From the Crash Pad manual: This 1⁄4” jack is the input for an external trigger. The Crash
Pad triggers on positive going pulses or clock signals ranging from 3V to 15V or
+/- 3V to +/-8V. Nearly all types of gate, clock and trigger signals that fall within
this voltage range will trigger the Crash Pad. Some examples of acceptable
trigger signals include electronic drum trigger pads, an electronic pulse (as
produced by the EHX Clockworks), a clock generator (as produced by the EHX 8
Step), gate signals from synthesizers such as V-Trigger, or sync signals from
drum machines. The input impedance is 2Mohms.
Back in the 1980s there was an External Trigger box which was simply a piezo transducer. It worked fine on all the drum units back then. I’d get some piezo discs and try them out. They’re easily mounted to just about anything.
