For my birthday I got a NiftyCase with various modules, and I also bought a Mr Phil Ter and a Monsoon for it. However, it lacked envelopes. I started looking for possible solutions, On my youtube channel you can see a bit more about the synthesizer, here I will describe the design and making of the module. The basic circuit I came up with is as follows:
There's two inputs and an output, one of the inputs is the gate signal, which starts the decay, and the other is the control voltage, which uses and LED to control a photoresistor which is in parallel with a potentiometer, both of which control the length of he decay. The output is multiplied by a certain amount (currently 1.55x, but I may go back to the idea of multiplying by 2x as in the circuit above.
The 85k resistor was changed to 1M, and I added a LED and 470 resistor to show the decay working after the OPAMP (an TL072). This circuit worked, but unfortunately most gate signals are terrible, so I had to add a special PIC10F206 circuit to clean that up. This circuit also discharges the capacitor quickly between uses, avoiding a wait time that was necessary otherwise.
The only improvement still needed is handling of triggers: if the gate signal is too short I want to change the behavior into a release instead of a decay, possibly with a check that the output signal has grown small enough.
Here is what it looks like on the outside, which required some drilling, which was definitely a new experience, and quite scary.
By now I learned that the CEO of the company that makes these specific panels isn't a very nice person, so I may have to reconsider my purchasing strategy here.
