Kastle v1.5

Experiment with lo-fi anywhere with this pocketable modular

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Traveling light? Bastl has got you covered. We've managed to pack a lot of energy and soul into a tiny box named the Kastle.

The Bastl Kastle is a mini modular synthesizer you can play anywhere. It's powered by just 3 AA batteries or a regular micro USB charger, and it offers a headphone output and 2 in/out ports for interfacing other gear, be it a keyboard, a groove box, or a Eurorack synth.

The device is DIY-friendly and ideal for beginners in synthesis, but it will also add unique functionality to any full-sized modular synthesizer system. It delivers the fun of modular at a low cost, and it fits into your pocket so you can play it anywhere.

As Sound on Sound’s Paul Nagle summarizes Kastle in his review: “It’s a stand-alone synth, a modulation source for your modular and can live in your pocket ready to impress your friends. You’d have to be pretty hard-hearted to find no pleasure in it.”

Be an operator with a pocket calculator

The Kastle has a unique digital lo-fi sound, and it can be melodic, rhythmic, as well as very noisy and droning, soft or harsh. It was designed to be fun on its own, but it thrives the most in an ecosystem of other modular gear.

The Kastle is an open-source DIY project which runs on two Attiny 85 chips that can be reprogrammed with an Arduino (simply Google "programming Attiny 85 with Arduino"). One chip is dedicated to sound generation while the other handles modulation. Several versions of firmware for the Attiny chips are available via GitHub.

The 1.5 version combines a complex oscillator and LFO with a stepped waveform generator.

The oscillator section has 3 sound parameters: PITCH, TIMBRE, and WAVESHAPE – all under voltage control, offering 6 different synthesis modes. It has a main output and a secondary waveform output. Both can be used independently or in combination. The 3 synthesis modes available through the main output are: phase modulation, noise mode, and track & hold modulation. There are 3 more modes on the secondary OSC output: phase distortion, tonal noise mode, and formant synthesis. Each mode utilizes one or two oscillators. The PITCH parameter controls the main oscillator, the TIMBRE sets the PITCH of the modulating oscillator, and the WAVESHAPE depends on the synthesis mode.

The voltage controllable LFO has a triangle and square output and a reset input. The stepped waveform generator is inspired by the Rungler circuit by Rob Hordijk. It can produce 8 different voltages either in random order or in 8 or 16 step looping patterns depending on how the BIT IN is patched.


  • complex oscillator
  • 3 synthesis modes on the main output: phase modulation, noise mode, track & hold modulation
  • 3 more modes on the secondary OSC output: phase distortion, tonal noise mode, formant synthesis
  • PITCH control with offset and CV input with attenuator
  • TIMBRE control with offset and CV input with attenuator
  • WAVESHAPE control with offset and CV input
  • voltage controllable LFO with triangle and square outputs and reset input
  • stepped voltage generator with random, 8-step, and 16-step loop mode
  • 2 I/O CV ports that can be routed to any patch point
  • the main output can drive headphones
  • 3x AA battery operation or USB power selectable by a switch
  • open source
  • possibility of exchanging different LFO and OSC chips
  • durable black & silver PCB enclosure


What's in the box

  • Kastle synth
  • 10 patch cables
  • user manual
  • sticker
  • for environmental reasons we are excluding the batteries in the package

Technical details

  • dimensions: 70 x 60 x 50 mm
  • power: 5V via Micro USB or 3x AA battery
  • FAQ


DIY kit

You can build your own Kastle with our DIY kit. Go through the complete assembly guide below and make sure that you understand all the steps.