JPH07239263A - Liquid detector - Google Patents

Abstract

PURPOSE:To widen the application range of liquid detector by detecting the capacitance being set by a liquid present between a pair of opposing electrodes and comparing the detected value with a reference value thereby determining presence of liquid in a pipe easily. CONSTITUTION:A pair of electrodes 2, 3 of copper plate, for example, are embedded oppositely to a pipe made of rubber or synthetic resin thus connecting a series/parallel resonance circuit 7 with a reference sine wave oscillation circuit 8 connected therewith. A positive pole side detecting circuit 9 is connected with the output terminal of the circuit 8 and a negative pole side detecting circuit 11 is connected with the output terminal of the circuit 7. When water is not present in a pipe, capacitance between the electrodes 2, 3 is very low and the negative voltage from the circuit 7 is lower than the positive voltage from the circuit 8 and when the sum of voltages is compared 14 with a reference voltage, an H level control signal is outputted to actuate an alarm means 20. When water is present in the pipe, the capacitance increases steeply to cause series resonance of the circuit 7 and an AC current flows from the circuit 8 to the circuit 7 thus notifying the presence of water.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid detecting device capable of reliably detecting whether or not a liquid such as water is flowing in a pipe or existing in a container.

[0002]

2. Description of the Related Art In recent years, a device for controlling the flow of various liquids such as water, for example, a spraying device for spraying a chemical solution in a greenhouse, a device for automatically detecting the water level in a bathtub, and controlling the water flow rate. In, the confirmation of the presence or absence of liquid is the most basic thing. When detecting the liquid, there is a very high social demand today for non-contact detection of the liquid.

[0003]

However, at present, as a generally known liquid detecting device, a proximity switch having a special sensor that reacts with a heavy metal when detecting the liquid to be used, for example, sewage containing heavy metal, is used. However, it is difficult to use it for general purpose liquid detection because it is not only limited in its application but also very expensive. For this reason, normally, a contact detection switch such as a float type or electrode type is often used.

However, the float type or electrode type detection switch detects the detection of the liquid by a mechanical means such as a float or an electrode rod, and the detected state is converted to an electrical means by using a mercury switch or the like. The converted signal is transmitted to the control device, and the control device sends an alarm signal or a command signal to a valve device or the like for stopping the flow of the liquid to notify the presence or absence of the liquid,
I try to control the distribution. However, for example, when detecting the water surface of a bathtub with a float-type detection switch, the float itself is installed in the bathtub, which not only interferes with bathing, but is often damaged by mischief, etc. Was difficult to install. Further, even in the electrode type, since a liquid is used as a conductive medium to energize between the electrodes, for example, in the case of installation in a bath, in consideration of the problem of electric leakage, safety must be sufficiently taken into consideration. However, there is a problem that its use place is limited.

Further, in the electrode type, it has been difficult to withstand long-term use because the detection sensitivity decreases as much as water stains and fouling substances adhere. Furthermore, the float type or electrode type detection switch can be installed and used in existing equipment, for example, grasping the distribution status of the liquid in the piping of the plant equipment that processes various liquids, Use to easily grasp the water leak status of water pipes that are piped to general households, factories, etc., because most of them are constructed with piping equipment using pipes, the presence or absence of liquid It was difficult to use as a detection device.

In view of the above various problems, the present invention detects a capacitance value set by a pair of electrodes facing each other and a substance existing between the electrodes, and detects the detected value and a reference value. The object of the present invention is to provide a simple electrostatic capacitance type liquid detection device capable of detecting the presence or absence of liquid by comparing with.

