The Sentinel is a PT2399-based pitch modulator, delay, reverb and drone verb designed by Arnoud Philip (~arph) as part of the DIYSB 10th Anniversary design contest. It's based on the Sewer Pipes ring-verb and the Noise Ensemble. Original thread here, schematic here. Should be able to squeeze it into a 1590B.
Note: LED2 in the layout is a bi-color LED that could be used externally as a rate indicator or even the on/off indicator. For the later, use the footswitch to break/connect the connection between the common cathode (middle lead) and the 1k resistor going to ground at the top of the board. Also, the 25k trimmer is a fine tune control for the Drone pot.