Adventures in DIY: Monsters at the (CV) Gate

A workaround for Arturia KeyStep
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This month’s adventure started as a workaround for an awkward feature on the Arturia KeyStep, an otherwise brilliant keyboard controller. The KeyStep offers some of the best-feeling mini keys I’ve played, as well as aftertouch. It’s also highly connectable, with USB, 5-pin MIDI, analog sync, and three types of control voltage (CV) output—pitch, modulation, and gate. You can even power it from an iPhone through Apple’s Camera Connector Kit.

The KeyStep’s awkward feature is its modulation ribbon, which shows no indication of the current value. Using a trick from the modular synth world, I soldered an LED to a ⅛-inch mono phone plug and connected that to the Mod jack. (To limit the current, I added a 1kΩ resistor.) When the ribbon is at its lowest setting, the light is off. As you slide upward, the light grows brighter.

You can connect to the other outputs, too. Plugged into the Pitch output, the LED brightens as you play higher. Plugged into the Gate output, it flashes as you play new notes.

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Next I mounted the circuit in a Japanese monster toy. Bandai’s Ultraman toys work especially well for this, because they’re made of flexible vinyl sections that snap apart for easy access. They cost about $6 online (see Parts List). For stability, I pried two strong magnets off an old electric toothbrush and hot-glued them inside the monster’s feet.

I liked the result so much that I built a second monster using a color-changing LED I pulled from a broken toy. (This LED is also available as RadioShack part #2760421.) Connected to the KeyStep’s Gate output, it flashes red when you press a key, then fades to blue and green as you hold the note. A built-in resistor makes hookup even simpler.

Tech tip: LEDs are one-directional. Connect the positive side (long leg) to the tip of the plug. If your synth uses bipolar control voltage (±5V), you can substitute a two-leg, bipolar LED that lights green in one direction and red in the other.

I’d love to shine a light on your inventions, as well. If you have a favorite project you would like to share, send details to Meanwhile, you can visit to see me build and play the CV monsters mentioned here.

Parts List

Vinyl monster toy (search Amazon for “Ultraman kaiju”)
T1-¾ (5mm) LED, 3–5V
5mm LED holder
1kΩ resistor (not shown)
2-conductor insulated wire
⅛" (3.5mm) TS miniphone plug
Solder, soldering iron, ¼" drill bit or reamer
(Optional) 2 magnets and hot glue