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Picture of Homebrew Digital Effects Pedal
Hi, my name is Colin. I am an avid guitarist, and I am always looking for cool new sounds. I'm a big fan of the guitarist Paul Gilbert, and I heard him play this crazy effect in one of his songs, so I decided I would try to emulate it. The result was a very long endeavor, but in the end a successful one.

 

I have created a digital loop pedal. The pedal itself is based upon the PIC16F877 microcontroller, and is complete with 256kb of RAM for storing sound. The unit achieves true bypass with the help of a relay, and runs on a standard 9V power supply.

If you want to build a pedal like this, be warned- it is a daunting task. The complexity of this project reached the upper limit of what my brain can autoroute on a prototyping board, and was very frustrating many times when it didn't work as planned.

In the end however, I learned a lot. I think this is a great project to try, but only try if you are really dedicated. For those interested, I would suggest you design your own, using mine as a template to know what works and what doesn't, rather than copying directly- though if you wish to copy, you are more than welcome.
 
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Step 1: Audio-only demo

Picture of Audio-only demo
This is an audio-only demo I recorded.



Step 2: Materials

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I used a number of ICs in the construction of this project, they are listed below.

  - PIC16F877a with 20MHz oscillator
  - 23k256 RAM
operating at 20MHz, interfaced via three-wire SPI
  - A standard 9V single throw relay (though I recommend double throw, they are more useful)
  - IRFD120 N-channel MOSFET for powering the relay
  - LM358 operational amplifier
  - 1 Mohm trim pot (for mixing ADC and unprocessed guitar when the pedal is not in "kill" mode)
  - a bunch of 10k and 20k ohm resistors for the DAC
  - 4 switches
  - Aluminum enclosure
  - Two LEDs
  - 3 10k potentiometers
  - lots and lots of stranded-core wire
   - Guitar audio jacks
  - Barrel plug socket for DC adapter

Resistances and capacitances are generally labeled on the schematic in the next step.

Step 3: Schematic, explanations

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This section addresses the circuits behind the project.

Explanation:


When the circuit is powered off, the relay is disengaged, and the guitar signal comes directly through the pedal, truly bypassed.

When the circuit is powered on, the relay remains disengaged and the signal is nearly fully bypassed, with most of the signal coming directly to the output, but some part of it is amplified through the op-amp (with gain set by the "gain" knob) and sent  to the PIC for ADC processing.

When the system goes into "record" mode, the ADC values are transmitted to the 23k256 for temporary storage using the "continuous write" mode, overwriting previous data if there is any there.

When the system switches back into playback mode, the relay is engaged. Audio from the guitar gets transmitted through the amplifier if the "byp/kill" switch is in "byp" mode, otherwise it is silenced. Data is rapidly read from the 23k256 and outputted to the DAC. The resulting signal needs no amplification and is directly mixed through the trim potentiometer to the output of the pedal.

The values of the potentiometers are read continuously by ADC channels on the PIC.

Step 4: Code, explanations

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The entire purpose of this pedal is to record a short segment of sound and then play it back in a loop continuously, until the foot-switch is pressed again.

This pedal has two recording modes and two playback modes.

Recording:

"Loop" mode

In this mode, a guitarist will hit the foot-switch to begin recording, play a riff, then hit the switch again to stop playback. In this way, a guitarist can define the length of a recording during the recording process, allowing for live/dynamic loop playback.

"Burst" mode

In this mode, the guitarist defines the recording length via a knob. When ready, the foot-switch is pressed to initiate recording, which continues until the time defined by the knob is elapsed. This mode is mostly targeted at extremely short recording lengths, which can make crazy alien or robot sounds.

Playback:

"Bypassed" mode

In bypassed mode, the guitarist can play along with his own recording. When a recording is playing back, the live guitar signal is mixed in with the recorded one, which allows for interesting dynamic playing.

"Kill" mode

In "kill" mode, the signal from the guitar is killed and not mixed into the output, which allows the guitarist to adjust the pitch shift or do whatever else without worrying about their guitar making noise during playback.

Step 5: Key Insights

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I learned a lot during the course of this project. In a lot of ways, the design process was comprised of trial and error. My hope in publishing this online is that other people will learn from and build upon what I have figured out over the course of this project.

When to use proto-board and when to use PCB:
This schematic was the upper limit of complexity I am personally capable of autorouting with my brain while soldering. When circuits get complicated enough to require multiple pages of schematics, pages of diagrams, and ages of experimentation, it's probably worth it to create a PCB.
The smartest idea would have been to design a PCB and then use surface mount components. The DIP ICs with sockets barely fit inside the container- I had to dremel parts of the aluminum case away to fit all the components! Needless to say it was a tight squeeze.

