MLS-5
A 5-loop, MIDI-Controlled Switcher
My custom-built pedalboard, featuring the MLS-5 on the top deck
Overview
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. Also, there is a two position DIP switch. When programming the Nano, these need to be off, otherwise they stay in the on position.
MLS-5 Microcontroller Section
The second component is the loop circuitry. Each loop is identical, with the output of each loop going 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. 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 for certain circumstances, but I chose non-latching so that I could tie the indicator LED to the same pin as the relay. Of course, only after completing it all did I realize how easy it would be to have a separate pin do the LED indication. Oh well.
MLS-5 Loop/Relay Circuitry
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!