Arduino Guitar Pedal
Abstract
This project involves replicating a low-fidelity, digital guitar pedal as shown on Instructables. The guitar pedal uses the Arduino Uno for digital signal processing of the guitar input and outputs a processed digital signal to a guitar amp. The focus of this project involved replacing the Arduino shield with a custom printed circuit board that connects to the Arduino Uno.
How the Arduino Guitar Pedal Works
Figure 1 - PCB Schematic
he guitar signal goes into the circuit via J1 connector and goes to the TL082 IC (U2) at pin 5 (via AC coupling, C6) for amplification by one of the op-amps. R18 and R14 control the gain of the amplifier, and the addition of C5 makes the op-amp act as an active low-pass filter. R2 and R9 create a voltage divider to set the reference voltage of 4.5V for the non-inverting inputs of the op-amps. The output from the op-amp (pin 7 of U2) connects to R23 and R19.
R19 is a potentiometer that controls the loudness of the clean guitar signal that goes out to the guitar amp via J3 connector. R23 scales the amplified guitar signal and that signal is AC couple to a resistor divider through C9. This resistor divider shifts the DC level of the guitar signal to around 0.5V, which allows the ADC to sample the signal via A0 pin onboard the Arduino Uno.
The switch, S1, with six resistor values connected to it allows the Arduino to read different analog reference voltages. This allows the user to select between different pre-programmed guitar effects onboard the Arduino Uno. R12 is a potentiometer that controls the tone of the guitar effect. Once the signal has been sampled and processed by the Arduino Uno, the Arduino Uno creates pulse-width modulated signals that go to pins D3 and D11 and go through resistors R13 and R10 for scaling. These signals go through the potentiometer R17 that controls the loudness of the guitar effect. The pulse-width modulated signals and the clean guitar signal mix in via the second op-amp in the TL082 IC (U2). This amplified mixed signal goes out to the guitar amp via AC coupling through C3 and the J3 connector.
Guitar Pedal Chassis
Figures 2 and 3 - Input and output jacks
The guitar signal into and out of the guitar pedal come from 1/4" stereo audio jacks on the side of the pedal labeled “IN” and “OUT.” The jacks also serve the purpose of turning on the circuit by completing the circuit from the negative terminal of the 9V battery to the ground connection on the PCB if a guitar pedal is plugged into the “IN” connection of the pedal.
Figure 5: Terminals of DPDT switch
The double pole, double throw (DPDT) stomp switch acts as a true bypass switch for the guitar. This means that the switch can either send the guitar signal to the PCB, or short the connection and send the guitar signal straight through to the guitar amp without adding any guitar effects. This is done by shorting to outer pair terminals of the switch, and connecting the middle pairs to the 1/4" jack and to remaining pairs the PCB (J1 and J3).
Conclusion and Discussion
Overall, the project was mostly a success. The guitar pedal functioned as explained by the Instructables after I verified the pedal using a guitar and guitar amp. The guitar pedal played different effects by using the FX switch (See Figure 6). However, the quality of some of the guitar effects was low quality or lagged in performance. This is due to the memory and analog to digital bit resolution limitations of the Arduino Uno. However, the quality of the guitar signal with the switch set to bypass to the guitar amp did not experience any significant attenuation.
Another issue I could not resolve, but did not affect the performance of the guitar, was not taking into account of all the physical dimensions of the materials. This included the vertical height of the PCB with the Arduino Uno attached, the length and width of the audio jacks, and the length and width of the battery. Because of this negligence, I could not fit all the contents of the pedal inside the chassis.
Reference
Downloads
- Presentation (PDF)
- BOM (PDF)
- Layout (JPEG)