![]() ![]() The AC mains is stepped down by transformer X1 to deliver a secondary output of 12V at 500 mA. Presets VR1 and VR2 are used for calibrating the circuit for under-and over-voltage, respectively. This low signal is sensed by the microcontroller and the LCD module shows ‘high voltage.’ When the voltage at pin 2 of N2 goes above the set voltage at pin 3, output pin 1 of N2 goes low. In normal condition, pin 1 of N2 is high. This high output makes pin P2.3 of IC2 low, which is sensed by the microcontroller and the LCD module shows ‘low voltage.’ When the voltage at pin 6 of N1 goes below the set reference voltage at pin 5 (say, 170 volts), output pin 7 of N1 goes high. In normal condition, output pin 7 of IC1 is low, making pin P2.3 of IC2 high. Two of its op-amps are used as comparators to detect under- and over-voltage. The LM324 (IC1) is a quad operational amplifier (op-amp). Using switch S3, you can manually switch on the motor. Pin P3.4 is used to control relay RL1 with the help of optocoupler IC3 and transistor T5 in the case of under-voltage, over-voltage and different water level conditions. Port pins P2.0 through P2.2 are used to sense the water level, while pins P2.3 and P2.4 are used to sense the under-voltage and over-voltage, respectively. The microcontroller is operated with a 12MHz crystal. Power-on-reset is achieved by connecting capacitor C8 and resistor R14 to pin 9 of the microcontroller. Switch S2 is used for backlight of the LCD module. Pin EA (pin 31) is strapped to Vcc for internal program executions. Pins P3.0, P3.1 and P3.6 are connected to RS (pin 4), R/W (pin 5) and E (pin 6) of the LCD, respectively. Eight pins of port-1 and three pins of port-3 are interfaced with data and control lines of the LCD module. The AT89C51 (IC2) is an 8-bit microcontroller with four ports (32 I/O lines), two 16-bit timers/ counters, on-chip oscillator and clock circuitry. It comprises operational amplifier LM324, microcontroller AT89C51, optocoupler PC817, regulator 7805, LCD module and a few discrete components. ![]() ![]() 1 shows the circuit of the microcontroller-based water level controller-cum-motor protector. ![]()
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