dear all,
i am designing a water level controller for my home and the device is working perfectly with less than 1mtr cable length ....
i am connecting stainless steel screws as the sensors and they are connected through 15mtr 5core(7strands each) cable.. the cable used is not shielded ...
the sensors are connected to P1 of 89c51 directly
now coming to the problem ...
when i use the longer cable (15mtr) the voltages at p1 are not pulled low by the common ground running with them till the tank or the sump (i am using active low signals as P1 is already pulled high inside)
(4 sensors and 1 common ground is to be dipped in the tank of water)
i see a phenomenon similar to capacitor charging on the port values and the values start getting values from 2.5V and settles at 2.94V when the sensors are dipped in water.

Things that i am considering and will try out tonight are:
1. a pull high or pull low resistor ( Mr. Erik please help me with this again )
2. connecting the sensors via a buffer like 74ls244 or 245.
3. Use sheilded cable.

PS i am not trying for changing the sensors for the time being nor going for AC kind of sensors ...
please try and help me out with my current problem ...
Mr Raj Shetgar ... what kind of cable had you used in your prototype ....
all responses are being awaited sincerely and at the earliest .....
regards
nitin