Adaptive filter of optical fiber sensor

Abstract

The optical fiber sensor adaptive filter comprises a front-end digital filter and a rear-end analog filter, the front-end digital filter adopts filtering of an FIR filter adaptive filtering coefficient, estimates an FIR length N through frequency deviation to obtain a correlation coefficient n, and sets N and a corresponding weight according to the correlation coefficient and frequency precision, namely, resolution requirements, so that the FIR filter adaptive filtering coefficient is obtained. And an additional momentum method is adopted to improve a weight value, over-fitting or under-fitting can be avoided, the filtering performance is ensured, a rear-end analog filter adopts attenuation, variable impedance acquisition, transformer primary frequency selection and transformer secondary induction tuning, and finally, noise interference can be effectively filtered through filtering of an adjustable harmonic device and filtering of an LC frequency selection network, so that the measurement accuracy is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic sensing, and particularly relates to an adaptive filter for fiber optic sensors. Background Art

[0002] Fiber optic sensors play an important role in multiple fields such as communication and industrial monitoring. In a fiber optic sensing system, in order to convert a physical quantity to be measured (such as temperature, pressure, displacement, etc.) into an optical signal for transmission and processing, modulation techniques are usually adopted. Modulation can be intensity modulation, phase modulation, frequency modulation, etc. In frequency modulation, a modulation frequency is introduced, which is related to the physical quantity to be measured. By detecting the frequency change of the modulated optical signal, the information of the physical quantity to be measured can be deduced. In addition, during the detection of certain physical quantities and subsequent signal processing, such as in the measurement of vibration, only vibration signals within a specific frequency range can be accurately detected. Subsequent signal processing such as filtering, amplification, and conversion also needs to be carried out within a specific frequency range.

[0003] During the fiber optic transmission process, it is inevitable to be affected by various noise interferences. Noise interferences will cause signal frequency deviation and waveform distortion, affecting the accuracy and reliability of the signal. Therefore, an adaptive filter for fiber optic sensors needs to be provided to effectively filter out the noise interferences. Summary of the Invention

[0004] In view of the above situation, to overcome the deficiencies of the prior art, the present invention provides an adaptive filter for fiber optic sensors. By adopting filtering with an adaptive filtering coefficient of an FIR filter for the received front-end modulation frequency signal and adopting attenuation + variable impedance acquisition + primary transformer frequency selection and secondary transformer induction tuning for the subsequent signal processing, and finally filtering through an adjustable harmonic filter and an LC frequency selection network, the noise interferences can be effectively filtered.

[0005] Its technical solution is as follows: It includes a front-end digital filter and a rear-end analog filter. The front-end digital filter receives the modulation signal of the fiber optic sensor, adopts filtering with an adaptive filtering coefficient of an FIR filter to remove noise and extract useful information, and is converted into an analog signal through a DA converter. The FIR filter internally includes an FIR controller, an adder, a multiplier, and an accumulator; The rear-end analog filter receives the analog signal, controls the intensity of the optical signal within a certain range through an attenuator, and then obtains a variable impedance through a tuner, selects a frequency at the primary of the transformer, and tunes the induction at the secondary of the transformer. Finally, it outputs after filtering through an adjustable harmonic filter and an LC frequency selection network; The steps for determining the coefficient of the adaptive filtering coefficient are as follows: Step 1, obtain the desired frequency characteristic signal; Step 2, obtain the input signal; Step 3, extract the frequency feature components of the input signal; Step 4, estimate the frequency deviation; Step 5, estimate the FIR length N according to the frequency deviation; Step 6, predict the corresponding weights of each convolutional layer; Step 7, the input at each moment is multiplied by the weight coefficient by a multiplier, and finally the accumulator performs superposition output to filter the signal.

[0006] Preferably, the FIR length N is to avoid overfitting and underfitting. The FIR length N and the corresponding weights are determined by the frequency deviation and the frequency accuracy. When the frequency deviation trend becomes smaller, the weights are adjusted positively. When it becomes larger, stop and determine the weight adjustment range. Fine-tune within the weight adjustment range, and the number of fine-tuning times is set to 1 / 2 of the frequency accuracy.

[0007] Preferably, it further includes introducing a momentum factor and improving the weight value correction process by using the additional momentum method. The specific method is: add a part of the previous or previous weight adjustment amount to the weight adjustment amount calculated according to the current error as the actual weight adjustment amount for this time.

