Time Based Effects like Slow Gear?

[thehallofshields]

Hey all,

Can anyone describe the type of circuit in which playing above a certain threshold activates an effect for a set period of time?
Unlike an Envelope Filter, we want the effect to switch on-off at a fixed interval (at some point along the descending RC curve) somewhere between 30-60 ms.

I'd like to chop the time-constant mechanism of a Boss SG-1, skip the section that kills the signal, and make a 'Send-Receive' type pedal so I can experiment with applying other effects to the attack.

Has anyone gone down this road of experimentation? I think I've got a great idea but I don't want to get ahead of myself.