Here is a circuit for an automatic water pump controller that runs the motor of the water pump. When the water level in the overhead tank (OHT) drops below the lower

limit, the motor turns on by itself. In the same way, it turns off when the tank is full. The circuit is simple, small, and cheap because it is made up of only one NAND gate IC (CD4011). It gets its power from a 12V DC power supply and doesn't use much power. There are two parts to the circuit: the controller circuit and the indicator circuit. The controller circuit is shown in Figure 1. Let's say there are two reference probes, "A" and "B," inside the tank. "A" is the lower-limit probe, and "B" is the upper-limit probe. The 12V DC power supply goes to probe C, which is the minimum amount of water that can always be kept in the tank. The lower limit "A" is connected to the base of transistor T1 (BC547), which is connected to the 12V power supply through its collector, and to relay RL1 through its emitter. Pin 13 of NAND gate N3 is hooked up to relay RL1. In the same way, the upper-limit probe "B" is connected to the base of transistor T2 (BC547), whose collector is connected to the 12V power supply and whose emitter is connected to pins 1 and 2 of NAND gate N1 and ground via resistor R3. NAND gate N2's output pin 4 is connected to NAND gate N3's pin 12. Resistor R4 connects the output of N3 to the input pin 6 of N2 and the base of transistor T3. The motor is turned on and off by connecting relay RL2 to the emitter of transistor T3. Circuit operation

If the tank is filled below probe A, transistors T1 and T2 don't work, and the output of N3 goes high. This high output turns on relay RL2, which drives the motor to pump water into the tank. When the water level in the tank is above probe A but below probe B, the base voltage of transistor T1 is provided by the water in the tank, and relay RL1 turns on to make pin 13 of gate N3 high. But the water in the tank doesn't give transistor T2 a base voltage, so it doesn't work, and the logic built around NAND gates N1 and N2 sends a low signal to pin 12 of gate N3. The result is that the amount of N3 produced stays high and the motor keeps pumping water into the tank. Automatic water pump controller: Indicator/monitoring circuit When the tank is full to the level of probe B, the water in the tank still gives base voltage to transistor T1 and turns on relay RL1, which makes pin 13 of gate N3 high. At the same time, water in the tank gives transistor T2 its base voltage, and the logic

built around NAND gates N1 and N2 sends a high signal to pin 12 of gate N3. The output at pin 11 of N3 goes low, which stops the motor from pumping water into the tank. When the water level in the tank drops below probe B but stays above probe A, the water in the tank still gives base voltage to transistor T1 and keeps relay RL1 on, which makes pin 13 of gate N3 high. But transistor T2 doesn't work, and the logic made up of NAND gates N1 and N2 sends a high signal to pin 12 of N3. So, the output of N3 stays low, and the motor doesn't move. When the water level goes below probe A, neither T1 nor T2 will work. When the output of NAND gate N3 is high, relay RL2 is turned on, and the motor starts pumping water into the tank again. Figure 2 shows the circuit for indicators and monitoring. It is made up of five LEDs that light up to show how much water is in the tank above. Since the water at the bottom of the tank gets 12V power, transistors T3 through T7 get base voltage and conduct to light up the LEDs (LED5 down through LED1). When the water level in the tank drops to level C, the transistor T7 starts to conduct, and LED1 lights up. When the water level in the tank gets to be one-fourth full, transistor T6 conducts, and LED1 and LED2 light up. When the water level in the tank gets to be halfway full, transistor T5 conducts and lights up LED1, LED2, and LED3. When the water level reaches 3/4 of the tank, transistor T4 conducts and lights up LED1 through LED4. When the tank is full, T3 conducts, and all five LEDs light up. So, one can tell how much water is in the tank by how bright the LEDs are (see the table). The LEDs can be put anywhere, making it easy to keep an eye on them. Note: The heights of probes A and B can be changed by the user to change how much water needs to be put in the tank. The stand and the screws for adjusting it should be insulated so that they don't short out.