I am a guitar enthusiast and a hobbyist player. The majority of my projects happen around guitar paraphernalia. I build my owns amps and some effect pedals.
In the past I played in a small band and convinced myself that I only needed an amp with a reverb, a clean channel and a dirty channel, and a tube screamer pedal to boost my guitar for soloing. I avoided having several pedals because I am sloppy and wouldn't engage the correct ones, I do not know how to tap dance.
The other problem that happens with having several pedals in a chain is that some of them are not true by-pass. As a result, if you don't use a buffer you will lose some definition in the signal, even when the pedals are not engaged. Some common examples of these pedals are: my Ibanez TS-10, a Crybaby Wah, a Boss BF-3 Flanger, you get the idea.
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There are digital pedalboards that allow you to setup individual buttons for a pre-defined combination of digitally simulated effects. But, dealing with programming a digital platform, loading patches, setups, etc. bother me big-time. Besides, they are definitely not true bypass.
Finally, I already have pedals and I like them individually. I can setup the pedal I want and change its presets without the need of a computer (or my phone).
I found a product by Carl-Martin called Octa-Switch that was exactly what I wanted, at almost $430 it was and still is not for me. Anyways, it is going to be the base of my design.
Diy Loop Effects Pedal
I think that it is possible to build a platform with my requirements, for less than a quarter than buying it from the store. I do not have an Octa-Switch, never owned one, or play with it, so I do not know what is inside. This is my own take.
For the schematics, layout and PCB design I will be using both DIYLC, and Eagle. I will use DIYLC for wiring designs that not need PCB, Eagle for the final design and PCB.
This simple circuit allows you to bypass a pedal using a 9-pin 3PDT Foot Switch and 4 input jacks (1/4 mono). If you want to add an on/off LED, you will need: an LED, a 390 Ohms 1/4 watt resistor, a battery holder for 9V, and a 9 volts battery.
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So the total, including the box, for this project is $11.36. It is the same circuit sold at eBay for $18 as a kit, so you would have to build it.
The way this circuit works is very intuitive. The signal from the guitar enters X2 (input jack). In rest position (effect pedal not engaged), the signal from X2 bypasses the pedal and goes directly to X4 (output jack). When you activate the pedal, the signal enters X2, goes to X1 (out to pedal input), returns through X3 (in from pedal output)and exits via X4.
Expanding on the simple on/off switch idea, I wanted to be able to bypass simultaneously more than 1 pedal. One solution would be to use a foot switch that have several DPDT in parallel, one switch per pedal to be added. This idea is impractical for more than 2 pedals, so I discarded it.
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Another idea would be to trigger several DPDT switches (one per pedal) at the same time. This idea is challenging because it means that one should simultaneously activate as many footswitches as pedals needed. As I said before, I am not good at tap dancing.
The third idea is an improvement on this last one. I decided that I could trigger low signal DPDT relays (each relay acts as a DPDT switch), and combine the relays with DIP switches. I could use a DIP switch with as many individual switches as relays (pedals) are needed.
In this way I will be able to select what relays I want to activate at any given time. In one end, every individual switch in the DIP switch will connect to the coil of the Relays. On the other end, the DIP Switch will connect to a single on off switch.
Looping Foot Pedal
Fig 1 is the complete schematic for 8 Relays (8 pedals), Fig 2 is the detail of the switch section of Relay 1 (K9), and the 3rd file is the Eagle Schematic.
It is easy to see that the bypass section (Fig 2) is exactly the same circuit as the one discussed in Step 1. I kept the same denomination for the jacks (X1, X2, X3, X4), so the explanation of how the bypass work is the same word by word than the one for Step 1.
In the complete schematics for 8 relays (Fig 1) I added switch transistors (Q1 - Q7, Q9), polarization resistors to set the transistors as On-Off switches (R1 to R16), an 8 switches DIP Switch (S1-1 to S1-8), an on/off switch (S2) and the LEDs that indicate what relays are on.
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In saturation VCE (DC voltage between collector and emitter) is approximately 0 V, so VCC is applied to the selected relays turning them on.
This part of the project could be done without the transistors, using the DIP Switch and the S2 to either VCC or Ground. But I decided to use the complete circuit so there is no need for further explanation when the logic part is added.
The diodes in reverse, parallel to the relays' coils, protect the circuit from the transients generated with the activation/deactivation of the relays. They are known as fly back or flywheels diodes.
Programmable 8 Looper
The next step was to think how to add more versatility to the idea. In the end I want to be able to have several possible combinations of pedals that are selected by pressing different foot switches. For example I want to have pedals 1, 2 and 7 working when I press one foot switch; and I want pedals 2, 4 and 8 when I press another.
The solution is to add another DIP Switch and another foot switch, Fig 3. Functionally it is the same circuit than the one explained in the previous STEP.
When pressing S2, those S1-X switches that are ON will activate the correct relays, BUT S3-4 and S3-8 will also be activated via the shortcut S1-2 // S3-2. Even though S4 is not grounding S3-4 and S3-8, they are grounded via S3-2.
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The solution to this problem is to add diodes (D9-D24) that will be opposing any short cut (Fig 4). Now in the same example when S2-2 is at 0 V D18 is not conducting. It does not matter how S-3 and S3-8 are setup, D18 will not allow any flow of current. Q3 and Q7 will remain off.
Although the simple circuit explained so far can be extended with as many DIP switches as combination of pedals are wanted, there is still a drawback. The user needs to activate and deactivate the footswitches one by one according to the combination required.
In other words, if you have several DIP Switches, and you need the pedals on DIP Switch 1, you have to activate the associated foot switch and disengage any other footswitch. If not, you will be combining the effects in as many DIP switches as you have active simultaneously.
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This solution makes the user's life easier in the sense that with only 1 footswitch you can activate several pedals at the same time. It does not require you to activate each effect pedal individually. The design can still improve.
I want to activate the DIP switches not with a foot-switch that is always on or off, but with a momentary switch that remembers my selection until I select another DIP Switch. An electronic latch.
I decided that 8 different configurable combinations of 8 pedals will suffice for my application and it makes this project comparable to the Octa-switch. 8 different configurable combinations mean 8 footswitches, 8 pedals mean 8 relays and associated circuitry.
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I chose the Octal edge triggered D type Flip Flop 74AC534, this is a personal choice and I assume that there might be other IC's that will also fit the bill.
According to the datasheet: On the positive transition of the clock (CLK) input, the Q outputs are set to the complements of the logic levels set up at the data (D) inputs.
Which essentially translates to: every time the pin CLK sees a pulse going from 0 to 1 the IC reads the state of the 8 data inputs (1D to 8D) and sets the 8 data outputs (1Q/ to 8Q/) as the complement of the corresponding input.
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In any other moment, with OE/ connected to ground, the data output maintains the value read during the last CLK 0 to 1 transition.
For the input switch I chose SPST Momentary Switches ($1.63 in eBay), and set them up as shown in Fig 6. It is a simple Pull down circuit, with a de-bounce capacitor.
At rest, the Resistor pulls the output 1D to VCC (High), when the momentary switch is activated 1D is pulled down to ground (Low). The capacitor eliminates transients associated with the activation/deactivation of the momentary switch.
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The last piece of this section would
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