Fluid Level Sensor

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
 Fluid Level Sensor - schematic

the probes. The AC signal is rectified and used to drive Transistor T1 that drives the relay. The relay is a 12-V type of your choice. Diodes D2 and D3 are both small signal diodes (1N4148). Diode D1 (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. The MC14093B is a CMOS 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. Please note: UnusedinputsMUSTbe 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. Unusedoutputs (10 & 11)MUSTbe 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 chrome bicycle spokes with very good success. The `Sensor` works via the capacitive method. Thanks, Tony, for publishing your Fluid-Level Sensor design. I`m using it to detect sewer line plugs (water backing up toward the access port), and hot water heater / clothes washer / AC condensate pump...

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