A Fluid Sensor

For Conductive Liquids: These may simply consist of Two Parallel Metal Plates. If the Container is Metal, that could act as one of the plates. NOTE: A Suitable metal is required for Acid or Alkaline Solutions. The Emitter of Q1 is the circuits `Voltage Reference` (Aproximately 4. 75 volts) and the circuit switches when the Base of Q4 gets a signal
A Fluid Sensor - schematic

of about 0. 4 Volts. When in use, an AC signal is passed through the liquid between the two probes. This detector than compares the fluids resistance to the internal resistance of RX. "CX", is Mainly used with Inductive Loads. On the Schematic it shows a Visual LED and a Pizo Speaker, But these could be replaced with a Relay. Note: Do NOT Use this Capacitor if using the Pizo Device or a Speaker. It Will Stop 4 to 5 Khz Frequency from coming through. "RX" Reference is typically 15K and it is coupled to a 0. 047 uF Cap. However it may be in a range from 1K to 100K. The. 047 capacitor is provided so there is no Net DC Voltage on the Probes. The use of an AC Signal also prevents any Plating effects from occuring on the probes. It is Possible to calculate the resistance of any aqueous solution of an electrolyte for different concentrations, Provided the dimensions of the electrodes and their spacing is known.

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