Here is a simple solution to prevent the wastage of water from overhead tank. It gives an audio visual indication when the water level rises towards the top of the tank so that the water pump can be switched off to prevent the overflow. This is also an ideal circuit to monitor the water tank with float valve operation.
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IC1 (741) is used as a Voltage comparator with the Inverting input (Pin2) connected to a potential divider R2 and R3 so that pin 2 gets half supply voltage. The Non inverting input (Pin 3) is connected to R1 and the probe B. So when the probes are free, the Inverting input gets higher voltage and the output of the comparator remains low. This keeps T1 out of conduction so that buzzer and LED remain off. The probes connected to points A and B are kept near the top of the water tank where maximum water level attains. When the water rises through the probes, Pin 3 gets a higher voltage through the moist probes and the output of comparator turns high. This triggers T1 and Red LED lights and buzzer beeps to indicate the overflow.
Image courtesy http://www.flickr.com/photos/thingo/2084324866/ Andrea.
Circuit can be powered by a standard 9-12 volt DC adapter. Use two brass nails as probes which can be inserted in to the water tank below the lid with 1 cm gap.Connect the probes with the circuit using thin plastic wires.
Note :One drawback of IC 741 is that some makes will not give zero output in the off mode and around 0.6-1 volt may appear at the output in the stand by mode. This can give a false alarm. This can be prevented by connecting an IN 4007 diode at the output in series with R4. Other Op Amps like CA3130, CA3140, TL0 71 etc gives Zero output in the off mode