Try This Water Level Controller

(Published in Electronics For You - February 1982 issue)

(Republished in Electronics Projects Volume 3)

During these days of high-rise apartments it has become necessary to store water in overhead water storage tanks. Since water pressure in most localities is not sufficient to carry water to the overhead tanks, water is pumped from ground-level tanks to overhead tanks for storage and use. However, to ensure a continuous supply it is essential that a check is kept on the water level in both the overhead tank and the ground level tank.

Here is a circuit which continuously monitors the levels in both the overhead and ground-level tanks, and switches on the pump whenever water level in the overhead tank falls below a predetermined level, and switches off the pump whenever it reaches the maximum preset level so that there is no spillage. Further, if the level of water in the lower tank falls below a preset low level, the circuit overrides the com­mands being sensed from the overhead tank and switches off the pump. This overriding facility is unique for this circuit and ensures the pump never runs dry. All these func­tions can be handled by a single 555 integrated circuit555 consists of two comparators, a flip-flop and an inverting amplifier. The lower comparator triggers whe­n ever the voltage applied to pin 2 of the 1C goes below 1 / 3 Vcc. The upper comparator triggers whenever the voltage applied to pin 6 exceeds 2/3 Vcc. A trigger pulse from the lower comparator drives the flip-flop to a low state, and the inverting amplifier drives pin 3 to a high state. This condi­tion continues until the flip-flop receives another trigger pulse from the upper comparator, when the output of the flip-flop goes high and the output at pin 3 goes low. How­ever, if pin 4 js brought to ground, the flip-flop is set to its normally high state and the output at pin 3 goes low, irres­pective of the signals from the upper and lower comparators.

The levels in the upper and lower tanks are sensed by electrodes mounted on the tanks. There are three electrodes in the upper tank—a reference rod (A) which reaches to the lowest point in the tank, another rod (B) which senses the low level of water and is of a length which just reaches the lowest preset level, and the third rod (C) which reaches the highest permissible level in the tank, as shown in Fig. 2.

Pure water is not a good conductor of electricity but. because of dissolved impurities, small currents can flow from the reference rod A to the rods which sense the upper and lower levels of water. Similarly, two rods are fixed in the lower tanks, viz, a reference rod (D) which reaches the lowest level in the tank, and another rod (E) which just reaches the level below which if water is drawn further the pump shall run dry.

Let us now consider the situation when the water level in the lower tank is high and both the rods are submerged in water. Since water is conductive, transistor T1 will get satu­rated and LED D1 will glow. The 1C will also be able to sense the signals from the two comparators.

Initially, the overhead tank is empty and as such there is no connection between the three electrodes A, B and C. Electrode B is connected to ground through resistor R5. Since electrode B is open, the potential at pin 2 is drawn towards ground. This triggers the lower comparator to sig­nal the flip-flop to go low and the output at pin 3 goes high. This operates the relay through transistor T2 and the pump starts operating.

  

When the level of water reaches the upper rod C, current begins to flow from the reference rod A to C via resistor R7 to ground. Since the resistance between rod A and C is lower in relation to R7, potential at pin 6 reaches nearly the supply voltage. This triggers the upper comparator to signal the flip-flop to go to a high state and consequently the output at pin 3 goes low. This de-energises the relay and the pump switches off. This condition will continue until the level of water in the overhead tank falls below the lower reference rod B, when again a pulse at pin 2 will energise the relay.

However, if during the pumping operation or thereafter, the level of water in the lower tank reaches a level lower than the rod E, the transistor T1 will go out of saturation, LED D1 will extinguish and the flip-flop will get a negative poten­tial through pin 4, immediately resetting the flip-flop to a high state. The input at pin 3 will be forced low thereby de-energising the relay and switching off the pump.

The pump can be switched manually by momentarily depressing switch S1. It can be switched off by depressing S2rCapacitors Cl and C2 help prevent false triggering due to line transients.