MFOS Mini-Controller (+/-9V to +/-15V)
Article by Ray Wilson
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Value Change Bulletin
The value of R7 has been changed from 1M to 10M. Doing so reduces the current through R7 and subsequently increases the
precision of the bus voltage when a key is pressed. A voltage divider exists between the resistor ladder and R7. By increasing
the value of R7 by a factor of 10 the output of the resistor ladder is loaded less (by 10X) when a key is pressed.
Features
- The MFOS Mini-Controller has a twenty nine note stylus keyboard (C - C - C to E)
- It can be used to play your Sound Lab Mini-Synth or 1V/Oct modular.
- The MFOS Mini-Controller supports four CV outputs (2 with portamento 2 without portamento) My prototype (shown) only used two.
- There is one trigger output and one gate output.
- Just the coolest thing EVER!
Quirks
- Rare, rare (did I say rare) mis-sampling that grabs the raw buss voltage. Remember you are jabbing
a PC trace with a piece of metal (slightly bouncy). But even so this thing is just too much fun to play with.
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Introduction
If you are into the synth hobby scene this is a "must have" you will be playing with this thing into the wee hours of the morning
especially if you built a Sound Lab and don't already have a keyboard controller or you are planning on building a Sound Lab.
This thing has added a new dimension to the fun I have with my Sound Lab. My son built one for his Sound Lab and he says he
can't put it down.
More Photos
MFOS Mini-Controller MP3 Samples
This is an intermediate to advanced project and I do not recommend it
as a first project if you are just getting started in synths or electronics.
Only the circuit and some explanation are shown here. A lot of project building
experience and electronics
knowledge and equipment ownership (scope, meters, etc.) is taken for granted.
If you are interested in building this project please read the entire page before
ordering PC boards to ensure that the information provided is thorough enough for
you to complete the project successfully.
MFOS Mini-Controller Schematic Page 1 PDF
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This circuit is identical to the one in the
Single Buss 1V/Octave Keyboard Controller.
See the circuit description there. This board has the addition of the resistors used in the keyboard
voltage divider. Whereas in typical keyboard pressing a key brings the resistor divider in contact with the
main buss we bring the bus to the divider via the probe used to touch the PCB pads that represent the
keyboard.
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MFOS Mini-Controller Schematic Page 2 PDF
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This is the full resistor divider that is used to produce the 1V per octave voltage necessary to control the
Sound Lab or other moddular synthesizer.
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| Approx. Current Consumption |
| +12V | 30mA |
| -12V | 30mA |
| +15V | 32mA |
| -15V | 32mA |
MFOS Mini-Controller PCB Parts Layout Legend View (Parts Side Shown) PDF
MFOS Mini-Controller PCB Parts Layout Value View (Parts Side Shown) PDF
MFOS Mini-Controller PCB Bottom Copper (Parts Side Shown)
MFOS Mini-Controller PCB Top Copper(Parts Side Shown)
MFOS Mini-Controller PCB Top Silk Screen
MFOS Mini-Controller Front Panel and Wiring
PDF
As you can see in the photos above I combined the MFOS Wall Wart Power Supply
with the MFOS Mini-Controller and also fed
the +12V, -12V and Ground out via banana jacks so I can power my Sound Lab Mini-Synth. It is very convenient.
You only connect the PCB points shown here and of course KBUS to the banana jacks so you can connect the wand
to them. There are several PCB connection points that are not used unless you score a real keyboard and then
use this board to build a regular keyboard. Then you might want to use TLA (Trigger LED Anode), GLA (Gate LED Anode), and KBTP (keyboard top)
as described in this article Single Buss 1V/Octave Keyboard Controller.
As far as the Mini-Controller circuit is concerned when you plan to use the touch pads everything is already pre-connected on the
PC board and you just leave these as they are.
MFOS Mini-Controller Project Parts List
- Using 1% metal film resistors everywhere will reduce temperature related drift.
