Try using a water sencor that is sold bt some alarm companies for monitoring the over flow conditiond of a sump pump. You can use the output of this device to interface with your microprocessor.
Hope this helps.
One thing to consider in this wireless application is the EM noise in your environment. If the area is electrically noisy, lots of motors and cabling etc, it may be better to run a twisted pair between the two areas. The sencing of the closing and opening of a relay could do the trick in the...
