In most of the cities and towns in our country the corporation or municipal water supply is restricted to a few hours in the morning and evening. So nowadays most of the urban houses are equipped with overhead water tanks to ensure 24-hour water supply.
Water to these tanks is pumped from a low—level tank which gets water fr0m the corporation supply lines. Level of water in these tanks is generally maintained by mechanical float valves. Maintenance of the minimum water level in the overhead tanks is not so easy. Usually 0nly after the water is comple tely finished will we come to know about that. The unit described here will switch on and switch off the pump so as to maintain the level within the desired limits.
Whenever the level reaches the lower set limit the pump is switched on. and it will be switched off when the level reaches the upper set limit. The unit can be powered from the domestic AC mains and consumes very little power. A special probe is however required to sense the level of water. The probe Fig. 1 shows the probe fitted inside a water tank. The probe can be made from a PVC tube with length equal to the depth of the tank. Three brass or copper rings are fitted to the tube with Araldite. The positioning of the upper and middle rings determines the upper and lower limits. Wire connections to the rings can be made from inside before fixing them. The bottom end of the tube must be sealed with a plastic cap and Araldite. A 3-core cable must be used for connecting the probe to the unit. This ensures complete protection of the leads from moisture and rain
The circuit & construction
The circuit (Fig. 2) uses four transistors, five diodes, seven resistors, three capacitors, one relay and transformert A When the tank is full the probe is completely immersed in water and all the rings touch the water. Thus points A and B and points A and C will be connected through water as The circuit & construction T The circuit (Fig. 2) uses four transistors, five diodes, seven resistors, three capacitors, one relay and transformert A When the tank is full the probe is completely immersed in water and all the rings touch the water. Thus points A and B and points A and C will be connected through water as treated water is a conductor of electricity. (Only pure water is an insulator.) Hence, transistors Tl and T2 get base bias and will be saturated. Their collector voltages will be now around 0.3 V. So transistors T3 and T4 will be cut off and the relay will be de-energised, cutting off the power to the pump motor. When the water level is in-between rings B and C, transistor T2 is cut off and Tl is saturated. The collector voltage of T2 is now very near to the supply voltage and transistor T4 gets saturated. Since T3, T4 and the relay are in series, in order to energise the relay both the transistors must be saturated. When the water level goes just below the B ring, the base bias to transistor Tl is cut off, thereby switching it off. Now its collector voltage swings to supply voltage and T3 is saturated. Sinc T4 is already saturated, the relay will be energised switching on power to the motor. The base of T1 also gets grounded through th N/O relay contacts. This ensures that during pumping, when the water level again reaches the B ring, transistor Tl is not saturated and T3 is switched off. As the motor goes on pumping, the tank is filled and the water level reaches the C ring. Transistor T2 is now saturated and T4 is cut off, thereby de-energising the relay. The power to the motor is now cut off and the grounding of Tl base is removed. This cycle will be repeated again as the water level goes below the B ring. Diode D3 suppresses the surge voltages developed across the relay coil. D1 and D2 increase the threshold of switching of T3 and T4. Capacitors Cl and C2 bypass any transients appearing at the base of Tl and T2 and increase the noise immunity of the system. The power supply is an ordinary full-wave rectifier. Neon saturated. Sinc T4 is already saturated, the relay will be energised switching on power to the motor. The base of Tl also gets grounded through th N/O relay contacts.
This ensures that during pumping, when the water level again reaches the B ring, transistor Tl is not saturated and T3 is switched off. As the motor goes on pumping, the tank is filled and the water level reaches the C ring. Transistor T2 is now saturated and T4 is cut off, thereby de-energising the relay. The power to the motor is now cut off and the grounding of Tl base is removed. This cycle will be repeated again as the water level goes below the B ring. Diode D3 suppresses the surge voltages developed across the relay coil. Dl and D2 increase the threshold of switching of T3 and T4. Capacitors Cl and C2 bypass any transients appearing at the base of Tl and T2 and increase the noise immunity of the system. The power supply is an ordinary full—wave rectifier. Neon lamp L1 indicates the ‘power on’ condition. lt must be noted that the current rating of the relay contacts must be at least seven times greater than the rated current of the motor. The PCB layout of the circuit is given in Fig. 3. The relay and the transformer can be wired externally. The whole unit can be mounted inside a suitable box. Wire connections to the unit can be made through terminal stations.
Adjustments
The circuit must be thoroughly checked before switching on. Set the trimpots to their maximum resistance position. Insert a piece of paper between the relay contacts connected to the base of `l`l. lmmerse the probe in water such that all the three rings are under water. Now adjust the trimpots such that the relay just energises. The unit may be mounted on the wall near the main switch for the motor.