- Where 1% metal film is specified 5% carbon comp will work but with more temperature drift.
- Usually biFET amps (quads, duals, singles) can be replaced with an equivalent from another manufacturer (except for the LF444).
- Capacitors can be film, ceramic, or silver mica.
MFOS Mini Controller Parts List
| Qty. | Description | Value | Designators |
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| 1 | TL071 Op Amp | TL071 | U1 |
| 2 | TL082 Dual Op Amp(s) | TL082 | U2, U6 |
| 2 | TL084 Quad Op Amp(s) | TL084 | U5, U4 |
| 1 | LF444 Quad Op Amp (do not sub) | LF444 | U3 |
| 11 | 1N914 Sw. Diode(s) | VALUE | D5, D4, D1, D2, D3, D6, D10, D7, D11, D8, D9 |
| 1 | 2N3906 | 2N3906 | Q1 |
| 2 | 2N5457 N(s) | 2N5457 | Q2, Q3 |
| 2 | LED(s) | LED | LED1, LED2 |
| 1 | Capacitor Ceramic | .001uF | C18 |
| 2 | Capacitor Ceramic(s) | .0047uF | C25, C24 |
| 1 | Capacitor Ceramic | .01uF | C1 |
| 15 | Capacitor Ceramic(s) | .1uF | C6, C7, C9, C11, C10, C12, C13, C14, C20, C2, C3, C4, C5, C16, C22 |
| 1 | Capacitor Ceramic | 22pF | C23 |
| 2 | Capacitor Ceramic(s) | 470pF | C21, C17 |
| 1 | Polystyrene or Polycarbonate (low leackage) Capacitor | .01uF | C19 |
| 3 | Electrolytic Capacitor (S)(s) | 10uF | C8, C15, C26 |
| 1 | Potentiometer | 1M | R11 |
| 28 | Resistor 1/4 Watt 1%(s) | 100 ohm | R37, R38, R39, R40, R41, R42, R43, R44, R45, R46, R47, R48, R49, R50, R51, R52, R53, R54, R55, R56, R57, R58, R59, R36, R60, R61, R62, R63 |
| 4 | Resistor 1/4 Watt 5%(s) | 100K | R23, R9, R32, R33 |
| 3 | Resistor 1/4 Watt 5%(s) | 10K | R15, R13, R21 |
| 1 | Resistor 1/4 Watt 5% | 150K | R22 |
| 2 | Resistor 1/4 Watt 5%(s) | 1K | R27, R25 |
| 6 | Resistor 1/4 Watt 5%(s) | 1M | R1, R19, R20, R10, R30 |
| 1 | Resistor 1/4 Watt 5% | 20 Ohm | R8 |
| 1 | Resistor 1/4 Watt 5% | 200K | R34 |
| 3 | Resistor 1/4 Watt 5%(s) | 20K | R24, R26, R16 |
| 1 | Resistor 1/4 Watt 5% | 3.9K | R2 |
| 1 | Resistor 1/4 Watt 5% | 300K | R31 |
| 1 | Resistor 1/4 Watt 5% | 30K | R12 |
| 1 | Resistor 1/4 Watt 5% | 39K | R4 |
| 2 | Resistor 1/4 Watt 5%(s) | 3K | R28, R29 |
| 1 | Resistor 1/4 Watt 5%(s) | 10M | R7 (changed from 1M to 10M Apr 2009) |
| 1 | Resistor 1/4 Watt 5% | 4.7K | R18 |
| 1 | Resistor 1/4 Watt 5% | 475 Ohm | R5 |
| 1 | Resistor 1/4 Watt 5% | 47K | R35 |
| 1 | Resistor 1/4 Watt 5% | 82K | R6 |
| 1 | Resistor 1/4 Watt 5% | 200K | R14 |
| 1 | Resistor 1/4 Watt 5% | 39K | R17 |
| 1 | Multi-turn Trim Pot | 1K | R3 |
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