Capacitive Liquid Level Sensor
This project, a capacitive
liquid level sensor
1989, uses an ingenious technique where the senor is located
outside the liquid container.
This is a very useful technique and has been used in later designs to
create a portable liquid level sensor for locating the exact height of
liquid in a plumbing system.
Normal capacitive liquid level sensors work by having the sensor within
the liquid and sensing the capacitance change when as they are exposed
when the liquid level falls. This has the disadvantage of plate
electrolysis when an ac signal is developed across the capacitive
sensor. This design completely avoids that problem.
Executive Summary of the Capacitive Liquid Level
A non-intrusive fluid level detector including a single point
capacitive sensor mounted on the outside surface of a receptacle such
that capacitive principles can be utilized to sense the level of a
liquid contained within the receptacle. The sensor assembly is disposed
in a substantially fixed position on the exterior wall of the
receptacle wherein the dielectric effect of the liquid changes the
effective capacitance of the sensing capacitor as the liquid rises and
falls within the receptacle.
This change in effective capacitance is detected by electronic
circuitry included in the detector device. In one embodiment, the fluid
level detector is directly mounted to a completely non-conductive
receptacle. In another embodiment, the fluid level detector is mounted
to a non-conductive window which is an integral part of a receptacle
fabricated out of a conductive material.
Background of the Capacitive Liquid Level Sensor
Fluid level detectors which sense the level of a liquid
contained within a receptacle are well known for use in conjunction
with automotive engines. Historically, such detectors have been made in
the form of float operated switches involving moving parts which are
subject to friction and wear.
Other devices utilize an electrical probe to detect fluid levels by
measuring the conductivity of the coolant. However, these devices
require complicated current amplifying systems because there is often
an insufficient amount of current passing through the electrodes to
power an indicator lamp.
In either of these systems, the measuring sensor is located in the
fluid where contaminants are likely to collect on the sensor and
disturb the fluid level measurement. Furthermore, the devices in the
prior art, as described, are categorized as "intrusive" in that they
require an opening into the receptacle containing the fluid.
This creates an additional potential for fluid leaks as well as
potential deterioration of the sensing devices.
Due to the problems discussed above, it has become desirable to employ
non-intrusive means to sense fluid levels contained in receptacles.
These non-intrusive systems typically involve wave propagation
techniques which are implemented through a transmitter/receiver system.
Typically, these systems involve the transmission of an ultrasonic
signal from a transmitting transducer through a fluid to a receiving
Such ultrasonic transmission systems require a liquid transmission
medium in order to carry the ultrasonic signal from the transmitting
transducer to the receiving transducer. Lack of signal at the receiving
transducer relates to a lack of liquid transmission medium, activating
a no fluid present indication.
However, a failure in the transducer pair or in their respective
electronic connections results in a lack of signal from the receiving
transducer which, in turn, activates the normal failure mode thereby
falsely indicating a dry or no fluid present state.
The increasing importance of monitoring fluid levels in automotive and
other applications is creating a need for more reliable non-intrusive
fluid level sensors. It is, therefore, important that a fluid level
detection system be developed which can provide reliable data and which
does not require contact with the fluid being measured.
Summary of the Capacitive Liquid Level Sensor
In accordance with the present design a non-intrusive fluid
level detector is provided for mounting on the exterior wall of a
receptacle. The present design provides single point sensing of
predetermined low levels of fluids, as in an automobile cooling system
or a windshield washer solvent reservoir, without contact with the
liquid being sensed.
The design also provides an electrical signal which can energize an
alarm light or other indicator. The fluid level detector of the present
design incorporates capacitive sensor means established relative to a
fluid receptacle in a substantially fixed relation on the outside wall
of the receptacle.
The capacitive sensor can be made to be an integral part of a printed
circuit containing electronic detection circuitry, thereby making the
device self-contained. This sensor assembly is mounted on the exterior
wall of the fluid receptacle at a position to sense the lower limit of
acceptable fluid level.
The present design utilizes capacitive principles to sense the level of
a liquid contained within a non-metallic receptacle. As a liquid rises
and falls in the container, the dielectric effect of the liquid changes
the effective capacitance of the sensing capacitor which is detected by
electronic circuitry coupled to the sensor. The device remains
activated whenever power is applied and provides an indication to the
user only when the low liquid level is detected.
The present design is a passive device in that the device monitors the
level of the liquid within the receptacle at all times and requires no
interaction or other monitoring by the user.
