Pedal-Pi is a lo-fi programmable guitar pedal that works with the Raspberry Pi ZERO Board. The project is totally Open Source & Open Hardware and made for hackers, programmers and musicians that want to experiment with sounds and learn about digital audio. 

 

You can code your own effects using standard C and get inspiration from the ready-to-use effects from the forum, like the Clean/Transparent, Booster/Volume, Distortion, Fuzz, Delay, Echo, Octaver, Reverb, Tremolo, Looper, etc.

The project was created with the aim of having fun and learning about guitar pedals. If you want to program digital audio effects in C without deep knowledge on DSP languages or electronics, this pedal is for you. But take also into consideration that it is only 12 bits and does not feel like a finished pedal that you could buy in a shop. 

  

pedal pi intro  

Specifications.

  • Based in Raspberry Pi Zero (1GHz ARM11 core).
  • Analog stages using MCP6002 rail-to-rail operational amplifier.
  • ADC: 12bits / Sampling Rate 50Ksps (MCP3202).
  • Output Stage: 12 bits (2x6bits PWMs running in parallel)
  • Pi Zero:
    • 1GHz ARM11 core.
    • 512MB of LPDDR2 SDRAM.
    • Micro-SD card slot.
  • Interface:
    • 2 Configurable push buttons.
    • 1 Configurable switch.
    • 1 programmable blue led.
    • True Bypass Foot-switch.
  • Connectors:
    • Input Jack, 1/4 inch unbalanced, Zin=1MΩ.
    • Output Jack, 1/4 inch unbalanced, Zout=100Ω.
    • Power supply: power taken from the Pi Zero board (micro-USB).

     

Review by Blitz City DIY:

 

How Does the Circuit Work?

This hat has three parts:

  • The Input Stage: Amplifies and filters the guitar signal making it ready for the ADC (Analog do Digital Converter). The ADC sends the signal to the PI ZERO using SPI communication. In the forum the topic "Using MCP3202 ADC with Raspberry Pi Zero" gives more details about the ADC-Pi ZERO connection.
  • Pi ZERO: It takes the digitalized audio waveform from the ADC and does all the Digital Signal Processing (DSP) creating effects (distortion, fuzz, delay, echo, tremolo...).  In the forum the topic "Basics of Audio DSP in C for Rapsberry Pi Zero" can assist you to learn the basics.
  • The Output Stage: Once the new digital waveform is created, the Pi Zero creates an analogue signal with two PWMs combined, the signal is filtered and prepared to be sent to the next pedal or the guitar amp.  For more info check the topic "PWM Audio on Raspberry Pi Zero".

pedal pi block diagram small

If you want to understand all the details of the circuit, please read the Pedal-Pi Circuit Analysis:

 

How to Program it?

The idea is to make it as easy as possible, the examples are programmed in C using the standard BCM2835 Libraries. The Operating System used is Raspbian.  All tools and programs are free/open source. In the forum there is a topic called "How to Start Programming Pedal-Pi" with more details.

Basic knowledge of Arduino, Linux and C are needed. Have a look to the real code of a Clean effect:

Clean Effect Pedal:

The block diagram of this code looks like this:

pedal pi software flow

The real code used looks like this:

// CC-by-www.Electrosmash.com Pedl-Pi open-source project
// clean.c effect pedal, the signal is read by the ADC and written again using 2 PWM signals. 
 
