I'd guess it's just a simple voltage comparator that triggers the sounder & LED if the input exceeds eg. 200mV or so, plus a high value pull-up resistor so the sounder is triggered if the sensor is open circuit, or not connected.
I'd guess it's just a simple voltage comparator that triggers the sounder & LED if the input exceeds eg. 200mV or so, plus a high value pull-up resistor so the sounder is triggered if the sensor is open circuit, or not connected.