[0007]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a liquid, for example, water at a predetermined position in a pipe through which the liquid flows.
A pair of electrodes are buried opposite to each other in an insulated state with a liquid flow path in between, and the electrodes are electrically connected to a liquid detection device described below. That is, the liquid detection device includes a series-parallel resonance circuit connected to a pair of electrodes, a reference sine wave oscillation circuit connected to the series-parallel resonance circuit and oscillating a frequency fixed at a fixed time, and the reference sine wave. A positive-polarity first detection circuit connected to the output end side of the oscillator circuit to detect an AC voltage corresponding to the frequency output from the oscillator circuit into a direct current, and smooth the pulsating current to output a direct current voltage. And a voltage dividing / integrating circuit and the serial / parallel resonant circuit connected to the first detecting circuit and the first voltage dividing / integrating circuit so as to have opposite polarities, and the AC voltage output from the serial / parallel resonant circuit is detected as DC. Then, the negative second detection circuit and the voltage dividing / integrating circuit for smoothing the pulsating flow are connected to the output terminals of the first voltage dividing / integrating circuit and the second voltage dividing / integrating circuit. The positive and negative DC voltages output from both output terminals are added. Compared circuit, and the voltage and the reference voltage output from the addition circuit "H"
Or a comparator circuit for outputting a control signal of "L" level.

[0008]

According to the present invention, the AC output of the fixed oscillation frequency which is constantly oscillated from the reference sine wave oscillating circuit is detected as the DC output by the first detecting circuit and the first voltage dividing / integrating circuit on the positive side. -Smoothed and output to the comparison circuit. On the other hand, 1
The capacitance between the pair of electrodes is set by the dielectric that exists between the electrodes, so the dielectric is liquid (for example, water).
In the case of, it greatly changes depending on the existence or nonexistence. The capacitance when there is no liquid between the electrodes is small, and the capacitance when there is liquid is necessarily large. Therefore,
When the capacitance increases due to the presence of the liquid, the negative side second detection circuit and the voltage dividing / integrating circuit connected to the series-parallel resonance circuit detect the negative side AC output as the DC output. The smoothed, positive DC output flowing to the positive polarity side significantly increases and flows. The positive and negative DC outputs are added up by an adder circuit in the comparison circuit, the voltage of the added value is compared with a reference voltage, and a control signal indicating the presence of liquid is output from the output end of the comparison circuit.

Further, the present invention corresponds to a DC output on the positive polarity side obtained by detecting and smoothing an AC output of a fixed oscillation frequency output from a reference sine wave oscillation circuit, and an increase / decrease in capacitance between electrodes. The liquid detection device is configured so that the AC output that is output as a result is detected and smoothed, and the DC output on the negative polarity side is added, and the summed output (voltage) can be compared with a preset reference voltage. For example, if all of the reference sine wave oscillator circuits are configured to output at a fixed oscillation amplitude, no problem will occur, but even if there is a difference in oscillation amplitude within the allowable range, Since the present invention is configured such that the DC voltages on the positive and negative sides can be added and the total voltage value thereof can be compared with the reference voltage, even if the oscillation amplitude varies or fluctuates, the output from the comparison circuit is adversely affected. To give There.

Further, according to the present invention, since the series-parallel resonant circuit is configured by connecting one variable capacitor and two inductors in series and in parallel, respectively, the resonant circuit has a series resonance and a parallel resonance. It will have two resonance points. Then, by setting the relationship between the two inductors such that the peak of the resonance frequency of the parallel resonance is lower than the resonance frequency of the series resonance, a large impedance change occurs between the series resonance frequency and the parallel resonance frequency. Can be formed. As a result, the operation in the series-parallel resonance circuit operates in the parallel resonance region and the impedance becomes large when the electrostatic capacitance between the electrodes is small, and conversely, in the series resonance region when the electrostatic capacitance is large. Since it operates, the impedance becomes small. Therefore, the output voltage from the series-parallel resonance circuit is extremely small when the electrostatic capacitance is small and increases when the electrostatic capacitance is large, so that the presence or absence of the liquid can be effectively detected. In addition, when the capacitance of the electrodes varies, the position of the series resonance point and the parallel resonance point of the series-parallel resonance circuit can be easily changed by variably adjusting the variable capacitor. The detection sensitivity of the electrostatic capacity of can be maintained well.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 2 and 4, a liquid detection device 1 according to the present invention includes a pair of liquid detection electrodes 2 and 3 made of a metal plate such as a copper plate, and a liquid detection circuit electrically connected to the electrodes 2 and 3. 4 and a power supply circuit 5 for supplying operating power to the detection circuit 4. Next, the configuration of each part of the liquid detection device 1 will be described. First, the electrodes 2 and 3 are, for example, as shown in FIG. 1, in a pipe 6 made of rubber or a synthetic resin in which a liquid (for example, water) circulates so as to face each other with a liquid circulation hole 6a interposed therebetween. It is buried at a position close to 6a.