#include <stdio.h>
#include <bcm2835.h> // Define Input Pins #define PUSH1 RPI_GPIO_P1_08 //GPIO14 #define PUSH2 RPI_V2_GPIO_P1_38 //GPIO20 #define TOGGLE_SWITCH RPI_V2_GPIO_P1_32 //GPIO12 #define FOOT_SWITCH RPI_GPIO_P1_10 //GPIO15 #define LED RPI_V2_GPIO_P1_36 //GPIO16 uint32_t read_timer=0; uint32_t input_signal=0; uint8_t FOOT_SWITCH_val; uint8_t TOGGLE_SWITCH_val; uint8_t PUSH1_val; uint8_t PUSH2_val; //main program int main(int argc, char **argv) { // Start the BCM2835 Library to access GPIO. if (!bcm2835_init()) {printf("bcm2835_init failed. Are you running as root??\n"); return 1;} // Start the SPI BUS. if (!bcm2835_spi_begin()) {printf("bcm2835_spi_begin failed. Are you running as root??\n"); return 1;} //define PWM mode bcm2835_gpio_fsel(18,BCM2835_GPIO_FSEL_ALT5 ); //PWM0 signal on GPIO18 bcm2835_gpio_fsel(13,BCM2835_GPIO_FSEL_ALT0 ); //PWM1 signal on GPIO13 bcm2835_pwm_set_clock(2); // Max clk frequency (19.2MHz/2 = 9.6MHz) bcm2835_pwm_set_mode(0,1 , 1); //channel 0, markspace mode, PWM enabled. bcm2835_pwm_set_range(0,64); //channel 0, 64 is max range (6bits): 9.6MHz/64=150KHz PWM freq. bcm2835_pwm_set_mode(1, 1, 1); //channel 1, markspace mode, PWM enabled. bcm2835_pwm_set_range(1,64); //channel 0, 64 is max range (6bits): 9.6MHz/64=150KHz PWM freq. //define SPI bus configuration bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST); // The default bcm2835_spi_setDataMode(BCM2835_SPI_MODE0); // The default bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_64); // 4MHz clock with _64 bcm2835_spi_chipSelect(BCM2835_SPI_CS0); // The default bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW); // the default uint8_t mosi[10] = { 0x01, 0x00, 0x00 }; //12 bit ADC read channel 0. uint8_t miso[10] = { 0 }; //Define GPIO pins configuration bcm2835_gpio_fsel(PUSH1, BCM2835_GPIO_FSEL_INPT); //PUSH1 button as input bcm2835_gpio_fsel(PUSH2, BCM2835_GPIO_FSEL_INPT); //PUSH2 button as input bcm2835_gpio_fsel(TOGGLE_SWITCH, BCM2835_GPIO_FSEL_INPT); //TOGGLE_SWITCH as input bcm2835_gpio_fsel(FOOT_SWITCH, BCM2835_GPIO_FSEL_INPT); //FOOT_SWITCH as input bcm2835_gpio_fsel(LED, BCM2835_GPIO_FSEL_OUTP); //LED as output bcm2835_gpio_set_pud(PUSH1, BCM2835_GPIO_PUD_UP); //PUSH1 pull-up enabled bcm2835_gpio_set_pud(PUSH2, BCM2835_GPIO_PUD_UP); //PUSH2 pull-up enabled bcm2835_gpio_set_pud(TOGGLE_SWITCH, BCM2835_GPIO_PUD_UP); //TOGGLE_SWITCH pull-up enabled bcm2835_gpio_set_pud(FOOT_SWITCH, BCM2835_GPIO_PUD_UP); //FOOT_SWITCH pull-up enabled while(1) //Main Loop { //Read the PUSH buttons every 50000 times (0.25s) to save resources. read_timer++; if (read_timer==50000) { read_timer=0; uint8_t PUSH1_val = bcm2835_gpio_lev(PUSH1); uint8_t PUSH2_val = bcm2835_gpio_lev(PUSH2); TOGGLE_SWITCH_val = bcm2835_gpio_lev(TOGGLE_SWITCH); uint8_t FOOT_SWITCH_val = bcm2835_gpio_lev(FOOT_SWITCH); bcm2835_gpio_write(LED,!FOOT_SWITCH_val); //light the effect when the footswitch is activated. } //read 12 bits ADC bcm2835_spi_transfernb(mosi, miso, 3); input_signal = miso[2] + ((miso[1] & 0x0F) << 8); //**** CLEAN EFFECT ***/// //Nothing to do, the input_signal goes directly to the PWM output. //generate output PWM signal 6 bits bcm2835_pwm_set_data(1,input_signal & 0x3F); bcm2835_pwm_set_data(0,input_signal >> 6); } //close all and exit bcm2835_spi_end(); bcm2835_close(); return 0; }

This is a graphical representation of the whole process:

pedal pi dsp1

 

Online Learning Material

We have prepared a series of articles for you to learn more about Digital Signal Processing for Audio, Raspberry Pi PWM Audio, ADC connection and guitar effects.

