Here is a circuit for an automatic water pump controller that runs the motor of thewater pump. When the water level in the overhead tank (OHT) drops below the lowerlimit, 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 gateIC (CD4011). It gets its power from a 12V DC power supply and doesn't use muchpower. 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-limitprobe. The 12V DC power supply goes to probe C, which is the minimum amount ofwater that can always be kept in the tank. Indicator

Figure 2 shows the circuit for indicators and monitoring. It is made up of five LEDsthat light up to show how much water is in the tank above. Since the water at thebottom of the tank gets 12V power, transistors T3 through T7 get base voltage andconduct 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 tankgets 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 upLED1 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. The LEDscan be put anywhere, making it easy to keep an eye on them. Note: By changing the heights of probes A and B, the user can change how much water needs to go into the tank. The stand and the screws for adjusting it should be insulated so that they don't short out.