Next, the liquid detection circuit 4 will be described. 3, the liquid detection circuit 4 includes a series-parallel resonance circuit 7 connected to the electrodes 2 and 3, a reference sine wave oscillation circuit 8 connected to the series-parallel resonance circuit 7, and an output terminal of the reference sine wave oscillation circuit 8. The first detection circuit 9 and the first voltage division / integration circuit 10 on the positive polarity side connected to the second detection circuit 11 and the second detection circuit 11 and the second detection circuit 11 connected to the series-parallel resonance circuit 7 in the negative polarity state. Voltage dividing / integrating circuit 12 and the first and second voltage dividing / integrating circuit 1
The output terminals of 0 and 12 are roughly configured by a comparator circuit 14 connected to the non-inverting input terminals of a comparator 13 having an adder circuit incorporated therein.

First, the series-parallel resonance circuit 7 includes electrodes 2,
Variable capacitor 1 for resonance frequency adjustment connected in parallel with 3
5 and an inductor L ₁ for parallel resonance and an inductor L ₂ for series resonance, which is configured by connecting the variable capacitor 15 and the inductor L ₁ in parallel, and connecting the variable capacitor 15 and the inductor L ₂ in series. Has been done. The reference sine wave oscillating circuit 8 is configured by incorporating an oscillator that outputs a fixed oscillating frequency (for example, 1 MHz), and its output end is located on the inductor L ₁ side of the series-parallel resonant circuit 7 and in the AC voltage component. Each of them is connected to the first detection circuit 9 via a capacitor C ₁ which functions as a high pass filter for removing the included DC component. The first detection circuit 9 comprises a resistor R ₁ and a diode D _1, and the reference sine wave oscillation circuit 8 to the capacitor C ₁
The AC output input via the resistor R ₁ is subjected to terminal processing (converting current into voltage) and pulsating (detected). The first voltage dividing / integrating circuit 10 is composed of voltage dividing resistors R ₂ and R ₃ and a capacitor C _2, and divides the pulsating flow input from the first detection circuit 9 with the resistors R ₂ and R _3, and further Resistor R ₃ and capacitor C ₂
The first detection circuit 9 according to the CR time constant determined by
The pulsating current input from is output as a smoothed direct current. The output from the first voltage dividing / integrating circuit 10 is output as a positive DC voltage.

Next, the second detection circuit 11 and the second voltage divider
The integrating circuit 12 has a reverse polarity (negative polarity) with respect to the first detecting circuit 9 and the first voltage dividing / integrating circuit 10 and is a connection point between the inductors L ₁ and L ₂ of the series-parallel resonant circuit 7. Is connected via a capacitor C ₃ which functions as a high pass filter, the second detection circuit 11 has a resistor R ₄ and a diode D ₂ , the second voltage dividing / integrating circuit 12 has a voltage dividing resistor R ₅ , Each of them is composed of R ₆ and a capacitor C _4, and a negative DC voltage is output from the output terminal of the second voltage dividing / integrating circuit 12. The second detection circuit 1
Since 1 and the voltage dividing / integrating circuit 12 operate in the same manner as the first detecting circuit 9 and the voltage dividing / integrating circuit 10, their functional descriptions are omitted.

As shown in FIG. 3, the comparison circuit 14 is positive,
Resistors R ₇ and R ₈ for setting the addition reference value connected to the output terminals of the first and second voltage dividing / integrating circuits 10 and 12 on the negative polarity side
Composed of an adder circuit configured by using a comparator 13 and a comparator 13, and calculates the sum of the positive and negative DC voltages input to the non-inverting input terminal of the comparator 13 and the reference voltage input to the inverting input terminal. It is configured to output a liquid presence / absence control signal for comparison. In addition, D ₃ is a diode for protection against a negative DC voltage input to the comparator 13. Also, FIG.
Reference numeral 16 denotes a relay operated by a control signal of "H" level output from the comparison circuit 14, and D ₄ is a protection diode for the relay 16.