[0008] Preferably, the attenuator includes an inductor L1 and a capacitor C1. One end of the inductor L1 and one end of the capacitor C1 are connected to the analog signal. The other end of the inductor L1 is respectively connected to the other end of the capacitor C1, the right end of the diode BD1, and one end of the capacitor C3. The negative electrode of the diode BD1 is connected to detect the signal amplitude, and the left end of the diode BD1 is connected to one end of the grounding capacitor C2; The tuner includes a potentiometer RW1, an inductor L, and a capacitor C4. The potentiometer RW1, the inductor L, and the capacitor C4 form a variable impedance network. The inductor L, the capacitor C4, and the primary coil of the transformer T1 form a frequency selection network. Among them, the high frequency is added to the primary coil of the transformer T1 through the capacitor C4, and the low frequency is added to the primary coil of the transformer T1 through the inductor L. The secondary coil of the transformer T1 and the varactor diode BD2 are connected in parallel to form an inductive tuning network. The upper end of the potentiometer RW1, the upper end of the inductor L, and one end of the capacitor C4 are connected to the other end of the inductor L1. The other end of the capacitor C4 is connected to one end of the primary coil of the transformer T1. The other end of the primary coil of the transformer T1, the lower end of the potentiometer RW1, and the lower end of the inductor L are grounded. One end of the secondary coil of the transformer T1 is connected to the upper end of the varactor diode BD2, and the other end of the secondary coil of the transformer T1 is connected to the lower end of the varactor diode BD2; The adjustable harmonic filter receives the output signal of the tuner and realizes notch filtering of the interference noise frequency components in the tuned signal, including a resistor R4 and an electrolytic capacitor E2. One end of the resistor R4 and the upper end of the electrolytic capacitor E2 are connected to one end of the secondary coil of a transformer T1. The negative electrode of the electrolytic capacitor E2 is respectively connected to the positive electrode of an electrolytic capacitor E3 and one end of a potentiometer RW2. The other end of the resistor R4 is connected to the positive electrode of an electrolytic capacitor E4. The negative electrode of the electrolytic capacitor E3 is respectively connected to the negative electrode of the electrolytic capacitor E4 and the other end of the potentiometer RW2. The adjustable end of the potentiometer RW2 is connected to the ground through a resistor R5. The adjustable end of a potentiometer RW1 is connected in parallel to the noise frequency signal, and the noise frequency signal is used to control the frequency of the notch filter. The LC frequency selection network is composed of an inductor LN and a varactor diode BD3, and finally outputs the filtered and conditioned fiber optic sensor signal through a capacitor C5.

[0009] The beneficial effects of the present invention: By adopting filtering with the adaptive filtering coefficient of the FIR filter for the received front-end modulated frequency signal, and adopting attenuation + variable impedance acquisition + primary frequency selection of the transformer and secondary induction tuning for the subsequent signal processing, and finally filtering through the adjustable harmonic filter and the LC frequency selection network, the noise interference can be effectively filtered. Among them, for the filtering with the adaptive filtering coefficient of the FIR filter, the length N of the FIR is estimated through the frequency deviation, the correlation coefficient n is obtained, and according to the correlation coefficient and the frequency accuracy, that is, the resolution requirement, N and the corresponding weights are set, and the additional momentum method is used to improve the weight value, which can avoid overfitting or underfitting and ensure the filtering performance. Description of the Drawings

[0010] Figure 1 is the step flow chart for determining the coefficient of the adaptive filtering coefficient of the present invention.

[0011] Figure 2 is the circuit schematic diagram of the present invention. Detailed Embodiments

[0012] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0013] The following is combined with the specification appendix Figures 1 to 2 to make a further detailed description of the specific embodiments of the present invention.