You can find more info in the Pedal-Pi Forum. 

 

cart logo  Buy Pedal PI online.

There are 3 options in the store:

  1. Order only the PCB. It uses easy-to-find standard components and you can build the kit yourself. The bill of materials is public with all the references and the Mouser part numbers.
  2. Order the PCB + Transparent Acrylic Cover.
  3. Order the Full Kit: This kit includes the PCB, Cover and all the components to build Pedal-Pi at home. You would only need to buy the raspberry Pi-Zero by yourself.

If you have any question, contact us.

pedal pi image

 Bill Of Materials:

The components are easy-to-find through hole parts with the minimum number of references:

Pedal-Pi Bill of Materials.
Reference Qty Value Description Mouser Reference
Capacitors
C2,C4, C6, C7, C8 5 6.8nF ceramic cap SR211C682MARTR1
C1, C10, C15 3 100nF ceramic cap SR211C104KARTR1
C3, C9, C16
3 4.7uF electrolytic cap ECE-A1EKA4R7 
C5 1 270pF ceramic cap K271J15C0GF5TL2
C11, C12, C13, C14 4 220uF electrolytic cap REA221M1CBK-0811P
Resistors    
R0, R1
2 1MΩ Resistor, 1%,1/4W MFR-25FRF52-4K7
R2, R3, R6, R9, R10, R15
6  4.7KΩ Resistor, 1%,1/4W MFR-25FRF52-100K
R4, R11
2  100KΩ Resistor, 1%,1/4W MFR-25FRF52-1M
R7 1 300KΩ
Resistor, 1%,1/4W MFR-25FRF52-300K
R12 1 50KΩ Resistor, 1%,1/4W MFR-25FRF52-49K9
R13, R14 2 300Ω Resistor, 1%,1/4W MFR-25FRF52-300R
Others    
RV1 1 500K resistor trimmer 3319W-1-504
D3 1 Led 3mm blue blue led 3mm SSL-LX3044USBC
U1 1 MCP6002
op-amp rail-to-rail, pdip8
MCP6002 
U2 1 MCP3202 12bit ADC, p-dip8 MCP3202 
IC Socket 2 dip 8 socket socket dor dip8 1-2199298-2
SW1 1 3DPT footswitch 3PDT footsitch 107-SF17020F-32-21RL
SW4 1 Toggle switch SPDT toogle switch 612-100-A1111
SW2, SW3 2 Pushbutton off-on pushbutton  
Conn1 1 40 pin header
2.54 pitch pin header
J1, J2 2 1/4 Jack audio stereo 6.35mm jack NMJ6HCD2
PCB + Cover + Screws  
Pedal Pi PCB 1     electrosmash store
Acrylic Cover 1     electrosmash store
M2.5x15 + 6 nuts 8      

 pedal pi start2 small

Frequently Asked Questions (FAQ):

Where can I buy the Raspberry Pi ZERO board?

Is Pedal-Pi compatible with all Raspberry Pi boards?

  • Not yet, we developed this project for PI ZERO but in theory it would be compatible with other Raspberry Pi boards.

Is Pedal-Pi suitable for bass players?

  •  Yes, but you need to swap 1 capacitor (C1) from 0.1uF to 0.47uF, allowing lower frequencies into the circuit, that is all. If you are buying the kit in our store, send us an email and we will include this caps for you.

Can I plug headphones directly to pedal Pi?

  •  No, pedal Pi is not an amplifier. It needs to be plugged to a guitar amplifier, or to a multi-effects (Line 6 pods, etc..).

 

  Thanks for reading, all feedback is appreciated    jr(at)ElectroSmash.com

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