The power supply circuit 5 shown in FIG. 4 includes a DC power supply 17 and, for example, a connector 18 having four conductive terminals a to d.
And capacitors C ₅ , C at the first conductive terminals a, b.
There is provided a noise filter 19 configured by combining ₆ and the inductor L _3, and the noise filter 19 is provided.
Prevents the high frequency signal of the AC output output from the reference sine wave oscillator circuit 8 from leaking to the outside. Further, the second conductive terminals c and d are provided with an alarm means 20 such as a light emitting diode and a buzzer, and an auxiliary contact 16a of the relay 16.
Is mounted, and the alarm means 20 is activated by the auxiliary contact 16a which is closed when the relay 16 is activated. Then, the reference sine wave oscillation circuit 8 of the power supply circuit 5
An operation power supply Vcc for operating the liquid detection circuit 4 is output from the output end of the noise filter 19 that removes noise from the.

Next, the operation of the liquid detecting device 1 of the present invention will be described. First, the case where the liquid does not exist in the pipe 6 through which the liquid flows will be described. The liquid in this case will be described by using water as an example, but before that, the preconditions of the present invention will be described. As is well known, the relative permittivity of water varies depending on the temperature, but is generally about 50 to 90 times that of vacuum. Therefore, when water is not present in the pipe 6, only air is present, so that the relative permittivity in the pipe 6 is significantly lower than that in the case where water is present (about 1/10). Less than). Due to the above, the capacitance C between the electrodes 2 and 3 embedded in the pipe 6 is extremely small. On the contrary, if water is present in the pipe 6, the electrostatic capacitance C between the electrodes 2 and 3 is large. Therefore, by detecting the change amount of the electrostatic capacitance C, It becomes possible to confirm the presence or absence of water.

On the basis of the above points, the present invention detects an amount of change in the electrostatic capacitance C by using an unknown electrostatic capacitance Cx,
The inductors L ₁ and L ₂ that resonate in series and in parallel are detected by utilizing the fact that the resonance frequencies at which they resonate in series and in parallel resonate depending on the presence or absence of water. That is, in the series-parallel resonant circuit 7 shown in FIG. 3, for example, the electrostatic capacity in the absence of water is Cx _0, and the electrostatic capacity in the presence of water is Cx _0.
When x ₁ is set, each series resonance frequency can be obtained by the following mathematical expressions (1) and (2).

[0019]

[Equation 1]

Here, fx ₀ : series resonance frequency without water fx ₁ : series resonance frequency with water S: capacity of variable capacitor 15 When the oscillation frequency (1 MHz) output from the reference sine wave oscillation circuit 8 is fr, the AC output (voltage) output from the output end e of the reference sine wave oscillation circuit 8 of the liquid detection circuit 4 is serially paralleled with the AC output (voltage). The series-parallel resonance circuit 7 and the reference sine wave oscillation circuit 8 are configured such that the AC output (voltage) output from the output terminal g of the resonance circuit 7 is satisfied by the following relational expression. That is, e> g when there is no water and e <g when there is water. As a result, when water is present between the electrodes 2 and 3 via the member of the pipe 6, a large capacitance C is generated between the electrodes 2 and 3.
Occurs, the variable capacitor 15 and the inductor L ₂
In the series resonance circuit formed by combining and, the series resonance frequency becomes lower as the capacitance C increases.

Next, a parallel resonance circuit in which the variable capacitor 15 and the inductor L ₁ are combined is set by setting the relationship of the inductances of the inductors L ₁ and L ₂ forming the series-parallel resonance circuit 7 to L ₁ > L _2. It is possible to set the peak of the parallel resonance frequency occurring at the frequency lower than the series resonance frequency. As a result, a large impedance change can be formed between the series resonance frequency and the parallel resonance frequency. Therefore, the fixed oscillation frequency fr output from the reference sine wave oscillation circuit 8 is the series resonance frequency f.
When the voltage is close to x, the amount of change in voltage is large, so that the AC voltage at the output end g of the series-parallel resonance circuit 7 becomes equal to the reference sine wave oscillation circuit 8.
It becomes higher than the AC voltage at the output terminal e. Also, the oscillation frequency f
When r is far from the series resonance frequency fx, the amount of change in voltage is small, so the AC voltage of the g portion becomes lower than the AC voltage of the e portion (under the condition of fx> fr).

