MBED_Guitar_Pedal

Page last updated 12 Dec 2014, by wes adams. 2 replies

This is basically a way to create a "generic" guitar pedal. By that, I mean that this is a great way to get a guitar signal into your mbed at the correct voltage, and then get the signal back out to send it through an amp. I've included code to generate an echo/delay effect, but the code can be changed to generate any number of effects.

Code

simple code

#include "mbed.h"

#define BUFFER_MAX   15000
#define BUFFER_MIN   50

AnalogIn fromGuitar(p19);
AnalogOut toAmp(p18);
AnalogIn buffer_size(p15);

unsigned short buffer[BUFFER_MAX];

int inv_gain = 3;
int delay = BUFFER_MAX;

void getValues(void) {
    delay = buffer_size*BUFFER_MAX;
    if (delay < BUFFER_MIN)
        delay = BUFFER_MIN;
}

int main() {
    int i;
    for (i = 0; i < delay; i++)
        buffer[i] += fromGuitar.read_u16();
    for (i = 0; ; ) {
        buffer[i] = buffer[i]/inv_gain + fromGuitar.read_u16();
        toAmp.write_u16(buffer[i]);
        i = (i+1) % delay;
        if (i == 0)
            getValues();
    }
}

Circuit Diagram

/media/uploads/Shizler/guitar_pedal_.png

  • R1,R7, R8
    • 2.2k
  • R2,R4
    • 100k
  • R3
    • 47k
  • R5, R6
    • 4.7k
  • R10, R11
    • 10k
  • C1
    • 47uF
  • C2
    • 2.2nF
  • C3
    • 4.7nF
  • C4
    • 470uF
  • C5, C6
    • 0.1uF

Comments

As stated before, this should be used as a guide to connect a guitar and amp to the mbed controller. Use this to create a skeleton guitar pedal that you can program to whatever effect you desire.

2 comments on MBED_Guitar_Pedal:

Michael Bawden
22 Feb 2015

Hello Wes,
nice project to go on with. What op-amps are you using and are they especially designed for audio (is there such a thing?) or are they just run of the mill op-amps?. I notice that the non-inverting input of U5 is not connected to anything, is this normal?
Cheers
Mike

Nestor Skiadas
14 Feb 2017

I know this is a year later, but if anyone is asking the same question, op-amps generally don't affect audio signals, especially at the levels this circuit is working at.

Please log in to post comments.