When the resistance value of a temperature, light, pressure,  or any other resistive sensors  Rs drops below a certain point  (adjusted by a preset potentiometer),  the SCR (silicon controlled switch) will be triggered. The sensor (Rs) and the potentiometer placement can be interchanged to get opposite action, where the SCR need to be triggered  at the increase of sensing resistor (Rs). Here is the schematic diagram of  the circuit:

RESISTANCE TRIGG FRED ALARM circuit schematic

[Circuit’s schematic diagram source: General Electric’s Transistor Manual]