This invention is not so new but so I took the liberty of posting my symple version, based on IC 4093. With that, I hope no one more is surprised by the lack of water in the water tank, just as I was.
Hope you enjoy.
Step 1: Electronics
2 x CI 4093;
2 x socket for IC;
7 x 500 ohm resistors;
7 x 2M2 ohm resistors;
1 x battery 9Volts;
1 x battery clip;
1 x normally open (I used a interuptor bell because I had already);
1 x mirror for the ringer switch;
1 x box to switch;
1 plate phenolite standard 10 x 5 cm (already drilled);
X 8 brass screws (they will be sensors);
Double-sided tape or screws to fasten the box to the wall;
network cable (the circuit was primarily designed for use with this cable because it has 8 wires inside: 7 for the sensors and 1for the the common).
OBS.: The length of cable will depend on the distance from the water tank and where the display will stay.
solder, soldering iron, etc ...
The heart of th circuit is a CI4093 that has four "NAND" doors. In this project, I used two.
Basically we have a port "NAND" with one input at a high level and other has a pull-up resistor providing a high logical level (1). When it is placed in that logic zero input, the inverter output goes to high level and turns the LED, illuminating it.
I used seven of the eight doors due to a limitation of the cable network.
The circuit board was done in a standard phenolite (stuck) with the components placed in the top. In a bottom, I made the connections using solder and a few pieces of wire cable.
For the panel, used a box that also served to house the circuit. The center button (normally open) is used to connect the circuit.
The circuit only works when operating the center button. The rest of the time he is in "stand-by ". The good news is that the battery will last a long time and no current through the water. Even when triggered circuit, the current is minimal and poses no hazard to health.