Next, in the series-parallel resonance circuit 7 shown in FIG. 3, assuming that the capacitance without water is Cx ₀ and the capacitance with water is Cx ₁ , the respective parallel resonance frequencies are as follows. It can be obtained by the following equations (3) and (4).

[0023]

[Equation 2]

Here, fy ₀ : parallel resonance frequency without water fy ₁ : parallel resonance frequency with water S: capacity of variable capacitor 15

As a result, the best oscillation frequency for detecting the presence / absence of water exists in the range of fy ₀ <fr <fx ₁ (see the shaded area in FIG. 5). As described above, in the series-parallel resonance circuit 7 as shown in FIG. 3, the parallel resonance point fy ₁ and the series resonance point fx ₀ shown in FIG. 5 are provided, and the series resonance frequency is between the resonance points fy ₁ and fx _0. By setting the difference between the parallel resonance frequencies, it is possible to set the range of the detection frequency regarding the presence / absence of water. That is, in FIG. 5, when the difference (frequency) between the parallel resonance point fy ₁ and the series resonance point fx ₀ is reduced, the amount of change in voltage depending on the presence / absence of water increases, and conversely, fy ₁ and fx _0.
The larger the difference from the above, the smaller the amount of change in the voltage will necessarily be. Thereby, the sensitivity adjustment for detecting the presence / absence of water can be favorably performed by changing the variable capacitor 15.

Next, the detection when there is no water in the pipe 6 (no water) will be described. When there is no water in the pipe 6, the capacitance C between the electrodes 2 and 3 is very small. Therefore, the series-parallel resonance circuit 7
The impedance of the inductor L ₁ that constitutes the parallel resonance circuit in the internal circuit becomes high, and the inductor L ₁ that does parallel resonance
_It becomes difficult for current to flow in ₁ and the impedance of the parallel resonant circuit is increased. Therefore, the negative AC voltage output from the output terminal g of the series-parallel resonance circuit 7 is extremely low because the parallel resonance frequency fy ₀ is lower than the oscillation frequency fr as shown in FIG. . On the other hand, since the oscillation frequency output from the reference sine wave oscillation circuit 8 is fixed, an AC voltage based on the oscillation frequency is constantly output from the output terminal e of the oscillation circuit 8. That is, as shown in FIG. 5, the AC voltage output from the output terminal e of the reference sine wave oscillation circuit 8 causes a parallel resonance to cause the AC voltage output from the output terminal g of the series-parallel resonant circuit 7 to be a lower voltage. Will be output (e> g).

The positive AC voltage output from the reference sine wave oscillation circuit 8 is supplied to the first detection circuit 9 and the first detection circuit 9.
Is detected and smoothed by the voltage dividing / integrating circuit 10 and is output as a DC voltage to the non-inverting input terminal of the comparing circuit 14,
The negative AC voltage output from the series-parallel resonance circuit 7 is applied to the second detection circuit 11 and the second voltage dividing / integrating circuit 12.
Is detected and smoothed into a DC voltage by the output and output to the non-inverting input terminal of the comparison circuit 14. The positive and negative DC voltages input to the comparison circuit 14 are summed and the magnitude is compared with the reference voltage. In this case, assuming that the reference voltage is 0V, the summed DC voltage has a high voltage on the positive polarity side.
An output terminal 4 outputs an "H" level control signal for operating the alarm means 20. When the "H" level signal is output, the constant voltage power supply Vcc is applied to the relay 16.
, The relay 16 is stopped, the auxiliary contact 16a inserted in the power supply circuit 5 of FIG. 4 is opened to stop the power supply to the alarm means 20 such as a light emitting diode or a buzzer, and the water in the pipe 6 is stopped. Notify that it does not exist. Therefore, for example, when the liquid detection device 1 of the present invention is provided in a hose of a drug spray car and the spray car runs unattended in a greenhouse to spray the drug, Since it is possible to immediately detect the situation and notify the operator, it is possible to reliably avoid a situation in which the drug spray vehicle is run unmanned with the drug running out.

