This circuit can be used to automatically keep the header tank filled. It uses a double-pole relay.
This is the transistor version of the circuit below. Here is the circuit using a 555:
These are the facts you have to remember. In our circuit, Pins 2 and 6 detect a voltage when water is not touching the probes, due to the 100k resistors.
When water touches the probes, neither pin "detects a voltage." Don't worry about pin 2 detecting 1/3 of rail voltage and pin 6 detecting 2/3 of rail voltage.
In our circuit the pins either detect a voltage or do not detect a voltage.
Pin 2 detects a LOW and pin 6 detects a HIGH. Pin 2 does nothing when it detects a HIGH and pin 6 does nothing when it detects a LOW.
When the water is LOW, as shown in fig 1, both pins 2 and 6 are HIGH and the output of the 555 is LOW.
As the water rises, as shown in fig 2, Pin 6 goes low but nothing happens to pin 3 except the chip "has been prepared via the internal flip-flop" to change when pin 2 goes LOW.
When the water reaches pin 2, as shown in fig 3, this pin "fails to see a HIGH," the output of the chip goes HIGH and the pump turns off. As the water level goes down, as shown in fig 4, pin 2 sees a HIGH but this does not change the 555 as pin 2 only has an effect when it goes LOW.
When the water level goes down further, as shown in fig 5, pin 6 sees a HIGH and because pin 2 is not seeing a LOW, the chip will change states. The output goes LOW
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