There are plenty of designs if you google AD envelope generator, or similar keywords. This may be the simplest: http://www.bridechamber.com/IF_EG_PCB_files/adar_board_doc.pdf
You can omit the lower half of the circuitry - everything connected to the right side of R22. That's for converting audio signals to gates. If you send a trigger (narrow pulse) to the T/G input, then you should get an AD response.