MLS-5+
A 5-loop, MIDI-Controlled Switcher
My custom-built pedalboard, featuring the MLS-5 on the top deck
Overview
UPDATED FOR 2026!
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What Has Changed?
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There have been several notable improvements and changes made to the new revision, including:
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Full Stereo Loops
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More responsive and robust switching
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Press and Hold Momentary functionality
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Exclusivity (flip/flop) switching
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MIDI-Thru
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Read below for more details!
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The core of my pedalboard is a Boss MS-3, which I quite like. However, the drive sounds were a little hit and miss for me, so the onboard loops got filled with overdrives pretty quickly and I wanted more. I decided that what I needed was a loop switcher that the MS-3 could control via MIDI. I set to work in the top secret lab with the only real constraints being that I had to be able to fit it all into a 1032l enclosure.
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Taking some measurements, I found that 5 loops was about as good as I could get, so that determined that. I then set to designing the relay control circuits for the microcontroller to work with. After a couple weeks, I had it all finished up. I now have 7 loops at my disposal and I couldn't be happier.
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Want to make one yourself? Everything you need to make it, including the microcontroller code, BOM, writeup, etc. can be found here. Let us know how you like it!
How It Works
The MLS-5 is really composed of two primary parts: the microcontroller and the loop circuitry. The microcontroller used in this case is the Arduino Nano (mine is actually a Chinese knockoff with the CH340G USB chip). I used the Nano because it is compact and has hardware serial support, which is best for dealing with MIDI. As seen in the schematic snippet below, there are a few components necessary for the MIDI path, such as the optoisolator, which helps provide stability for the signal. Additionally, I have separate 5V regulators so that I can keep the Arduino power rail separate from the relay power rail. You can also see the MIDI through connections which are taken from the optoisolated output.
MLS-5+ Microcontroller Section
The second main component is the loop circuitry. Each loop is identical, comprised of two DPDT relays controlled by a single output from the microcontroller. This allows for true stereo, true bypass switching of each loop. The output of each loop goes to the input of the next, with the exception of the last loop, which goes to the output jack. The working principle is rather simple. A BJT is used as a resistor whose state we can change for each relay. When no power is supplied to the base of the transistor, no power flows through it, so the relay is off. When we apply a voltage to it, power flows, which activates the relay and enables the loop. The diode across the + and - terminals of the relay is for stability.
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Some relay switching uses latching relays, which only requires a voltage pulse to cause them to change state. Latching relays work well, but I chose non-latching so that I could tie the indicator LED to the same pin as the relay. The design could be modified for latching relays, but that would require 3 pins of the MCU per loop (relay on, relay off, LED indicator) as opposed to 1.
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As seen in the figure below, there is also a resistor divider so that there is a path for the BJT base voltage to be pulled to ground when the loop is turned off. Additionally, there is a flyback diode that will help with voltage spikes and instability.
MLS-5+ Relay Control Circuitry
The actual wiring of the relays is a true bypass configuration and is rather boring to look at. Don't believe me? Look below:
MLS-5+ Loop Circuitry
One of the new features of this version is the addition of exclusivity switching. This feature allows you to designate loops that operate exclusively, meaning that when you turn it on, all other loops that are also exclusive are turned off. This is very useful if you have a loop for a clean tone and another for a drive tone that have separate effects. When you switch your drive loop on, the clean one turns off, making it easy to switch multiple effects on/off simultaneously. This is similar to the "flip-flop" mode of the Gigrig Quartermaster. An idea for future modification is to use on/off/on switches allowing for there to be two different exclusivity states, but that's for another time. This feature from a schematic standpoint is just a switch that connects power to the appropriate input pin.
MLS-5+ Exclusivity Switches
A feature that is included that is the antithesis of sexy is hardware debouncing of the switches. This makes the switching more reliable by not accidentally triggering due to the bouncing of the switch contacts. It's a couple of resistors and a capacitor that basically form a low pass filter that filters out any high frequency bouncing of the switch contacts.
MLS-5+ Switch Debouncing
Want to make one yourself? Everything you need to make it, including the microcontroller code, BOM, writeup, etc. can be found here. Let us know how you like it!