In some countries, the water supply Ad is irregular at most times; in many other countries only during a drought. A means of making this less inconvenient is offered by the present circuit. It needs two water tanks: one, T2, at ground ley or even underground, and the other, Ti in the loft or at least appreciably higher than T2. Tank 2 gets filled by a pump from tank 1 to ensure that there is sufficient water pressure. The circuit shown ensures that the water in tank 1 is kept at a given level; if the water drops below that level, the pump will be switched on. There is protection in case tank 2 is empty. The circuit is operated by a number of sensors mounted in the tanks. Each tank contains a non-corrosive or insulated pin or straight piece of stout wire, R. Tank 1 has two sensors. P and Q. each consisting of a small, non-corrosive disk. Tank 2 has one sensor, S, which is iden-tical to those in tank I. Sensor P indicates when tank1 is full: Q signals when tank 1 is empty; S shows when tank 2 is empty (no water at all). Pins R is connected via a resistor to the pos-itive supply line, while the sensors are linked to the inverting input of three opamps, IC1-IC3. The non-inverting inputs of these amplifiers are supplied with a reference voltage derived from potential dividers. Sufficient water between a pin and a sensor causes a virtual short-circuit that results in a high level at the inverting input of the associated opamp. A relay is used to switch the pump on and off: its normally open contact operates the motor and its normally closed contact is linked to the output of IC2.
When tank 1 is full, the inverting input of all three opamps is at a high level: the outputs of all three are then low and the relay is not energized. when the water level in the tank drops, the output of IC1 goes high, but since the output of IC2 is low the relay will remain inoperative. When, however, the water level drops below sensor Q. the output of IC2 will go high. Transistor T1 is switched on and the relay is actuated so that the pump is switched on and the output of IC2 is open circuited. The high level at pin 6 of IC1 will ensure that T1 remains switched on. When tank 1 is filled to the level of sensor P. the resulting low level at the output of IC1 will cause the relay to be de energized. so that the pump is switched off.
When tank 2 is empty, the output of IC3 becomes high. which switches on T2 so that T1 does not get any base current. Conse- quently, the relay can not be energized. The reference voltage for each of the opamps can be preset with P1-P3 to obtain the required switching pattern. which of course depends also on the sensors used and the composition of the water.
Since there is a direct voltage at the sensors, these must be inspected regularly. Some advice: the carbon electrodes of a discarded dry battery do not resolve in water and are non corrosive.
The current drawn by the circuit is determined primarily by the relay coil: the BC517 can switch up to 400 mA.
The opamps draw only a few mA.
