Using this circuit diagram can be designed a very simple electronic liquid level indicator . The circuit is built using ULN2803 and ULN2003 control circuits (which are Darlington transistors).
Each base of them is connected to a sensor consists of a carbon rod or a strip of aluminum or copper. These sensors are mounted in the vessel with liquid at the desired level.
The 15 LEDs on the control outputs are arranged to form a bar to represent an indicator, easy to read (when you press S1).
Due to relatively high current value (about 300 mA) and limited battery capacity do not use indicator continuously (as lead to rapid erosion of sensors).