I thought some could use this information.
I have modded a mini powerhorn so that it has an isolated pair of contacts that can turn other things on while the siren is triggered from the home security system.
I took the 2 white wires off of the piezo speaker in the mini and wired 1 to the coil part of a 12v reed relay, the other wire i put a diode in line before connecting to the other end of the coil on relay. The contacts of the reed itself are wired to a female headphone jack or similar jack. A small hole is drilled in the side of the case for the jack to stick out. Everything is hot glued in place. The case is reassembled and that is it.
I have a separate siren that is powered by a 12v transformer. In place of the switch that turns on the siren, I wire in a male headphone jack, this is then plugged into the modded mini. Now when the siren trips, it connects the wires as though it were a switch.
The mini is also silent because the piezo is disconnected, this stops from drawing attention to it, which stops from the power being disconnected.
the separate siren is wired into the cold air return of the furnace. which makes for a really really loud siren that sounds when the system is triggered.
I have never felt that the mini or large powerhorn were really that loud, but with this mod, it can be as loud as your new external siren.