This circuit liquid level sensor is using a simple and common operational amplifier IC 741 used as a comparator. When the sensor two-fluid levels (you can use two small pieces of PCB or drivers of some of them) are not found in the fluid of the output voltage of operational amplifier 741 is negative and the T1 is blocked as a result 0 volt voltage resistor R5. If the liquid level sensor is placed in the fluid of the output voltage of operational amplifier 741 is about a few hundred milli-volts and the transistor T1 conducts, which is an output voltage in the resistance around R5 4 volts.
