Unique Water Pump Controller

Here is a simple solution for automatic pumping of water to the overhead tank. Unlike other water-level indicators,  it  does not use probes to detect the water level and hence there is no probe corrosion problem. It has no direct contact with water, so the chance of accidental leakage of electricity to the water tank is also eliminated. Two important advantages of the circuit are that the water level never goes below a particular level and no modification in the water tank is required.
Fig.  shows the circuit of the water-pump controller. The circuit uses an LDR-white LEDs assembly to sense the water level. It forms a triggering switch to energise the relay for controlling the pump. The LDR-LEDs assembly (shown in Fig. 2) is fixed on the inner side of the cap  of  the  water tank without making contact with water. The light reflected from  the water tank is used to control the resistance of LDR1.

When the water level is high enough, light from the white LEDs (LED1 through LED3) reflects to fall on LDR1. This reduces the resistance of LDR1, increasing the voltage at the non-inverting input (pin 3)  of IC1. IC1  is used in the circuit as a  voltage comparator. Resistors R4 and R5 form a potential divider to fix half of supply voltage to the inverting input of IC1.

Normally, when the water tank is full, LDR1 gets more of reflected light because the distance between the water level and the face of LDR1 is minimal. When white light falls on LDR1, the voltage at the non-inverting input (pin 3) of IC1 increases and its output goes high. This high output makes pnp transistor T1 non-conducting and the relay remains de-energised. LED1 also remains ‘off.’ Since the water-pump power supply is connected to the normally-open (N/O)  contacts of  relay RL1, pumping is stopped.

When water level falls, the amount of  light reflected to LDR1 decreases and its resistance increases. This reduces the  voltage at pin 3 of IC1 and its output goes  low. This  low output from IC1 makes transistor T1 conduct. Relay RL1 energises to close the N/O  contacts and the motor  starts pumping water. LED1 glows to indicate the pumping of water.

Assemble the circuit on a general-purpose PCB and enclose in a suitable  cabinet. Solder the white LEDs-LDR1 assembly on a separate PCB and use a separate power supply for it. Mount LEDs behind the LDR. Otherwise, light from the LEDs will  affect the working of the circuit. Connect LDR1 to the main circuit board at ‘A’ and ‘B’ points.
Fix the LEDs-LDR1 assembly on the inner side of the water-tank cap as shown in Fig.  3. Orient the LEDs and the LDR such that when the water tank is full, the light emitted from the LEDs and reflected  from the water surface falls directly on  LDR1.  The  distance between the upper level of water and the LEDs-LDR setup should be minimal, ensuring that water doesn’t touch  LDR1. Otherwise, the circuit  will  not function properly. By using more white  LEDs, this  distance  can  be increased. Cover the LDR with a black tube to increase its sensitivity.

You can fix the main unit at a convenient place and connect it to the LEDs-LDR  assembly through wire. Select the relay according to the horse-power (HP) of the water pump. After  arranging the setup (with  maximum water in the tank), adjust VR1 until LED1 stops glowing. In this state, the relay should de-energise. When the water level decreases, the relay automatically energises to connect mains to the motor and it starts pumping water.