Water to these tanks is pumped from a low—level tank which gets water fr0m the corporation supply lines. Level of water in these tanks is generally maintained by mechanical float valves. Maintenance of the minimum water level in the overhead tanks is not so easy. Usually 0nly after the water is comple tely finished will we come to know about that. The unit described here will switch on and switch off the pump so as to maintain the level within the desired limits.
Whenever the level reaches the lower set limit the pump is switched on. and it will be switched off when the level reaches the upper set limit. The unit can be powered from the domestic AC mains and consumes very little power. A special probe is however required to sense the level of water. The probe Fig. 1 shows the probe fitted inside a water tank. The probe can be made from a PVC tube with length equal to the depth of the tank. Three brass or copper rings are fitted to the tube with Araldite. The positioning of the upper and middle rings determines the upper and lower limits. Wire connections to the rings can be made from inside before fixing them. The bottom end of the tube must be sealed with a plastic cap and Araldite. A 3-core cable must be used for connecting the probe to the unit. This ensures complete protection of the leads from moisture and rain
The circuit & construction
The circuit (Fig. 2) uses four transistors, five diodes, seven resistors, three capacitors, one relay and transformert A When the tank is full the probe is completely immersed in water and all the rings touch the water. Thus points A and B and points A and C will be connected through water as The circuit & construction T The circuit (Fig. 2) uses four transistors, five diodes, seven resistors, three capacitors, one relay and transformert A When the tank is full the probe is completely immersed in water and all the rings touch the water. Thus points A and B and points A and C will be connected through water as treated water is a conductor of electricity. (Only pure water is an insulator.) Hence, transistors Tl and T2 get base bias and will be saturated. Their collector voltages will be now around 0.3 V. So transistors T3 and T4 will be cut off and the relay will be de-energised, cutting off the power to the pump motor. When the water level is in-between rings B and C, transistor T2 is cut off and Tl is saturated. The collector voltage of T2 is now very near to the supply voltage and transistor T4 gets saturated. Since T3, T4 and the relay are in series, in order to energise the relay both the transistors must be saturated. When the water level goes just below the B ring, the base bias to transistor Tl is cut off, thereby switching it off. Now its collector voltage swings to supply voltage and T3 is saturated. Sinc T4 is already saturated, the relay will be energised switching on power to the motor. The base of T1 also gets grounded through th N/O relay contacts. This ensures that during pumping, when the water level again reaches the B ring, transistor Tl is not saturated and T3 is switched off. As the motor goes on pumping, the tank is filled and the water level reaches the C ring. Transistor T2 is now saturated and T4 is cut off, thereby de-energising the relay. The power to the motor is now cut off and the grounding of Tl base is removed. This cycle will be repeated again as the water level goes below the B ring. Diode D3 suppresses the surge voltages developed across the relay coil. D1 and D2 increase the threshold of switching of T3 and T4. Capacitors Cl and C2 bypass any transients appearing at the base of Tl and T2 and increase the noise immunity of the system. The power supply is an ordinary full-wave rectifier. Neon saturated. Sinc T4 is already saturated, the relay will be energised switching on power to the motor. The base of Tl also gets grounded through th N/O relay contacts.
This ensures that during pumping, when the water level again reaches the B ring, transistor Tl is not saturated and T3 is switched off. As the motor goes on pumping, the tank is filled and the water level reaches the C ring. Transistor T2 is now saturated and T4 is cut off, thereby de-energising the relay. The power to the motor is now cut off and the grounding of Tl base is removed. This cycle will be repeated again as the water level goes below the B ring. Diode D3 suppresses the surge voltages developed across the relay coil. Dl and D2 increase the threshold of switching of T3 and T4. Capacitors Cl and C2 bypass any transients appearing at the base of Tl and T2 and increase the noise immunity of the system. The power supply is an ordinary full—wave rectifier. Neon lamp L1 indicates the ‘power on’ condition. lt must be noted that the current rating of the relay contacts must be at least seven times greater than the rated current of the motor. The PCB layout of the circuit is given in Fig. 3. The relay and the transformer can be wired externally. The whole unit can be mounted inside a suitable box. Wire connections to the unit can be made through terminal stations.
Adjustments
The circuit must be thoroughly checked before switching on. Set the trimpots to their maximum resistance position. Insert a piece of paper between the relay contacts connected to the base of `l`l. lmmerse the probe in water such that all the three rings are under water. Now adjust the trimpots such that the relay just energises. The unit may be mounted on the wall near the main switch for the motor.