Figure 1 : (a,b) is a schematic
of the circuitry of the present invention. figure b is a schematic of
an alternative capacitive sensing circuit for the capacitive liquid
View larger image here
Description of the Capacitive Liquid Level Sensor
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.
Figure 2 : A-e are signal
diagrams of signals at certain test points in the schematic diagram of
fig. 1 for the capacitive liquid level sensor
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
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 values of capacitors C3 and C4 which enables the circuit to
determine if there is sufficient fluid in the receptacle 10.
Referring again to FIG. 1A, voltage comparator A2 compares the voltage
at test point 2 (TP2) with the reference voltage set by resistors R2
and R3. FIG. 2 (B and C) illustrate that when the voltage at TP2
decreases below the reference voltage V1, due to the discharge of
capacitor C3, the output of the comparator A2 reference voltage at TP3
drops to zero.
The time between the point where capacitor C3 begins to discharge and
the output of the comparator A2 drops to zero is used as a reference
time T1. Similarly, the comparator A3 compares the square wave output
of the comparator A2 with the voltage decay signal across resistor R10
due to sensor capacitor C4 at test point TP4.
As shown in FIGS. 2C-2E, if the discharge time T3 of capacitor C4 is
longer than the reference time T1 established by the comparator A2, the
output of the comparator A3 remains high. Conversely, if discharge time
T3 is shorter than reference time T1, due to a faster discharge rate in
C4, the output of the comparator A3 pulses low for a period of time in
which the discharge voltage at TP4 is less than the output voltage of
the comparator A2 (TP3). In application, this pulse is actually a
current pulse rather than a voltage pulse.
The output of the comparator A3 is an open collector type output which
allows capacitor C5 to charge through resistor R14. If the output of A3
pulses low, capacitor C5 discharges to ground and the voltage at test
point 5 (TP5) goes to zero.
Finally, the reference voltage set by resistors R2 and R3 is compared
to the voltage at TP5 through the comparator A4. If the voltage at TP5
is high, representing capacitance C4 to be of a relatively large value
and corresponding to a sufficient fluid in the reservoir, the output of
A4 will pull to ground and transistor TR1 will not conduct.
However, if the voltage at TP5 is low, which means that capacitance C4
is not of a sufficient value, the output of A4 will bias transistor TR1
such that current will flow through the collector and energize lamp L1.
In operation, when the fluid level in the receptacle is full, capacitor
C4 takes a certain length of time to discharge; when the fluid is at a
lower level, capacitor C4 takes a shorter period to discharge. Thus, as
the fluid level in the receptacle decreases, the capacitance of
capacitor C4 also decreases, which, in turn, decreases the discharge
time of capacitor C4.
This causes a current pulse to be output by comparator A3 during the
period in which the discharge voltage from capacitor C4 is less than
the voltage output from comparator A2. The current pulse causes
capacitor C5 to discharge, thereby allowing the output of comparator A4
to rise and allowing current to flow through transistor TR1 to lamp L1.
When current flows through lamp L1, the indicator lamp is lit.
Figure 3 : A and b are exploded
perspective views of a preferred embodiment of the liquid level sensor
of the present invention; and for the capacitive liquid level sensor
An exploded perspective view of the present design is shown in FIG. 3
(A and B) where sensor plate P1 is shown as an integral part of the
assembled unit. In an alternative embodiment, capacitor plate P1 can be
physically separated from the electronic circuitry of the design such
that the sensor plate P1 is attached to the fluid receptacle and is
electronically coupled to the corresponding electronic circuitry
located at a position which is isolated from the receptacle.
Figure 4 : A and b are
two-dimensional views of alternative mounting configurations of the
present invention for the capacitive liquid level sensor
In accordance with the present design, as shown in FIG. 4 (A and B),
the sensing capacitor can be mounted to the receptacle 10 in various
ways depending on whether the receptacle 10 is made of a conductive or
a non-conductive material. In one embodiment of the design, shown in
FIG. 4A, the fluid receptacle 10 is completely made of a non-conductive
material and the sensing capacitor plate P1 is mounted directly to the
surface of the receptacle 10.
In another embodiment of the design, shown in FIG. 4B, the receptacle
10 is made of a conductive material except for a relatively small
window 12 of non-conductive material making up a portion of a wall of
the receptacle 10 upon which the capacitive plate P1 is mounted.
The fluid level detector of the present design permits the monitoring
of a level of a fluid contained in a receptacle wherein there is no
engagement between the sensing device and the liquid. The disclosed
design is not to be limited by what has been particularly shown and
described except as indicated by the present claims.
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