Theories

We can use the logic of making an automatic light detector to make a simple water level indicator.We just need to remove LDR from the circuit and put the wires into water, then it simply works as water level indicator. There is a specific resistance between the wires that are dipped into water and conduction of electricity through water gives biasing voltage to base of transistor. The sensitivity can be adjusted using VR1. Put 470K variable resistance in VR1. My project: Buy similar kit VEK2639...

A voltage divider consists of two resistances R1 and R2 connected in series across a supply Voltage Vs. The supply Voltage is divided up between the two resistances to give an output Voltage Vo which is the Voltage across R2. This depends on the value of R2 relative to R1: If R2 is much smaller than R1, Vo is small (low, almost 0V) (because most of the Voltage is across R1) If R2 is about the same as R1, Vo is about half Vs (because the Voltage is shared about equally between R1 and R2) If...