K line accessory, volt power to use?

Gotcha, momentary use. thanks much.

There are (at least) two ways to limit the "ON" time of the accessory.

1. A resettable thermal-fuse (aka PTC) that cuts power to the accessory after some number of seconds.  After power is removed the fuse cools down and "resets".  The accessory is then ready for action again.  These resettable fuses are less than $1 and simply wire in-line with the accessory.  Unfortunately, it takes some measurement/experimentation to select the right value unless someone has already identified a suitable part.

2. Some kind of timer that detects 9V AC and enables a relay that "times out" after 15 seconds (or whatever).  After 9V is removed the timer resets and the accessory is ready for action again.  This could be done with a couple low-cost eBay modules - say $5 total but a couple weeks wait to get them from Asia.  Or maybe $10-15 for the same items from a U.S. source.  No soldering, but you need to make a half-dozen or so wire connections between modules and your accessory.

I can elaborate on either method if there's interest.