I will Teach you how to make a simple water sensor circuit

Step 1: Gather Parts

Parts List
1 General Purpose NPN Transistor $0.10
1 50K Trimpot $0.15
1 47ohm Resistor $0.01
1 LED $0.08
1 Proto Board or You can etch A PCB
1 3 volt power source battery, wall wart, whatever

Step 2: Learn the Layout

[Play Video]

Read Circuit Diagram

Step 3: Create PCB (skip Step If Using Proto Board)

The Schematic is made with ExpressPCB which is a free PCB Layout Software you can download it here
Than Download the PCB file and open and print it and make your PCB. Here are a few links to Instructibles that show you how to make a PCB
water sensor.pcb is 3v
water sensor2.pcb is 12v
these PCB schematics are free to use if I see them being sold buy anyone but me you will be contacted
5pcb
Cheap and Easy Toner Transfer for PCB Making
The Saltwater etch process

Step 4: Build It

Build time will update once I make the PCB

Step 5: Connect to HHO Cell or Any Other Water Sensing Probe

Here are some mock ups of how to connect it to a HHO Cell or just detect water.

Step 6: 12 VOLT CIRCUIT

Here is how to setup the circuit to run on 12 volts from a car battery the circuit is still the same just some resistors are added to the probes to protect the transistor and a larger resistor to protect the LED