I used my battery driven radio controlled alarm-clock (3v) a 12v 190mA 102dB alarm siren with 4 piezos in it. A 12v battery for the piezo. All I needed then was one of many KSP2222A transistors I recycled from a defect ATX power supply, and a 680Ohm resistor.
I opened the clock and found where it sent out 2v pulses when the alarm sounded. I soldered one wire there (the positive wire) and one to the clocks ground.
This pulse now triggers the transistor and it allows power to the siren as long as the pulse lasts.
Very simple, very cheap. You don't even need a pcb for this.
You can learn to find your resistor's value for your load here: