Katoomba: Triple LFO

 

Katoomba is the triple LFO that featured previously on this blog, but with its own panel and custom PCB. It has the following features:

* Power usage: -12V 43mA/+12V 46mA.

* A medium speed LFO (0.01 Hz to 2 Hz) with triangle wave, square wave (0-5V) and square wave (-5-5V) outputs.

* A slow speed triangle LFO (0.001 Hz to 0.3 Hz).

* A fast speed LFO (1 Hz to 200 Hz) with two triangle wave outputs (0-5V and attenuated -5-5V).

Each LFO has an LED indicating its current polarity and the panel is language agnostic (although it still reads left to right).

The Katoomba was designed in EasyEDA (schematic will be at the bottom of this blog post) based on the circuit from David Haillant with modifications to allow multiple outputs at the same time and different voltage levels. This version uses a 16-pin voltage connector, because it is more convenient for my setup, but it doesn't use 5V, so it could use a more standard 10-pin connector as well.

In this version I decided to use the smaller potentiometers to allow more room for the cables, but since these have the same footprint as the other potentiometers this can always be changed.

Black box - Gate to trigger

There are a number of cases where it is preferred to have a short trigger rather than a gate signal. This is yet another project for which a PIC10F206 was used. It uses all inputs and outputs, with one pair triggering each time the gate goes high (+2.1V or higher) and the other pair triggering each time the gate goes high OR low. This latter option can be used to double a clock if needed. Both can operate in the audio rate range, although it falters when the triggers get the same width as the incoming square wave.

The source code is here. The circuit just uses 2.2k jacks connecting things to the microcontroller, relying on the internal diodes to protect against voltages outside of the 0-5V range (since they can handle 20mA, this shouldn't be a problem at all with these resistors. Of course it would push the circuit out of specification, as it can't protect against other disturbances anymore).

Black Box - Decay

 The decay of the black box is different from the decay that I made for eurorack. That decay had the voltage drop as the capacitor filled, this decay is the more classic model which first quickly charges the capacitor (using a 200 Ohm resistor) and then discharges it through the potentiometer with an added resistor for safety.

The trigger is made using another PIC10F206, because it allowed precise control over the charging time, which needed to be around 5ms, as the capacitor I chose was a 4.7uF ceramic one. I used an MCP6272 to buffer the output as well as control the LED. Finally I used my classic transistor inverter circuit using the 2N2222 to feed the signal back into the trigger jack. This means that if no cable is plugged in the decay will feed on itself: each time the voltage drops below 600mV it will trigger itself again. This means it can basically be used as a kind of LFO.

Because of the code, there is a possibility that this fails if the speed is increased too much. After this happens it will not restart until a normal trigger is applied. This can, of course, be done using the 0-5V dial of the black box itself. In the code there is a loop to charge the capacitor. The reason why the same if statement appears twice is to time the loop to be close to 5ms.

Here is a (hopefully correct) version of the schematic.

Black box - Metallic noise generator

 The black box noise generator is quite similar to the earlier metallic noise generator that was in the drum module. The changes are that it doesn't handle the decay (there is a separate decay part in the black box) and it allows for reducing the number of oscillators used with a second input.

In this picture the decay module and trigger module aren't present yet.

The "N" button (and gate) allow switching the metallic noise to the next effect. The "A" is the CV of the VCA, which consists of a single transistor (like in the original schematic). "O" is the output, and "S" is the input that reduces the number of oscillators.

The new source code is here. You can hear how it sounds here.