedal had a few false starts. One of the most prominent issues happened when the pedal was powered on with a bright LFO setting (which translates to very low delay resistance). The PT2399 is a popular digital delay chip, used in a variety of applications, from karaoke machines to delay pedals (like this one!). It has a mechanical limitation, however, in the chip's design: the delay resistance needs to be at a certain minimum level (~2kΩ) during the chip's start-up (approximately 400 ms), otherwise the chip won't start-up. Read that again: the chip won't start. This means a dead-sounding delay pedal, an unhappy user, and a sad builder. One way to meet this requirement would be to add a resistor in series with the current delay-control-line, which would avoid leaving the line below the resistance threshold. The drawback to this approach is that the minimum-allowable delay time is limited by this threshold. Enter ElectroSmash and their wonderful analysis of the PT2399, which contains a very clever circuit designed to show very high resistance for a set period of time, then a very low resistance, allowing a sort of "resistor-switch" to happen! The circuit can be found near the end of the linked article, and is an RC-filter controlling a simple NPN transistor. While the capacitor is charging over the τ time period (defined by τ = R x C), the transistor is off, and the collector-emitter connection shows a very high resistance to the delay-control-line. Once the capacitor is charged, the transistor turns on the high resistance disappears, leaving only the LFO's delay-control signal.
Pedal Assembly
Once the boards are each assembled, it's time to start fitting them together. As I build more of these, this gets easier, since I can write down the length of each wire and don't have to measure each time.
Power Junction Board
With five component boards (counting the 3PDT switch board), connecting power and ground can get a bit messy. During the first few iterations of this process, I tried to tie all the ground/power wires together manually, leading back to one connection to the power jack. This works for two or three component boards, but at five (!!) there are a lot of extra connections being made - each of which is a potential failure point! Avoiding wire-to-wire connections was my goal. My initial attempt was to solder each ground connection to a small piece of perf-board, dedicated to ground connection. This works alright, but is difficult to fix after the fact (with seven connections in one blob of solder!) and I found I had a tendency to melt the wire shielding while doing so.
Enter SIP sockets: by making the power junction board socketed, I avoided melting any wire shielding, and ended up with a more fixable setup as well. The connections do need to be sealed, though, so I employ hot-glue (which, as well as being an excellent adhesive, is non-conductive -- perfect for my work!) to hold the wires in place and shield any unwanted contact.
Shakey Legs
And after all that work, we get an excellent looking pedal and functioning pedal, inside a hand-painted enclosure, made with a dash of love and magic.
Demos
Demos can be found in the Swamp Witch / Shakey Legs folder of my google drive.
Special Thanks
First thank you goes to my art director and partner, Kara Smith, who has created some really wonderful artwork to bolster and support my work. Next, a huge and fairly unending thank you to my circuit friend and companion in the pedal building journey, one special unnamed goose.
Finally, thanks to Luke Jolly for his photography services and for putting up with the noise and chaos of living with a pedal-builder.
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