Eowave Weather Drones

I built a pair of Weather Drones!  It is a 24hp complete ‘synth voice’ for Eurorack.  It is only available as a DIY kit.  Weather Drones is composed of a digital VCO, LFO, Envelope Generator, Noise, and Slewed Random, together with an all-analog Filter.  Here’s my block diagram.

Weather Drones Block Diagram


The oscillator is digital, implemented by software in a PIC microprocessor, and includes a cluster of triangle waves, wave folding, and CV over output level (VCA).  The oscillator is only heard through the VCA, which has +5 volts normalized to the level CV jack (labelled VCA) to maintain full level when nothing is patched to VCA.


  • Mod – attenuator for FM input
  • Freq – initial pitch
  • Spread – number of oscillators in cluster
  • Fold – wave folds


  • CV IN – 1 volt per octave pitch control
  • FM IN – attenuated pitch control
  • VCA – CV (0 – 5V) for output level
  • OUT – oscillator output – modular levels (+/- 5 volts)

Demo of manual operation of the oscillator.  We hear first the triangle, then wave folding, then spreading, then all combined.  The triangle wave contains some digital artifacts (noise).  But the filter can reduce them, and this is a drone machine, so doesn’t matter much to me.  The cluster spread is the killer feature.


Demo of oscillator pitch modulated by LFO, followed by level (VCA) modified by the envelope generator, triggered by the LFO.


Demo of the oscillator pitch controlled by the random slew.


The filter is a fully analog 2-pole design.  It does not self-oscillate at high resonance settings.


  • Freq – center frequency – large center knob
  • Feedback – resonance
  • Mod – attenuator for Mod 1 input


  • In – filter input, normally connected to the oscillator out
  • Out – filter output – modular levels
  • Mod 1 – attenuated cutoff FM, normally connected to the LFO triangle out
  • Mod 2 – unattenuated cuttoff FM

Demo of oscillator through the filter with manual knob twiddling.


LFO speed is manually controlled.  It has three outputs and an LED indicator of the triangle level.  Since behind the scenes it impacts the rate of change of the slewed random output, I consider the Slew to be a part of the LFO.  The LFO goes into the audio range.


  • Speed – LFO rate
  • Slew – adjusts the slew rate of the Slew output


  • Square symbol – Square wave output (0 – 8 volts)
  • Triangle symbol – Triangle wave output (0 – 8 volts)
  • Slew Out – Slewed random voltage (0 – 10 volts)

Demo of the LFO square wave output at audio rates going through the filter.

Envelope Generator

The AD envelope generator has manual control over attack and decay and includes a LED level indicator.  A manual output attenuator can set the maximum envelope to between 0 to 9 volts.  An external trigger must be patched in.  If a new trigger is received before the decay phase ends, a new attack is started.  Decay begins as soon as the attack phase ends, regardless of whether the trigger input is held high.


  • Attack – attack time
  • Decay – decay time
  • Level – output level


  • Trig – trigger in
  • Out – envelope out



  • Noise – digital noise (0 – 8 volts) pulse wave

Demo of the noise going through the filter.


The kit has good instructions and is easy to build.  The single PC board has through hole components mounted on the back side, with jacks, pots and LEDs mounted on the front.

I made a few parts substitutions.  I used my stock of 1% resistors instead of the ones supplied.  I decided to socket the three op amps.  And I used a keyed, shrouded power header.

The most important substitution was to use polystyrene capacitors for the 330pf instead of the supplied ceramics.  I could only do one, which enabled a comparison.  The filter with the polystyrene capacitors has much better definition and sound.  I will replace the ceramics in the other one.

For my own edification I drew out a schematic diagram, prior to starting the build.  That helped me to double check all the parts and to understand the circuit to some extent.  It it a very clever digital-analog design, with control inputs all having safety protection and outputs all buffered.  I’m not including the schematics, since Eowave didn’t publish them.


Weather Drones is a very cool collection of synthesizer modules in a small package.  It could be a nice entry point into modular synthesizers for a DIY builder.  I’ll be installing mine into a larger box and start patching!  I will post more recordings.


One Response to Eowave Weather Drones

  1. Richard says:

    Update to the post to suggest using polystyrene capacitors for the 330pf in the filter circuit. Filter sounds much better with them.

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