For high temp. alarm.
you use one temperature sensor (for ex. pt-100 , range 0 -150 Celsium).
This sensor connected with transmitter which transmitter use input PT-100 and output 4-20mA.
So you have the signal signal and one choice is to use plc with analoge input .
So you connect the signal direct in one input of plc.After work with software and connect one horn in one output of plc .( i have photoes but i cannot find them now , i must organised for help you)
The other easier choice is to connect the electric signal direct with one controller which has input 4-20mA and also digital output.
look this photo ( not so good ) one temp. controller.