Here is another interesting hobby circuit built around the popular and inexpensive 555 IC. It is nothing but a water sensor, which you can be configured as a rain alarm, water tank overflow alarm, aquarium water leak indicator, etc. It can be powered from any 6VDC supply. Initially, when DC supply is fed to the circuit the Red LED remains in OFF state. However, when presence of water is detected between the sensor probes (A&B), the LED starts blinking at a slow rate.
An application example
This circuit can be wired to flash the LED when the water contained into a plastic tank has reached the desired level. It should be mounted on top of the plastic tank by means of two small needles , acting also as sensor probes. If a deeper sensing level is needed, the needles can be extended by means of two pieces of flexible wire.