The water level controller based on the microcontroller cum Motor Protector controls the motor on and off conditions depending on the water level in the upper tank (OHT), which status is shown on an LCD module. The circuit also protects the motor from high voltages, low voltages, mains fluctuations and dry running.

The water level controller circuit based on microcontroller cum Motor Protector consists of the LM324 operational amplifier, the AT89C51 microcontroller, the PC817 optocoupler, the 7805 controller and an LCD module. The P2.0 to P2.2 connection pins of the AT89C51 (IC2) are used to detect the water level, while the P2.3 and P2.4 pins are used to detect undervoltage and overvoltage, respectively. Pin P3.4 is used to control the relay RL1 with the help of the optocoupler IC3 and the transistor T5 in case of undervoltage, overvoltage and different water level conditions. The LM324 (IC1) is a quad-op (op-amp) amplifier. Two of its amplifiers are used as comparators to detect sub and overvoltage. Under normal conditions, the output pin 7 of IC1 is low, causing the pin P2.3 of IC2 to be high. When the voltage at the N1 pin 6 goes below the reference voltage set at the pin 5 (eg, 170 volts), the N1 output pin 7 goes high. This high output causes the P2.3 pin of IC2 to be low, which is detected by the microcontroller and the LCD module shows 'low voltage'.

Under normal conditions, pin 1 of N2 is high. When the voltage on the pin 2 of N2 passes above the voltage set on the pin 3, the output pin 1 of N 2 drops. This low signal is detected by the microcontroller and the LCD module shows 'high voltage'.

The VR1 and VR2 presets are used to calibrate the circuit for sub and overvoltage, respectively. When tank water rises to contact the sensor, the BC548 transistor base rises. This high signal drives the transistor BC548 in saturation and its collector low. The low signal is detected by the IC2 microcontroller port pins to detect empty tank, dry sump and full tank, respectively.

When the water in the tank is below sensor A, the engine will turn on to fill water in the tank. The LCD module will display 'motor on'. The controller is programmed for a time interval of 10 minutes to check the dry running state of the motor. If the water reaches sensor B in 10 minutes, the microcontroller leaves the dry running condition and allows the motor to continue pushing the water into the tank. The engine will remain on until the water reaches sensor C. It will then stop automatically and the microcontroller will go into standby mode. The LCD module will show "full tank" followed by "standby" after a few seconds. The "standby mode" message is displayed until the water in the tank passes below sensor A. In case the water does not reach sensor B in 10 minutes, the microcontroller will go into dry operation and stop the engine during 5 minutes, to cool. After five minutes, the microcontroller will restart the motor for 10 minutes and check the status on sensor B. If the water is still below sensor B, it will enter dry operation AND the LCD module will display 'dry-sump2 'The same procedure will be repeated and if the dry running condition persists, the display will show' dry-sump3 'and the microcontroller will not start the motor automatically. Now you have to check the water line and manually reset the microcontroller to start the operation. Throughout the procedure, the microcontroller checks for high and low voltages. For example, when the voltage is high, it will scan for about two seconds to check if it is a fluctuation. If the voltage remains high after two seconds, the microcontroller will stop the motor running. Now you will wait for the voltage to stabilize. After the voltage becomes normal, it will still check for 90 seconds whether the voltage is normal or not. After the normal condition, it will pass into the standby mode and the above procedure will begin.