As the alarm rings, trigger pulses appear at the input of the water pump motor controller circuit shown in Fig. 1. A low-pass filter is connected at the input to avoid false triggering of the circuit due to excessive EMI produced by the pump motor and the relay contacts. Timer LM555 (IC1) is configured as a monostable multivibrator. The voltage at its output (at pin 3) goes high immediately after receiving trigger pulses (at pin 2), and it remains in this state for about five seconds, as set by the values of R3 (47k) and C3 (100µ, 25V). Diode D1 gets forward biased and sufficient base current flows through R4 and R5 to drive transistor T3 into saturation. A free wheeling diode (D2) is used to protect transistor T3 from the reverse current caused by self-induction of the relay coil (RL1).
Transistor T3 immediately switches on the relay. This, in turn, switches on the pump motor by the relay and at the same time the LED is turned on. The water is pumped by the motor and starts filling up the overhead water tank from the underground reservoir.
The light produced by LED continuously falls upon the LDR. This causes a decrease in the resistance value of the LDR and the base of transistor T2 (BC547) gets a high voltage (about 10V DC), which results in conduction of transistor T2. Since transistor T2 is biased as an emitter follower, the emitter voltage also becomes high. Due to this voltage feed, transistor T3 (SL100) remains in saturation.
Motor controller in action
Thus, once the circuit is triggered from the output of IC1, it will stay continuously on even when the input is withdrawn. As such, the pump motor will continuously remain on through the relay contact. (Note. Reflection of light from any moving/still object near the LDR affects the proper functioning (latching) of the circuit. The value of VR1 (20k) has to be chosen for proper functioning of the circuit and according to the amount of light received in your circuit from the surroundings.)
As the water level in the overhead tank rises, the empty vial floating along the water level inside the PVC pipe also goes up. When the water level reaches the desired level, the floating vial interrupts the path of light falling on the surface of LDR. Thus, the resistance of LDR increases sharply, resulting in voltage reduction at the base of transistor T2. Hence voltage at the emitter of T2 also goes low, which reduces the base current of transistor T3. This pulls T3 into cut-off region and both the LED and the relay are switched off. Ultimately, the AC supply via the relay to the pump motor is also cut off and the motor stops.
The circuit (excluding pump motor, alarm clock, and sensor assembly) costs around Rs 80.