This water-level indicator uses a 7-segment display, instead of LEDs,
to indicate the water level (low, half and full) in the tank. Moreover,
a buzzer is used to alert you of water overflowing from the tank. The
circuit shows the water level by displaying L, H and F for low, half and
full, respectively. The circuit uses five sensors to sense the
different water levels in the tank. Sensor A is connected to the
negative terminal (GND) of the power supply. The other four sensors (B through E) are connected to the inputs of NOT gate IC 7404. When there is a high voltage at the input pin of the NOT gate, it outputs a low voltage. Similarly, for a low voltage at the input pin of the NOT gate, it outputs a high voltage.
When the tank is empty, the input pins of IC 7404 are pulled high
via a 1-mega-ohm resistor. So it outputs a low voltage. As water starts
filling the tank, a low voltage is available at the input pins of the
gate and it outputs a high voltage. When the water in the tank rises to
touch the low level, there is a low voltage at input pin 5 of gate N3
and high output at pin 6. Pin 6 of the gate is connected to pin 10 of
gate N9, so pin 10 also goes high. Now as both pins 9 and 10 of gate N9
are high, its output pin 8 also goes high. As a result, positive supply
is applied to DIS3 and it shows ‘L’ indicating low level of water in the
tank. Similarly, when water in the tank touches the half level, pins 4
and 5 of AND gate N8 become high.
As a result, its output also goes high and DIS2 shows ‘H’ indicating
half level of water in the tank. At this time, pin 9 of gate N9 also
goes low via gate N4 and DIS3 stops glowing. When the water tank becomes
full, the voltage at pin 1 of gate N1 and pin 3 of gate N2 goes low.
Output pin 3 of gate N7 goes high and DIS1 shows ‘F’ indicating that the
water tank is full. When water starts overflowing the tank, pin 13 of
gate N6 goes low to make output pin 12. The buzzer sounds to indicate
that water is overflowing the tank and you need to switch off the motor
pump. Assemble the circuit on a general-purpose PCB
and enclose in a suitable box. Use a non-corrosive material such as
steel strip for the five sensors and hang them in the water tank as
shown in the circuit diagram. Use regulated 5V to power the circuit.
Source: EFY Mag