Adventures in DIY: Motorize Your Music

By David Battino ,

Fig. 1. Connecting the keyboard’s CV signal to the base of the transistor (B) enables a higher current to flow from the collector (C) to the emitter (E), providing enough power to run the motor.

I’ve long enjoyed driving LEDs with MIDI and control voltages. This month, I started driving electric motors from a keyboard, as well. My circuit, which costs pennies, uses one transistor, one resistor, and a diode, with an LED for show (see Figure 1). The transistor functions as a remote-controlled amplifier or switch, using the small current from the keyboard’s CV output to route a larger current from a battery pack through the motor. (Even small DC motors need far more juice than CV or MIDI can provide.) Notice the “flywheel” diode bridging the motor. Because an electric motor is a generator in reverse, it sends an electric spike when you cut the power. The diode protects the transistor from that surge.

The motor’s buzz and whir add a fascinating acoustic layer to synthesized sounds. For even more variety, you can wrap a flag of tape around the shaft so it thwacks other objects, or tape the motor to a piece of sheet metal and amplify the vibrations with a contact mic.

Fig. 2. Here my KMI QuNexus is playing a softsynth over USB and a motor from its CV output. In the foreground is a 3V motor I salvaged from an electric toothbrush.

My first experiment was to route the keyboard’s CV pitch output to the circuit, so that playing higher notes spun the motor faster and made the LED brighter (see Figure 2). Increasing the resistor’s value gives you more keyboard range to control the motor’s speed. With no resistor, I could play a few notes higher after the point the motor started. With 1kΩ, I got about an octave before the speed maxed out. The intervals didn’t match notes on the keyboard, but I could play simple melodies as well as warp the speed with the keyboard’s pitch bender. In this configuration, the motor spins constantly once you press a high-enough note.

Connecting the keyboard’s CV gate output to the circuit was interesting too: The motor spun at a constant speed for as long as I held a key. I’m planning on using this concept to trigger one of those solar-powered hula dancer toys as a fun performance visual, though the sound itself makes a unique percussive layer for drums, bass lines, and lead sounds.

I also tried triggering the circuit from MIDI by swapping the NPN transistor for a PNP, which gates the voltage when the base goes low. However, the MIDI pulse was too brief to move the motor. For more background on transistorized motor control, including a cheeky rant on why MOSFETs are a better choice, see batmosphere.com/transistors.

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