The M-104 MXR Distortion + aka Distortion Plus aka D+ is a distortion guitar pedal designed by MXR and released between 1978 and 1979. The original stompbox did not have external power jack or indicator LED. Jim Dunlop bought the MXR licensing rights and currently manufactures reissues of this classic MXR distortion effect (with a power jack and an LED). 

The MXR Distortion + uses germanium diodes and its sound could be defined as mild fuzzy distortion, like all of the 70's rock and 80's metal recordings that made this iconic pedal famous. The design of this circuit was reliable and used later on as a starting point for the M-133 MicroAmp which is, in fact, a un-distorted redesign of this (previous) M-104 MXR Distortion+ pedal. 

pedalSHIELD MEGA is a programmable guitar pedal that works with the Arduino MEGA 2560 and MEGA ADK boards. It includes a 1.3 inches OLED screen, a True Bypass footswitch, 2 programmable push-buttons and an analog input/output stage. The project is Open Source & Open Hardware and aimed for hackers, musicians and programmers that want to learn about DSP (digital signal processing), guitar effects, and experiment without deep knowledge on electronics or hardcore programming.

You can program your own effects in C/C++ with the standard Arduino IDE tool and get inspired using the library of effects posted on the pedalSHIELD MEGA online forum.

The Klon Centaur in an overdrive guitar pedal designed by Bill Finnegan with the help of 2 MIT Electronic Engineers between 1990 and 1994. The initial idea was to improve the TubeScreamer transient response and the midrange-bass frequencies in order to create a big open sound with a hint of tube clipping: the so-called transparent overdrive.

The design process took more than 4 years and the result was a mythic $329 hand build pedal that was running in production for 15 years. Nowadays the original models are highly appreciated, you just need to check eBay to see how high it goes.

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. 

The MXR 102 Dyna Comp is a compressor effect guitar pedal, released in 1972 by MXR and rapidly becoming popular because of its price and ease of use. The electronic circuit was also used as a reference design on many other compressor effects like the Ross Compressor, T-Rex CompNova, Ibanez/Maxon CP5/CP9/CP10 and boutique pedals. This model is neither the most silent nor the most hi-definition piece of gear but it has real character and it's easy to find.

This pedal produces dynamic range compression, this is basically smooth out volume differences between notes; giving more body and level to weak notes so the sound does not change drastically no matter what the real signal level is, giving better articulation and sustain to the guitar. The pedal also adds its own coloring and tone to the signal.

pedalSHIELD UNO is a lo-fi programmable guitar pedal that works with the Arduino UNO / Genuino UNO Board. The project is Open Source & Open Hardware and aimed for guitarists, hackers, and programmers that want to learn about digital signal processing, effects, synthesizers and experiment without deep knowledge on DSP, electronics or hardcore programming.

You can program your own effects in C/C++ or get inspired using the ready-to-play effects from the online forum.

The Boss DS-1 is a distortion guitar pedal released in 1978 by Boss. It was the first distortion pedal made by Boss with a distinctive hard-clipping sound perfect for high gain rock and metal still keeping the character of the original guitar. The Boss DS-1 together with the TubeScreamer and the ProCo Rat cover almost all the spectrum of tastes for vintage classic rock/hard rock guitar distortion.

Referring to the electronic design, the current Boss DS-1 has a simple circuit: some transistor stages with an op-amp core and hard clipping diodes. This analysis is focused in the latest pedal schematic revision which is easy to find and prone to modifications.

1Wamp is a one Watt small guitar amplifier based on a JFET guitar pre-amp, the Big Muff Pi tone control, and the LM386 power amplifier. This portable amp is an open hardware project designed by ElectroSmash using only free / open-source tools.

The preamp stage with two J201 transistors is designed to give a tube-like sound, the BMP tone stage is able to produce a big range of tones and the LM386 output stage can drive any kind of speaker, from headphones to a Marshall 2x12 cabinet.