ELEKTRONIK

Generally overhead tanks are filled with water by operating a 3-phase pump in conjunction with starter, manually. You have no control over the level to which it may be filled. It may cause overflow or water level may remain too low when you switch off the pump motor. Manual operation of the motor pump starter is thus not advisable. Present circuit may be added to the existing manual 3-phase starter of the pump motor so that it may be operated automatically as well as manually. Four sensor probes made up of brass or stainless steel stiff wires or rods may be hung firmly in the tank. Probes marked E and L are hung with their bottoms near the bottom level of the tank at which we wish to start the pump. The probe H is hung with its bottom at a level at which we wish to stop the pump. Probe M is hung with its bottom end in between those of probes E and H. It is used for indication (only) of middle level. Sensors M, L, and H are connected with resistors R1, R2, and R3, whereas probe E is directly connected to ground. Resistors R1, R2, and R3 are connected to 12V supply through resistors R4, R5, and R6, respectively. The connections of various ICs, gates and other components are shown in the figure. The N/C contacts of relay RL1 are conne- cted in series with the ‘off’ push button of 3-phase st- arter of pump mo- tor. The N/O contact of relay RL2 is connected in parallel to the ‘on’ push button of the 3-phase starter. Circuit shown within the dotted lines is the optional circuit of the water level indicator. When the wa- ter level in the ta- nk is below pro- bes E and L, output of gate N1 is logic low and ou- tput of gate N2 is latched to logic high level. Simu- ltaneously output of gate N3 goes logic low and is inverted by inverter N4. This is differentiated by R-C network for- med using resistor R8 and capacitor C2, and the resulting po- sitive going pulse inverted by inv- erter N6 is given to trigger pin of timer IC2. IC2 is wired as a monostable flip-flop. The output positive going pulse appearing at pin 3 energises relay RL2 for short duration to operate the 3-phase starter for the pump motor. As the water starts filling in the tank and ultimately touches probe H, the output of gate N3 goes high. This is differentiated by resistor R7 and capacitor C1. The positive going pulse appearing at resistor R7 is inverted by gate N5 and applied to trigger pin of monostable flip-flop IC1. The output of IC1 is used to energise relay RL1 via driver transistor T1. This results in opening of N/C contact in series with the stop button of starter and thus switching off of the pump motor. Water level status is displayed using IC5 and IC6 in conjunction with 7-segment display DIS-1. The display indicates F for full, H for half, L for low level, and E for empty condition. It may be noted that probes E and L must be fitted at a level slightly higher than the exit (vent) plug of the