Lonesoulsurfer introduces Acid Drip, an innovative open-source DIY groovebox designed for electronic music enthusiasts and producers. This project centers around a Raspberry Pi RP2040 microcontroller, combining a versatile acid bassline synthesizer with a feature-rich drum machine into a single, compact device. It offers a unique opportunity for users to build their own hardware instrument, providing both a creative tool and an educational experience in electronics and embedded systems. Consequently, it appeals strongly to those who enjoy hands-on projects and customization.

Core Hardware and Architecture

The core of the Acid Drip is the RP2040 microcontroller, commonly found in the Raspberry Pi Pico. This powerful dual-core processor is crucial for running both the synth and drum machine simultaneously without performance bottlenecks. The device integrates an ILI9341 320×240 TFT display for a clear user interface and 16 Cherry MX pads for tactile control. Moreover, it includes three analog potentiometers for real-time sound manipulation. The architecture wisely splits tasks, with Core 0 handling audio synthesis, pad scanning, and sequencer logic, while Core 1 is dedicated to TFT display rendering and drum sample buffer filling. This separation ensures efficient operation and a responsive user experience.

Acid Bassline Synthesizer

The Acid Drip’s acid bassline synthesizer, built upon the Mozzi audio library, features a comprehensive 16-step sequencer. Users can perform detailed step editing, including toggling steps on/off, adding accents, and applying glide effects. Furthermore, the note for each step can be precisely set using the CUT potentiometer, with scale-quantization ensuring musicality. The synth offers various voices such as SAW, SQR, SINE, PWM, and their ‘CSAW’ (complex saw) counterparts, providing a wide palette for sound design. In addition, the ‘Acid Walks’ easter egg mode provides 8 preset patterns inspired by classic acid tracks, each with pre-loaded key, tempo, length, and sound settings.

Advanced Acid Synth Features

The dedicated FUNC mode significantly expands the acid synth’s capabilities. Here, users can adjust the root note (KEY), load preset riff patterns (RIFF), select synth voices (SOUND), and apply various note walk patterns (WALK) like OCTWAVE or RANDOM. Moreover, an FX assign sub-mode allows for the selection and assignment of effects such as Oct Up, Retrigger, and Stutter to individual steps. The Accent Edit mode is particularly noteworthy, repurposing the three pots to fine-tune the accent envelope’s cutoff boost, resonance boost, and decay-time multiplier, offering deep control over the characteristic acid sound.

Integrated Drum Machine

Alongside the acid synth, the Acid Drip features an 8-instrument drum machine, which is a port of the popular DrumKid engine. It includes essential drum sounds such as Kick, Hi-hat (closed), Snare, Rim, Tom, Bass 2, Clap, and Open hat. Users can perform per-drum editing by holding a drum pad and turning the pots to adjust pitch, sample length (crop), and volume. The Beat Machine also includes its own FUNC mode, offering global controls for pitch multiplier, bit crush depth, global decay scaling, velocity humanization, and an independent tempo. Similarly, 16 preset drum patterns, ranging from TECHNO to HIP-HOP, are available, facilitating quick groove creation.

Synchronization and DIY Aspects

The Acid Drip supports external synchronization via its GP2 pin, although this pin is shared with the drum audio output, requiring a physical SPDT switch to select the desired function. This flexibility allows the groovebox to integrate seamlessly into existing hardware setups. The project is entirely open-source, with all firmware, hardware designs, and parts lists available on GitHub. Consequently, while the software is free, building the Acid Drip requires sourcing components, soldering, and flashing the firmware, making it a rewarding challenge for DIY enthusiasts. The project’s dependencies include the Mozzi audio library, Adafruit GFX, and Adafruit ILI9341, all installable via the Arduino Library Manager.

Features

• RP2040-based dual-core architecture
• 16-step acid bassline sequencer with accent and glide
• 8-instrument drum machine with 16 preset patterns
• ILI9341 320×240 TFT display
• 16 Cherry MX pads for tactile control
• 3 analog potentiometers for real-time sound shaping
• Multiple synth voices (SAW, SQR, SINE, PWM, etc.)
• FX assign sub-mode with per-step effects
• Accent Edit mode for detailed envelope tuning
• Per-drum editing for pitch, crop, and volume
• External sync input (shared with drum audio out)
• Open-source firmware and hardware designs

Price
The Acid Drip is an open-source DIY project. The firmware and hardware designs are available for free.

More info: lonesoulsurfer | Acid Drip