I must have for an application, a water detector with most reliability. I don't have many place, and i don't will µC or any special chip. I will something easy and why not make a modular system who can usable for most application! I must have one transmitter and many receiver that i have level to control! For the transmitter, i used a NE555 who generate a square signal at 2 KHz into the liquid for not damaged the probe by electrolysis. A second probe receive the signal through the liquid. The range of the signal can be more or less, that is function of the conductybilities of the liquid. No problem! The signal go on a comparator, so whe can adjust the level Under the mesured level. For the integration of the square signal, i used a reactivated monostable with a recurring of 3 cycle. Whe have a continuous signal in presence of liquid. A second monostable is for have a continuous signal when we have waves on the liquid to Watch. A first example (detecteur de liquide) is the minimal system for make work this system. You can use it for Watch water on a water well or into a tank . No water, the pump don't work. A second example (vide cave) is for a pump to raise up water, for emptying the cellar at a flood. Water go from the low probe to the middest probe. The pump goes on until the low probe don't have water. When the high probe have water, you can can make a second pump or an alarm. That say you that the water go high to fast or the pump have a problem. The transmitter module measure 27x23mm and the receiver 50x35mm. One transmitter and as mutch as receiver that you have level to check, a logic control ( with or without µC) for the output, and you have your own application ! Mickael sorry for my English, i am french.