Digital to Analog Converters:
I discovered that building your own DAC can work well, even with as few as 8 bits. Initially I bought a DAC IC, but it needed complicated biasing circuits, so I built one with a bunch of resistors, following the model shown here. Individual results may vary, but I played reasonably high quality sound from my iPod through it, and it fared well. Not the highest quality audio you've ever heard, but it represents the sound in a listenable manner.

RAM:
RAM is a very good way of storing and retrieving random access data quickly. I bought 23k256 20MHz SPI-interfaced RAM, totaling 256kb, which is 32kB. This ram runs off of 3V, so I ran all digital logic at 3V. Unfortunately, my PIC could only be programmed at 5V, which resulted in the accidental destruction of a few RAM chips. Good thing they are cheap!

Relays:
Always buy double-throw relays. When you need to solve a problem with a circuit, a lot of times it can be done if you have a double throw relay!

Op-amps/amplification circuitry
I had a lot of problems with my amplification circuitry. In the end, put capacitors everywhere you can anywhere you are biasing things, and separate biased supplies if you want to reduce interference between different signals being amplified separately.

COLD SOLDER JOINTS:
When you get an untrustworthy-looking solder joint during soldering, be sure to REHEAT the joint! A lot of times, if you don't do that, you can get a "cold solder joint", which can behave as an open circuit even if it passes cursory visual inspection! This has caused me problems with two different projects now, and it is an unspeakable evil that should be avoided at all costs!

Step 6: Improvements

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For the future, many improvements may be proposed, including:

 - Simple software-only modifications could make the pedal create a "delay" or "echo" effect
 - Expression pedal substitution for the shift knob (already planned)
 - Low pass filters on the output of the DAC

Any suggested improvements would be more than welcome!

Step 7: Notes

I have included here for reference selected notes from my notebook, in case they are in some way relevant to anyone.
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Hi! What relay do you use? Can you give me details?

Regs, Olgierd
lcantarini3 years ago
I've start make digital design about this Colin's job. Due the difficult to make this schema, from the Colind picture, please, feedback me about some mistake.
Thank you
Luca
Did you built it using that schema? Does it works fine??
selp lcantarini2 years ago
There's something i tried to understand : Inputs and outputs !

C3(+)Input is the audio input (from guitar)
Out is audio output (to amp)
both jack has to be solder to those input / output and to the ground ! Am I right ?

I think VFF is 12V
What is VEE ?? 3.3V ??

I see 3 potentiometers but not any switch ... Where are they ?

Please correct me if i'm wrong ! thanks
Really great DIY project!
I am not so familiar with microcontrollers programing, and seems it is to say only way to go, not like to build some digital echo or even reverb with PT 2399 ...
So, now if I would build that (I would really), for sure I need to make (to copy) only the same microcontroller circuit like your at least.
It would not be too hard?
I did not noticed you shown option how to add another sounds over first record, did you got idea about that?
Regards,
Peter
dunnos2 years ago
I am so building this with an avr! I am also going to go through hell to try and add overdubbing. :) Wish me luck!

Really nice project!
mattomatto2 years ago
Hey,

I appreciate you taking the time to write this up, it's not easy working this stuff out without an electronic engineering degree! It's helping me understand how to work with audio. I'm using a dspic33F with the mplabx IDE and C16 compiler all from Microchip, with an aim to mess about with audio loops stored in RAM. I just had a quick question..

Is there some sort of peripheral library that isn't referenced in your code? I see functions like 'output_low(PIN)' but I can't find that in any of my libraries. Is that that same as writing 0 to the latch register for that pin?

Cheers, Matt

starguitar2 years ago
Haha! Awesome! I want to build one, its like [url="http://http://www.mwfx.co.uk/judder"]this pedal/url]. I wish i could program on my mac. Kind of like the yamaha dds 20m from the 80s.
Hraaig3 years ago
Hi There.

Its really Hard to read your schematics.
Can you send the schematics via mail or make a digital version of them please?

sry for my bad english i hope you understand :)

Thank you
Hraaig
tdavis223 years ago
So I really love this looper and how it sounds. Love the very quick loop you did in the video. I am looking to buy one but I'm on a very tight budget. How much do you think it would cost to make one of these? And I don't have any electrical engineering knowledge, so how much do you think somebody would charge to make one of these?
Just a few questions from a beginner guitar player, Thanks.
arik_1233 years ago
Hi Colin
Could you please send the schematic of your amazing project.
I would like to build this effect.
TX
arik_123@walla.com
gmoon5 years ago
Hey, I like it! The playing and the style isn't my thing, but it sounds really good.

