Fluid-Level Sensor

Parts List:

     R1 = 470K                N1,N2 = MC14093B
     R2 = 15M*                   T1 = 2N3906 (these will work also: PN200, 2N4413)
C1 - C4 = 2N2 (2.2nF)                        (NTE159, ECG159, BC557, BC157, TUP)
     D1 = 1N4001                 Ry = Relay (12V or matching supply voltage)
   D2,3 = 1N4148             Sensor = Stainless Steel probes, brass, chrome, etc.

The above circuit uses an ac-sensing signal to eliminate electrolytic corrosion on the probes. The ac signal is rectified and used to drive Transistor T1 that drives the relay. The relay the 12-V type of your choice.
Transistor T1 can also be a TUP. Check out the TUP/TUN document for a large selection of European transistors and what this system is all about.
Diodes D2 and D3 are both small signal diodes (1N4148). Diode diode D3 (1N4001) eliminates transients and possible sparking over the relay coil. Do not use a signal diode for this but a rectifier diode like the 1N4001 or other types of the 1N400x series.
Resistor R2 controls the sensitivity. Also your choice. Select one between 10 and 22 Mega-ohm, or use a trim-pot.
The MC14093B is a mos p-channel quad 2-input NAND schmitt trigger. The supply voltage can be between 3.0 and 18Vdc. It is pin-for-pin compatible with the CD4093. The capacitors are standard ceramic types but try others if you have them available.

The unused inputs MUST be tied to an appropriate voltage level, either ground or +12V. In this case, tie input pins 8, 9, 12, and 13 to either ground or +12v. The unused outputs (10 & 11) MUST be left open. You can use them as spares when needed.
In regards to the sensor, use your imagination. Stainless steel would be preferred but try other materials too. Depending on what type of fluid you use it for you naturally would choose your type of sensor which would resist corrosion for that particular fluid. I often use chromed bicycle spokes with very good success.

The "RESET" switch in the circuit is optional. The relay can be replaced with anything you like; buzzer, lamps, other relays, etc. A small capacitor of about 10nF (nanoFarad = 0.01uF)