Next, detection when water is flowing (with water) in the pipe 6 will be described. When water flows in the pipe 6, the electrostatic capacitance C of the electrodes 2 and 3 increases rapidly as compared with the case where water does not exist. This is because the relative permittivity of water is about 10 times higher than that of air. When the electrostatic capacitance C between the electrodes 2 and 3 increases, the inductor L ₂ that forms the series resonance circuit in the series-parallel resonance circuit 7
Of the series parallel resonance circuit 7 due to the low impedance of the series resonance resonance.
The impedance of the series resonance circuit is lowered because it easily flows into the inside. As a result, the negative AC voltage output from the output terminal g of the series-parallel resonance circuit 7 is at a position where the series resonance frequency fx ₁ is higher than the oscillation frequency fr output from the reference sine wave oscillation circuit 8, as shown in FIG. Exists in
It is output as a high voltage.

On the other hand, since the frequency oscillated from the reference sine wave oscillation circuit 8 is fixed, an AC voltage based on the oscillation frequency is constantly output from the output terminal e thereof. When this result is confirmed by FIG. 5, it is found that the AC voltage output from the output terminal e of the reference sine wave oscillation circuit 8 is lower than the AC voltage output from the output terminal g of the series-parallel resonant circuit 7. This is because the series resonance frequency fx is the oscillation frequency f as described above.
Because it resonates at a position higher than r (e <
g).

Therefore, the first detection circuit 9 and the first voltage divider
The positive DC voltage output via the integrating circuit 10 and the negative DC voltage output via the second detection circuit 11 and the second voltage dividing / integrating circuit 12 are added by an adding circuit in the comparison circuit 14. When added, the summed value is summed up as a negative DC voltage. Therefore, when this is compared with the reference voltage (0 V), it is a voltage lower than the reference voltage. A "level control signal is output. When the "L" level control signal is output, the relay 16
Is supplied with a constant voltage power supply Vcc, and the relay 16 operates, whereby the closed state of the auxiliary contact 16a is continued to energize the alarm means 20. Therefore, the presence of water in the pipe 6 can be easily confirmed by the operation of the alarm means 20.

Next, the electrostatic capacitance C between the electrodes 2 and 3 which changes depending on the presence or absence of water can be confirmed by the following calculation formulas in the case of no water and the case of water. First, the electrostatic capacitance Cx ₀ between the electrodes 2 and 3 when there is no water can be obtained by the following mathematical expression (5).

[0032]

[Equation 3]

Next, the electrostatic capacitance Cx ₁ between the electrodes 2 and 3 in the presence of water can be obtained by the following equation (6).

[0034]

[Equation 4]

Here, the symbols used in the above equations (5) and (6) are: dielectric constant of vacuum: ε ₀ relative permittivity of water: εrw relative permittivity of resin: εrc permittivity of air: εra Area: P Distance between electrodes: ls Thickness of resin interposed between electrodes and water: lc.

Therefore, Cx ₁ -Cx ₀ is the electrostatic capacitance between the electrodes 2 and 3 which changes depending on the presence or absence of water. In the present embodiment, an example in which water is used as the liquid has been described, but the present invention is not limited to this, and ethyl alcohol, nitrobenzene,
It is also possible to detect a chemical liquid such as methyl alcohol, and the detection of these liquids can be easily performed by setting the area of the electrode according to the relative dielectric constant of the liquid to be detected.

