Single Buss 1V/Oct Keyboard PCB

The MFOS Single Buss Keyboard controller is SUPER useful for converting old organ keyboards or anything really into a 1V/Oct keyboard for your DIY Eurorack or other modular synth!

This is for a PCB only, suitable for multiple formats.

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  • One Volt Per Octave Keyboard Controller
  • Requires Only Single Buss Keyboard (or other single switch per note mechanism)
  • Four CV outputs
  • Two Outputs With Glide
  • Trigger and Gate Outputs
  • LED Drive For Gate And Trigger Indicators
  • Great For Performance Art projects
  • A True Classic Analog Synth Design


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This old-school, vintage style single buss 1V/octave keyboard controller is very versatile. As a normal keyboard all you need is an old organ keyboard that has one switch per key that closes to a common buss. Or you can modify an old piano keyboard section with reed switches and magnets to make you own keyboard. Or mount reed switches on yourself or whatever and make a really cool looking controller that you play with magnets glued to your fingers for your next performance art gig. As long as you have a series of resistors with switches to close the junctions to a common bus you have a controller that can use this circuit to sample and hold key presses.



Sample & Hold Performance - Output Voltage Droop

The control voltage output of any analog based keyboard that uses a sample and hold which stores a voltage on a capacitor will eventually droop as any leakage path to the capacitor will eventually cause it to do so. These numbers represent what I believe is typical performance for this keyboard controller circuit when used with a .01uF polystyrene cap for C19 and the LF444CN for IC U3. It is important to clean ionic soils from the PC board using flux remover especially in the vicinity of these components as well as the 2N5457 sample switch transistors Q2 and Q3.


Third C up pressed: Ideal Voltage 2.000V

Keydown 2.0038V
30 seconds later 1.9972V
30 seconds later 1.9897V
30 seconds later 1.9819V
30 seconds later 1.9748V
30 seconds later 1.9672V
30 seconds later 1.9598V

MIDI to CV modules do not suffer from output voltage droop and neither does the diode matrix keyboard project found on the site. I don't sell a PC board for the diode matrix circuit and it requires a dedicated switch per key (not switched to a buss) to function.

Specs & Downloads

Specs & Downloads

100 MFOS
Depth No
+12V No
-12V No
+5V No
Additional Resources Build Guide


Customer Reviews (1)

works greatReview by David
I used this board to turn an old Paia Stringz n Thingz into a CV keyboard. It was really easy to implement. I added a rotary switch with 1.2K resistors to change the octave of the entire keyboard range, which also works well. highly recommended. (Posted on 12/17/2016)

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