A simple thermometer built with operational amplifiers and a normal or protective diode such as 1N4148, instead of the temperature sensor can be made using electronic diagram below.
This liquid detector circuit diagram is designed using common electronic components. The liquid detector may operate an active buzzer, to make a sound when is reached a certain level of water.
Because water sensor and control circuit for buzzer are located on the same printed circuit board, indicator, together with 9 V battery and buzzer can be mounted in a compact case.
This infrared detector circuit is designed using the PID20 integrated circuit manufactured by Siemens (which converts thermal radiation into electrical impulses), an operational amplifier and a few electronic components.
Size of the output signal of PID20 is determined by the task at pins 3 and 4.