Is the PIC 16 bit ? (or a DSP chip?) Because the sample quality is much better then most 8-bit audio projects I've heard. Even more amazing 'cause your using a serial RAM...
colin353 (author)  gmoon5 years ago
Thank you! It actually is 8-bit recording. I used the PIC's internal ADC, which can handle 10-bits, but chose to use 8-bits because the SPI RAM was byte-addressable (and also for 10bit recording I'd need more ram).
hi there, hope you are well, as i am sure your no stranger to this, i was hoping to pick your brains on the digital loop pedal that you made, here are my questios as follows, - how are you programming your microchip is it an arduino? - is the unit capable of layering or overdubbing ? - can more memory be attached to the unit? - have you ever used the ardiono boards? - could something similar be achieved with the ardiono? -does the guitar go through a pre-amp? - how much did your unit cost in the end? sorry for all the questions lol but i am realy intregued as this is by far the best diy unit out on the entire internet to date and would like to base my project on yours and make adaptations. many thanks, sorry for all the questions lol, marc webb
colin353 (author)  marcjwebb3 years ago
The arduino is based upon the AVR micro controller. This project is achieved via the PIC architecture, which is slightly different, but anything done in this project could be replicated in principle by an AVR micro controller.

The PIC is programmed via a PicKit2. You can google that to see what it means. As for layering, it would require memory that is twice as fast and twice as long, but it would be possible with a faster PIC. I don't know what overdubbing means.

Preamplification is achieved by a two stage operational amplifier design, pretty straightforward. Actually the design presented here is terrible, but any operational amplifier when used correctly would suffice.

The cost I don't recollect, I imagine it was less than $40.
gmoon colin3535 years ago
How about the PIC itself (aside from the ADC sampling resolution)--is it a 16-bit uC, or an 8-bit?

Sometimes you can hide low res sampling by piping into reverb, etc., but this really does sound great...
colin353 (author)  gmoon5 years ago
The PIC itself is actually an 8-bit microcontroller.

I never considered the reverb idea, perhaps I should think about that. Thanks for the advice! 
gmoon colin3535 years ago
Wow-excellent for an 8-bit banger...
This is pretty impressive! Have you considered using the dsPIC family, as they're built for this kind of digital signal processing? I am not entirely familiar with them, but they're inherently more powerful, and you'd be able to build yourself at least a 16 bit DAC…

Well done, I look forward to seeing more of your work!
Barnaby
anassundala4 years ago
NICE PEDAL BRO...
CAN U SEND TO ME SCHEMATIC PLEASE
IF U WANT,U CAN SEND TO ahmad_dimarzio@yahoo.com
tq b4
lufox154 years ago
Great Work! i have a question which is the sample time that you can get with the pic16f877?.
I was trying to make a similar project but i can only get a maximun sample time of 625Hz.
Thanks.
microman1714 years ago
Hey Colin,

I'm working on my own digital pedal based on the PIC18F26J11 (Just what I had laying around). I'm not sure what effect I plan to try to create, but It's fun to have a go.

At the moment, I am trying to get the chip to act as a buffer, all I want it to do is read the analog into a variable, and then output it to the DAC (Mine uses 4.7k and 10k resistors, which I know gives a lower quality signal). I have a lot of trouble with noise, and have been using 100nF capacitors in places to see what I can do.

I have found I should have one series with the input, series with the output, and then one to bypass the output to ground.

I have tried the opamp circuit you have presented, but I have no idea what it's purpose is, how it works, or if mine even worked the way it was supposed to.

Could you please let me know how it was supposed to work, or even just tell me how the opamp is configured (-ve voltage amplifier?). I guess it is supposed to change the guitar signal into a 0 - 3.3v representation of the signal?


Cheers,

MM
colin353 (author)  microman1714 years ago
Hello microman,

You are correct, the op amp is there only to amplify the input guitar signal to something a little more usable for the ADC, since an unmodified guitar signal usually has a very small amplitude.

From what I remember (this was a long time ago) I just biased the guitar input up to something like 1.7 V, then amplified by a pretty large gain. In this particular configuration the gain is negative, but that's alright, it just changes the phase of the signal.

If you are going to try to do something similar, probably you should design the amplifier yourself though, looking at my old schematic, there are a lot of improvements that could be made to it that would improve the sound quality. Like I said, it was made a long time ago, I didn't understand too much about circuits back then :P

Let me know when you finish your project, I'd love to hear more about it.

Anyway, good luck (and thanks for making your question specific, you wouldn't believe how many people come on here and ask me for "help building the pedal" without specifying what they don't understand about it!)
Hey Colin,

A bias means to set the upper limit right? Sorry, I'm new to opamps. At one point, I was inverting the signal before the output, and found it made no change at all.

I'm looking at building a flanger effect at the moment, as it looks doable. I need about 20mS worth of recording, and then mix that into the live feed.

It's great you posted your instructable, as it has been a stepping stone. A friend suggested I make a digital pedal, and I leapt right in knowing all about ADC and DAC. The problem came with noise, and was solved with capacitors.


