Capacitive liquid level sensor circuit

Posted on Feb 7, 2014

FIG. 1 (A) illustrates the circuit diagram of one embodiment of the fluid level detector of the present design. The supply voltage for this circuit is typically provided by a 12-volt automobile battery which is reduced to a 5-volt DC source voltage by means of a voltage regulator comprising the combination of resistor R1 and zener diode Z1. Capacitor C1 serves as a filter for this voltage supply regulator. The detector of FIG. 1A incorporates an amplifier A1 in conjunction with a resistor/capacitor network R4 and C2 and resistors R5 and R6 to form a square wave oscillator. A reference voltage is supplied from the voltage regulator where the reference voltage value is determined by the voltage divider circuit R2/R3. This voltage serves as a reference voltage for the square wave oscillator and voltage comparators A2, A4 located in the circuit, as will be described.

Capacitive liquid level sensor circuit
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The output of the square-wave oscillator described above, is shown as the square wave of FIG. 2 (A). The oscillator produces an alternating electrical output which causes capacitors C3 and C4 to charge through diodes D1 and D2, respectively, where capacitor C4 is a sensor capacitor and capacitor C3 is a reference capacitor. As stated, capacitor C4 is the sensor capacitor wherein the two conductive surfaces of the capacitor are plate P1 and the residual ground of the receptacle 10 through its mounting arrangement. The dielectric of the capacitor is the fluid in the receptacle 10 such that the capacitance value of C4 varies relative to the fluid level in the receptacle. The capacitance value of capacitor C3 is adjustable to match the value of capacitor C4 when a predetermined liquid level is sensed, i.e., at the critical level. FIG. 1B shows an alternative embodiment of the present design wherein plates P2 and P3 are added to the configuration of sensing capacitor C4. The plates P2 and P3 are connected to circuit ground. A lower level capacitance is provided by plates P1 and P2, and an upper level capacitance is provided by plates P1 and P3. When the voltage from the oscillator circuit at test point 1 (TP1) makes a transition from positive to negative, capacitors C3 and C4 begin to discharge through resistors R9 and R10, respectively. It is the difference in voltage decay across resistors R9 and R10 due to the capacitance...

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