Water level controller circuit - Help

[jpreddy]

Hi,

I built water level controller circuit using nand gate and 555 timer. Without connecting to the externally rods on the over head tank it works fine. But when i connect the external rods and test it failed.
I noticed few thing in my circuit which are not behaving as i expected, for sensor inputs i have pull up which is in order of 1 Mega ohms, whenever the water level is low pull up circuit provides me high value, when the water is filled and crossed the sensor it should give me 0v, since at the bottom of tank i have connected 0v to one of the rod.
Before connecting this to rod. my inputs to the nand gate are fine by directly shorting the sensor wire, but when i connect it to external rod, i see the voltage is not 0v it is somewhere 1.8 v, which is not expected.

I tried changing my resistor value more and less but it dint gave me the required value.

Any clues are appreciated.

Thanks
Jpreddy

[pebe]

Can you give us a circuit showing how you have wired up the 555?