Cheers,

MM
colin353 (author)  microman1714 years ago
A bias means a DC voltage added to the alternating signal from the guitar. You have to bias it up to halfway so that you can amplify the negative part of the alternating signal too, since your power supply is probably not bipolar if you are driving digital electronics.

Here is a link to another circuit I designed (much better quality amplification actually) which shows the use of bias:
http://www.getlofi.com/?p=2945

if you look at the left/right channel amplification, the 1uF capacitors serve to prevent DC from flowing through from the inputs. Effectively, it just adds a DC voltage to the input signals so that they can be properly amplified.

Anyway, good luck with that. 20 ms of recording time should be totally possible, even at high sampling rates, so looks like you are on the right track. Let me know if you have any other questions.
Hey Colin,

I spent a whole lot of time learning, and in the end I designed my own opamp circuit. There is a 10k input resistor and an 8.2k feedback resistor. The +ve input is at 1.03v through the use of a 10k and a 22k divider. My simulation predicted this will create a 0v to 3.3v wave from a -1.8v to 1.8v input.

I still have two 100nF caps on the output, one to ground and one to output. These seem to cut down on noise dramatically.

It was an excellent experience.


Cheers,

MM
colin353 (author)  microman1714 years ago
I'm glad you managed to make it work! Link me to your instructable when it's done, I'll read it for sure.
Hey Colin353 this is the Rhino, i really want to build this pedal, can you help me with the schematic, and programming. You Rock, Paul Gilbert also
colin353 (author)  Baseball24_74 years ago
I would be happy to answer any specific questions you have about the project.
Do you have a clearer drawing of the schematic.
RogerClark4 years ago
I Colin. I was trying to build a something similar using a PIC16F690. But just to do effects. But I've realised that I don't have enought internal RAM and that the PIC16F690 can't address external ram (easily). So I may need to upgrade to something from the 18F series. Are you getting a fast enough data rate to your RAM using the SPI ? I was hoping to sample at at least 20Khz, and crank the PIC's clock up as high as it would go, but access speed of external ram would be a serious issue for effects like reverb etc which need a large buffer which can be accessed for virtually every output sample. BTW. I don't know if you've seen this article on a final year project at Cornell http://courses.cit.cornell.edu/ee476/FinalProjects/s2003/Gjr8tl74/ECE476FinalProject/Index.html
colin353 (author)  RogerClark4 years ago
Hello, You will definitely need to use external RAM, the PIC has nowhere near the amount of internal RAM you will need. I used serial RAM under the delusion that I could access it arbitrarily quickly. In the end, the time required to access the RAM was the limiting factor in the sound quality of the device. If you used a parallel ram, it might be fast enough to do 20kHz. I was clocking my PIC at 20MHz, but I know for sure there are very cheap PICs that can do 48Mhz. I did a bit of research on 32 bit PICs for a nicer quality digital audio project, you can get ones that go to 80MHz. The other thing to keep in mind is the nyquist minimum frequency that you really need to reproduce the guitar sound well. If you are putting your pedal first in the pedal chain (to minimize weird harmonics), then you only have to sample at twice the maximum frequency of the guitar, which (I believe) is well below 20kHz. Anyway just do the math on the SPI for whatever ram you are looking at. You will probably need an SPI clock cycle per bit and you can probably do your SPI clocking at about a quarter of the clock speed of the device for a standard PIC. Anyway this message ended up being much longer than I originally intended! Haha, I guess I am secretly still interested in this stuff. Keep me posted, and good luck!
Thanks for the reply, I totally agree with all your points. My project started off just being a practice amp that I was going to put inside the guitar (to hook up to headphones), but then I had the idea of using the 16F690 that I had in my box of components, and of course things sprialled on after that. But I can see that the 16F690 is not suited to this application, because it doesn't have lot of intenal ram. I'm still going to carry on with this project as I may be able to do some basic effects without ram e.g tremulo or I may be able to do some FFT stuff that may be able to do some wierd stuff to the sound. I'll also think about whether its worth starting a separate project to do it properly, and whether a PIC microcontroller is the way to go or whether a different one eg AVR would be better. I'll let you know how I get on.
somorastik4 years ago
have you updated the software features echo and delay?
ski4jesus4 years ago
ha! well i have an actual stratocaster! :p lol nice guitar!
colin353 (author)  ski4jesus4 years ago
mine is real too! I love my stratocaster...
yah they rock. i would like to build this project but i dont have the parts. sick guitar!
hahaman85 years ago
can you scan it? its kinda hard to see
or make a digital version of the schematic. Ty!
justine35635 years ago
man what did you attach to the potentiometer above to let it change pitch?capacitor?thanks alot!
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