The circuit uses a 555 timer in a stable mode with R1=220 ohms, R2= 150 ohms and C1=1 uF. As we know, the frequency of operation of the IC 555 in a stable mode depends on the values of R1, R2 and C1. By calculating the frequency of the given a stable circuit, we get the frequency to be around 1.18KHz. The frequency at which it operates is in the audio frequency.
The 1K Resistor R3 whose ends are connected to pin-4 and ground disables the circuit by default and it enables when the water reaches its full level when the probes get dipped in water.
The two probes which are shown in the circuit should be kept at the high level for the water. The stable multi vibrator in the circuit is normally disabled and it gets enabled only when the probes touch the water. The distance between the probes should be less than a few centimeters to ensure that the conduction between the probes will take place when water is touched to these probes. When the water level rises to the height of the probes, then the 555 circuit will get enabled and the output of the 555 timer produces a square wave output with a frequency of about 1.18Khz. This output is given to the mini loudspeaker which then beeps at an audio frequency of 1.18KHz.
The 1K Resistor R3 whose ends are connected to pin-4 and ground disables the circuit by default and it enables when the water reaches its full level when the probes get dipped in water.
The two probes which are shown in the circuit should be kept at the high level for the water. The stable multi vibrator in the circuit is normally disabled and it gets enabled only when the probes touch the water. The distance between the probes should be less than a few centimeters to ensure that the conduction between the probes will take place when water is touched to these probes. When the water level rises to the height of the probes, then the 555 circuit will get enabled and the output of the 555 timer produces a square wave output with a frequency of about 1.18Khz. This output is given to the mini loudspeaker which then beeps at an audio frequency of 1.18KHz.
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