LDR Based 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. 1 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 curpumping 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 horsepower (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.

Schematic:

Components Required:

Components	Values	quantity
R1-R3	100 ohm	3
R4-R5	12k ohm	2
R6	220 ohm	1
VR1	10k ohm	1
C1	220uF/25v	1
C2	470uF/25v	1
D1	1N4148	1
D2	1N4007	1
T1	BC558	1
IC1	uA741	1
RLA	See text	1
S1	On/off switch	1
Batt	9v battery	1
LED1-LED3	White LEDS	3
LED4	Red LED	1
LDR	---	1