Although the present invention has been described with reference to the example in which the electrodes 2 and 3 are embedded in the pipe 6, the present invention is not limited to this, and for example,
As shown in FIG. 6, the electrodes 2 and 3 are embedded in a pair of openable and closable holding members X and Y for holding the pipe 6, and the holding members X and Y are fulcrumed by a spring member using a handle 21. It is opened and closed with O as the center so as to be sandwiched by the pipe 6 as shown in FIG.
The presence or absence of the liquid may be confirmed by displaying the detection result on the surface of the box 22 in which the sandwiching members X and Y are attached and the detection circuit is built therein. The present invention can be realized even when used as the portable liquid detection device 1a.

[0038]

Since the present invention is constructed as described above, it has the following effects. (1) According to the present invention, a negative direct-current voltage detected and smoothed into direct current that varies according to a change in electrostatic capacitance between electrodes, and a positive polarity obtained by detecting and smoothing with a fixed oscillation frequency. Since it is configured to output the control signal that determines the presence or absence of liquid by adding the DC voltage of It is convenient because it can be detected. (2) Since the present invention is configured to add the positive and negative DC voltages and compare the voltage of the added value with the reference voltage as described above, the oscillation frequency is output, for example. Even if an error occurs in the output fixed oscillation frequency in the reference sine wave oscillating circuit, the error can be corrected and compared with the reference voltage, so that the liquid can be accurately detected. (3) According to the present invention, in the series-parallel resonance circuit, the variable capacitor and the two inductors are connected in series and in parallel to form the circuit, and therefore, the series resonance frequency and the parallel resonance frequency are changed according to the change in the capacitance between the electrodes. As a result of being able to form a large impedance change with the resonance frequency, it is possible to increase the output voltage from the series-parallel resonance circuit to a minimum when the capacitance between the electrodes is small and to increase it when it is large. Therefore, the presence or absence of the liquid can be effectively detected. (4) In addition, according to the present invention, even if a difference occurs in the detection sensitivity of the electrodes, the variable capacitor is variably adjusted to easily move and adjust the positions of the series and parallel resonance points with respect to the oscillation frequency. Therefore, the detection sensitivity of the electrostatic capacitance between the electrodes can be favorably maintained for a long period of time.

[Brief description of drawings]

FIG. 1 is a schematic view showing a usage state of a liquid detection device of the present invention.

FIG. 2 is a block diagram showing an electric circuit of the liquid detection device of the present invention.

FIG. 3 is an electric circuit diagram of the liquid detection device of the present invention.

FIG. 4 is a power supply circuit diagram of the liquid detection device.

FIG. 5 is a waveform diagram showing a relationship between a series resonance frequency and a parallel resonance frequency.

FIG. 6 is a front view of a usage state showing another embodiment of the present invention.

FIG. 7 is a front view showing the same state before use.

[Explanation of symbols]

1 Liquid Detection Device 2, 3 Electrodes 4 Liquid Detection Circuit 7 Series-Parallel Resonance Circuit 8 Reference Sine Wave Oscillation Circuit 9, 11 First and Second Detection Circuits 10, 12 First and Second Dividing / Integrating Circuits 14 Comparison Circuit 15 Variable Capacitor 19 Noise Filter 20 Alarm Means L _{1 to} L ₃ Inductor

Claims (2)

[Claims] 1. A direct electrode formed by connecting a pair of electrodes arranged in a predetermined space, a variable capacitor and a pair of inductors in series and in parallel, and connecting both ends of the variable capacitor to the electrodes. A parallel resonance circuit, a reference sine wave oscillation circuit having an output terminal connected to one of the series-parallel resonance circuit, and an AC voltage output from the reference sine wave oscillation circuit which is connected to the reference sine wave oscillation circuit and is detected and smoothed. Detecting and smoothing means for outputting a positive DC voltage, and a detecting and smoothing circuit connected to the series-parallel resonant circuit for detecting and smoothing an AC voltage output from the series-parallel resonant circuit and outputting a negative DC voltage. And a comparison control means for summing the respective DC voltages output from the two detection and smoothing means, comparing the summed DC voltage with a reference voltage, and outputting a predetermined control signal. A liquid detection device characterized by the above. 2. The liquid detection device according to claim 1, wherein the pair of electrodes are attached to a member through which a liquid flows or a holding member for holding the member with a predetermined space therebetween. .