I. Overview/Features
This module is a keyboard, only mounted with your other modules in your modular synthesizer
rather than in a separate housing. It does this by using a small number of push-buttosn instead of
traditional keys. This an be used like a traditional keyboard to play melodies, or it an be used a
pre ision voltage referen e for voltage ontrolled parameters in other modules.
CONTROLS
-Pit h sele t buttons. There are 12 buttons orresponding to a hromati s ale. They are arranged
like a typi al keyboard with sharp/flat notes in a seperate row. Whenever two buttons are pressed at
the same time, the lower in pit h of the two will be output.
-O tave sele t knob. This knob sele ts whi h of five o taves the keyboard will output. There are 4
LEDs above the knob whi h give visual feedba k for whi h o tave is a tive.
-Slew knob. This knob adjusts the rate of slew/glide/glissando between notes. Fully
ounter lo kwise is no slew at all.
OUTPUTS
-CV output. This output goes from 0 to 5VDC in 1/12th volt intervals to orresond to the 1v/o tave
standard ommonly used in synthesizer modules.
-Trigger output. This is a short pulse output whenever a pit h sele t button is pressed. It alternates
between 0 and 5VDC
-Gate output. This pulses up whenever a pit h sele t button is pressed and remains high (5V) until
the button is depressed.
II. Sc ematic.
The s hemati is on the next page. I'll briefly des ribe the different parts of the ir uit to
help people understand the module. At the top enter is the 16F689 PIC mi ro ontroller (the PCB
has it labeled as a 16F685, this is a typo and should be ignored) whi h is the heart of the module.
On the far left, we see the o tave display LEDs. Pin2 of the PIC is onne ted to the
athodes of the red LEDs and the anodes of the green LEDs, so this allows it to ontrol whi h set of
LEDs are a tive, by either being a urrent sink for the reds or sour e for the greens. Pin 13 ontrols
the anode of the first red LED and the athode of the first green, and Pin 3 does this for the se ond
set. This allows us to ontrol 4 LEDs with only 3 pins.
To the right of these, we see the swit h matrix. Pins 14 through 16 are the outputs. And pins
4-7 are the inputs. When a single output pin is turned on, if any of the four swit hes that it is
onne ted to through the diodes is losed, then the orresponding input pin will read this. This
allows us to get 12 swit hes from only 7 pins.
Below the swit h matrix to the left are the two potentiometers. Ea h is onne ted to +5V
and ground on it's outer lugs and it's wiper is onne ted to a pin on the mi ro ontroller. The wipers
are also filtered by a .01uf apa itor. Slew is onne ted to pin 19 and O tave is onne ted to pin 18.
To the right of the swit h matrix we see the gate and trigger outputs. Ea h of these onsist
of a se tion of the TL074 op-amp wired as a unity gain buffer, the output of whi h goes through a
1K resistor to the orresponding output ja k and a 1K urrent limiting resistor to an LED. The
trigger output omes from pin 10 and gate omes from pin 12.
Above these outputs we see the DAC and CV output ir uitry. The 4921 is a serial digital-
to-analog onverter (DAC). It re eives data from the mi ro ontroller on pins 2, 3 and 4 and then
outputs a orresponding voltage on pin 8. This voltage is then onne ted to a pair of inverting
amplifiers in series. The first has a trimpot in it's feedba k path to fine-tune the output voltage.
Finally on the bottom we see the power supply onne tions. There are eurora k and MOTM
power onne ter footprints, these are onne ted to a low pass filter of a 10 ohm resistor and 10uf
apa itor. The power rails go to the TL074 and a 7805 voltage regulator.
