Use the relay to control the water pump to drive the pump motor must be connected to the work of the relay controlled by the normally closed contact in series with the implementation of the power circuit, as shown in Figure (b) below. When the tank when there is no water, the relay is the release of state, normally closed contact connected to the circuit just to make the motor work, so the water being pumped to the tank. The question is how to control the water level If the low level probe 1,2, 6,7 receiving end of the relay input signal, once the water level rose to a high level, the 6,7-side Jibei water conduction, following electrical work, pull the armature, the original implementation of the normally closed contact circuit will disengage the motor stopped; It is our hope that program. However, the pump stopped working, the water level dropped, once the water level and high-level probe disengagement, because the relay disconnect between the 6,7-side, the release of the relay armature, normally closed contacts and connected to the motor started to work , so frequent on-off, which does not meet the requirements of our control. Careful analysis of this process, the liquid level increasing, and 6,7-side off, we need to release the relay armature, normally closed contact connected, but the decline in the liquid phase, 6,7-side disconnected, do not want to release the relay armature, and how to maintain the operation of the relays in the end imagine there is another switch, connected between the ends 6,7, 6,7 end when the water level dropped so disconnected, the switch to turn , then the problem is solved. This switch can be controlled by the relay contacts of the other knife on the to select (which can maintain the original pull-state relay circuit is called self-protection circuit in the electrical engineering). For example, the button to manually control the motor is running, used this circuit, as shown in (b) below. When SA1 by touch, the relay work, SJ2 connected to the motor work; while S1 is also connected, the relay will pull in a steady state.

To achieve high and low level control towers, is in need of this design ideas. The experimental circuit shown in Figure (b) below. Probe 1,2-side circuit terminal connected respectively with 7,6, while the 7-side and 3 side (metal tank) in series between a pair of normally open relay contact . Put the level drops below a brief analysis of the process.

When the water level before the relay has been energized, normally closed switch (normally closed contact) SJ1 disconnected, the pump stops working, normally open switch (normally open contact) SJ2 connected, 7 and 3-side similarities end. When the water level dropped to probe below the high beam position, disconnect between the 1,2-side, but the 2,3-side conduction through the water between 2 and 6 connected together side, 3 and 7 side by side SJ2 interlinked, so 6, 7, the server is still connected, the relay control circuit is still some work to maintain the current status of the relay's pull. Pumps still work until the end of the water level dropped to below 2, no conduction between 2,3, 6,7 will be lost between the channels, so the relay release, SJ1 again connected, the pump began to work. So forth, you can control the water level in the high and low level between.