Mosha Eurorack Modules #3: Attack/Release and Decay

 The Attack/Release and Decay is a 6 HP module, that, as the name implies, as an Attack/Release section and a Decay section. The output voltage is 8V to 0V. The Decay (in purple) accepts either triggers or gates as inputs, the Attack/Release (in gold) section requires a gate input.

There are are knobs to control the Attack, Release and Decay, and two red LED indicating the current voltage on each. There is also a single CV input for the Attack/Release section. A positive voltage will affect the Attack (the higher the voltage, the shorter the attack) and a negative voltage will affect the Release (the lower the voltage, the shorter the release). At 0V the CV doesn't influence the outcome.

The circuit diagrams shown are mostly used for the simulations, the actual diagrams are slightly more complicated. However, they give an idea on how things work. The Decay takes the gate input which is low-pass filtered by a large capacitor and a potentiometer. A diode prevents it from triggering negative voltages and the output is buffered. There is a second buffer for the LED.

The Attack/Release circuit diagram is even less complete, as it doesn't show how the potentiometers are individually connected to the outputs of the microcontroller in order for them to be controlled separately. 

In order to ensure that there is a clean input for both the Decay and the Attack/Release there is a PIC10F206 microcontroller that accepts the messy input signal and cleans it up. For the Decay this means converting a short trigger into a longer gate output and ensuring that the capacitor is discharged quickly (over a 100 Ohm resistor) when the signal needs to be reset. The source code is here for the Decay and here for the Attack/Release.

The reason for a micro-controller solution was to have more control over the outputs and how they work. However, a solution using an OPAMP in comparator mode would work equally well (or even better, as it would allow higher voltages without amplification, which could be done using the buffer. The CV control was done using a light sensitive resistor and an LED, with the LED being hooked up to the CV. By having one LED forward and one LED backward it was possible to control both with a single jack.