Find code, schematics and practical info on my GitHub repo: mcu-relay-controller.
What would the classic guitar effect pedal be without a way to enable it or disable it? While most players are unlikely to put much thought into the most mundane feature of their stomp box, i.e. the on-off switch, it is a feature whose importance is eclipsed only by its banality. Presumably, most pedal users simply want the switch to work, and spend the majority of their time and mental effort obsessing over tone... err, practicing.
And then there are those of us who obsess over technical minutiae, hence this treatise on effect bypass. While treatise may be a bit hyperbolic, it is a topic that does offer a satisfying amount of implementation nuance; it is, at a minimum, an interesting engineering exercise.
I like to tinker in DIY audio electronics. For years, I spent a lot of time engaged with diyAudio. Later, when I resumed my too-long-dormant hobby of guitar playing, I applied that love of DIY electronics to making effects for my guitar. I've built countless pedals from vendors such as Aion, PedalPCB, and MadBean.
Eventually, I came two realize there are two different kinds of on/off switches: the "soft-touch", and the "clicky". Those two categories are really only applicable from the user's perspective. That is, when actually cycling the pedal between bypass and engaged, is there a noticeable, tactile "click" or "snap" in the button's action? Or is the button's movement smooth and consistent throughout its whole range of action?
Based on my experience, and from a more technical, or behind-the-scenes perspective, the clicky switches are mechanical in nature, while the smooth/soft-touch switches are momentary. See the primer section for more information about switches.
Mechanical switches (in particular, the nearly-ubiquitous 3PDT), are relatively cheap, and perhaps more importantly, trivial to use in a guitar effect pedal.
On the other hand, pedals that use momentary (typically "SPST") switches have decidedly more complex bypass schemes. In this scenario, the footswitch is simply used as a "user interface" component: it does not actually change the effect between engaged and bypass, but instead sends a message (or "signal", to use more technical parlance) to other circuit components that says, "the user would like to turn the effect on (or off)".
Exactly what happens with that signal, i.e. the "please change to on or off" message, is where things get interesting...
When we use a momentary SPST-type footswitch, something else does the actual circuit switching, i.e. routing your signal either through the effect, or bypassing the effect entirely and sending it on to the next effect or amplifier. Loosely speaking, that "something else" typically falls into one of two categories:
The classic analog Boss pedals are typically of the first variety. Effect switching is done electronically, through the use of transistors. This is generally very reliable (Boss quality and longevity speaks for itself), as well as reasonably simple and cost-effective. One major downside is that the effect will not work at all (won't even allow for a bypassed signal) if it does not have power. In other words, an unpowered effect pedal that uses electrical switching (such as classic analog Boss pedals) in your effect pedal chain is no different than having your cable unplugged. Another potential downside is that a poorly designed buffer can impact your bypassed signal. That is, the expectation for an effect pedal in bypass mode is that it should be no different tonally than not having that effect at all (as though it were replaced by a short cable). But a poorly-designed or implemented bypass buffer can have unintended tonal effects.
A relay bypass, on the other hand, being mechanical in nature, allows for "true bypass" switching. Since connections are physical (as opposed to electrical), it can operate in bypass mode even without power. Buffers are therefore optional; if no bypass buffer exists, then the effect in bypass mode is indeed truly the same as being a short length of wire.
A relay is, functionally, the same as the 3PDT mechanical switch, in that it physically makes one or more connections between wires. The difference is how the switch is operated or activated: the 3PDT mechanical switch requires physical force, i.e. the user's foot. The electronic relay requires an electronic signal.
Broadly speaking, there are two relay types: latching and non-latching. Non-latching is conceptually similar to the momentary footswitch, in that there is a default state that requires no power, while the other state is actively only as long as a continous force (electrical force, i.e. current) is applied. Guitar effects that use the non-latching relay are typically designed to have the relay's default (no power) state be bypass mode, and the active (power required) state for effect engaged. The nice thing about this is that it acts as a built-in failsafe: say the battery of your boost pedal dies in the middle of your solo: you'll lose the boost, but will at least still have a viable signal going to your amp!
The main drawback to non-latching relays in guitar effect pedals is the added power (current) requirement. Generally, they are unsuitable for battery-powered effects. A typical relay might use about 20mA of current. By comparison, most analog overdrive/distortion effect circuits take well under 20mA.
On the other hand, the latching relay is conceptually similar to the 3PDT switch, in that it only requires force to change states, but once the change is made, no more power is required.
The most common mechanical switch used in guitar effect pedals is the three-pole, double-thow (3PDT). As the name implies, the 3PDT switch is compresed of three "single-pole, double-throw" (SPDT) switches that work together in parallel. To visualize the SPDT switch, imagine you hold a wire in your hand; in front of you are two more wires. Your job, as the SPDT switch, is to always maintain a connection between the wire in your hand and one of the two wires before you. Whenever you are asked to switch, you move the connection from one wire to the other. Congratulations, you're now a SPDT switch.
Now imagine you have two clones. Each clone has a wire in his hand, and also two wires before him. Between the three of you, there are nine total wires: three held by you and your clones, and a pair before each of you. Whenever you are asked to switch, all three of you switch from one wire to the other at the same time. Congratulations, you and your clones are now a 3PDT switch!
So what does a momentary switch look like? The kind most typically found in guitar effect pedals is the single-pole, single-throw (SPST). To act this out, you will old one wire in your hand, and there will be but a single wire before you. With the SPST switch, you had two "states" the wire in your hand was always connected to one of the wires before you; except for the brief moment when you switched wires, the wire in your hand must always be connected. However, with the SPST momentary switch, your two states are either connected (i.e. "closed") or not-connected ("open"). Furthermore, your arm is connected to a spring, so that in one state, you have to push against the spring. Therefore, the normal state for you, as a switch, is to keep your arm relaxed, and not pushing against the spring. When you are asked to "switch", you do so only for a moment, and then the spring pushes your arm back. The only way to keep your arm flexed against the string is if you are continually commanded to do so.
(A further qualification of SPST momentary switches is normally open or normally closed. Normally open means there is by default no connection; the switch is connected (i.e. closed) only as long as the spring is compressed. Normally open is the exact opposite.)
There are of course, in general, countless other varieties of switches. Even when constraining the scope to effect pedals, there are a few other types we sometimes encounter. But 3PDT mechanical and SPST momentary are two of the most common.
The implementation of effect switching using a 3PDT switch is trivial and well-documented. In short: solder the nine lugs to the right place on the effect's PCB and/or I/O jacks. Done!
However, the implementation of a switching scheme that uses a SPST momentary switch is decidedly more complicated.
From an engineering perspective, we must start with a list of requirements to drive our design. I think the first two requirements - reliability and predictability - are the absolute most important; the other requirements cannot compromise those in anyway. From the user's perspective, if the effect cannot be reliably and predictably switched on or off, it's worthless.
While I've never experienced a 3PDT switch failure, reports of them abound on the Internet. But a simplistic look at rated cycles numbers from switch manufacturers is compelling: standard 3PDT swtiches are rated for around 30,000 to 50,000 cycles. Most electronically controlled relays are rated in the millions of cyles, maybe even 10s of millions. Intuitively, a relay-based approach is more reliable, because the switching mechanism is decoupled from the user's foot. In other words, with a 3PDT switch, the most critical device (i.e. the one that controls whether or not your effect turns on and off reliably) is the same component that takes the most abust (i.e. your foot stomping on it). Whereas with a relay bypass, the switching mechanism is electronically controlled, and not physically or mechanically coupled to the footswitch.
I also happen to prefer the soft-touch momentary switches.