[0014] Optical fiber sensor adaptive filter, including a front-end digital filter and a back-end analog filter. The front-end digital filter receives the modulated signal of the optical fiber sensor, filters it using the adaptive filtering coefficients of the FIR filter to remove noise and extract useful information, and converts it into an analog signal through a DA converter (i.e., digital-to-analog conversion). The FIR filter internally includes an FIR controller, an adder, a multiplier, and an accumulator. The FIR controller is the control core, receives the desired frequency characteristic signal and the input signal (i.e., the signal with the modulated frequency output after measurement by the optical fiber sensor), estimates the frequency deviation, estimates the FIR length N and the corresponding weights, and sends control signals to the adder, multiplier, and accumulator. The multiplier multiplies the input signal by the weight. When N is symmetric, the adder first performs addition and then multiplication. The accumulator accumulates the weighted signals output by multiple multipliers to obtain the filtered output signal; The back-end analog filter receives the analog signal, controls the intensity of the optical signal within a certain range through an attenuator, then obtains a variable impedance through a tuner, selects a specific frequency range through the primary frequency selection of the transformer, induces tuning in the secondary of the transformer, and finally filters through an adjustable harmonic filter to filter out the detected noise frequency signal, and then filters out the signals outside the specific frequency range through an LC frequency selection network and outputs; The steps for determining the coefficients of the adaptive filtering coefficients are as follows: Step 1, obtain the desired frequency characteristic signal; Step 2, obtain the input signal; Step 3, extract the frequency characteristic components of the input signal. For example, the sweep frequency analysis method can be used to perform frequency sweeping within a certain frequency range and analyze the signal amplitude and frequency components; Step 4, estimate the frequency deviation. For example, the correlation method can be used. By calculating the correlation between two signals, the frequency deviation between them can be estimated. When the frequencies of the desired frequency characteristic signal and the input signal are close, the correlation between them will be high; otherwise, the correlation will be low; Step 5, estimate the FIR length N according to the frequency deviation. When the coefficient n of the correlation is set to be greater than 0.7 and the frequency accuracy (i.e., the resolution requirement) is high (for example, for an optical fiber vibration sensor, generally, the distance resolution of an optical fiber vibration sensor can reach the micron level. For a high-performance optical fiber vibration sensor, its distance resolution can reach below 0.1 micron. For some relatively simple optical fiber vibration sensors, their distance resolution is only in the hundreds of microns level or micron level, and below 0.1 micron is set as a high requirement), set N to (1 - n) * 2 / 0.1. To avoid overfitting and underfitting, the FIR length N and the corresponding weights are determined by the frequency deviation and frequency accuracy, and the weight is adjusted positively when the frequency deviation trend becomes smaller, and stops when it becomes larger. Determine the weight adjustment range and fine-tune within the weight adjustment range. The number of fine-tuning times is set to 1 / 2 of the frequency accuracy; Step 6, estimate the corresponding weights of each convolutional layer, and initially set them to equal weights, that is, (1 / N)*100%; Step 7, the input at each moment is multiplied by the weight coefficient by a multiplier, and finally the accumulator performs superposition to output the filtered signal.

[0015] Based on the above solution, it also includes introducing a momentum factor and improving the weight value correction process by using the additional momentum method. The specific approach is: a part of the previous or previous weight adjustment amounts is superimposed on the weight adjustment amount calculated according to the current error as the actual weight adjustment amount for this time.

[0016] Based on the above solution, the attenuator controls the intensity of the optical signal within a certain range, that is, attenuates the analog signal after DA conversion of the FIR digital filter, that is, the modulation frequency signal, which is controlled by the amplitude of the detection signal. Here, the amplitude of the detection signal can be the amplitude of the output signal of the fiber optic sensor obtained through the peak detection circuit, or the amplitude of the modulation frequency signal obtained by amplitude discrimination by the amplitude discriminator. The specific structure adopted is: an inductor L1 is connected in parallel with a capacitor C1, a diode BD1 is connected in series with a capacitor C2 and then connected in parallel with a capacitor C3 to form an attenuator. The diode BD1 is a varactor diode, and the voltage corresponding to the amplitude of the detection signal is applied to the negative electrode of the varactor diode to adjust the impedance to achieve attenuation. It can also be 1SV172, and the current corresponding to the amplitude of the detection signal is applied to the cathode. As the current increases, its impedance decreases, and the impedance is adjusted to achieve attenuation. It includes an inductor L1 and a capacitor C1. One end of the inductor L1 and one end of the capacitor C1 are connected to the analog signal. The other end of the inductor L1 is respectively connected to the other end of the capacitor C1, the right end of the diode BD1, and one end of the capacitor C3. The negative electrode of the diode BD1 is connected to the amplitude of the detection signal, and the left end of the diode BD1 is connected to one end of the grounded capacitor C2; After that, the tuner obtains a variable impedance through the potentiometer RW1. The potentiometer RW1 is a voltage-controlled potentiometer, which is regulated in the reverse direction by the amplitude negative feedback of the detection signal. The inductor L, capacitor C4, and the primary coil of the transformer are used for frequency selection to select a specific frequency range. The secondary of the transformer induces tuning and restores it within the optical signal intensity frequency range. The induced frequency range can be achieved by applying a voltage to the negative electrode of the varactor diode BD2. It includes the potentiometer RW1, inductor L, and capacitor C4. The potentiometer RW1, inductor L, and capacitor C4 form a variable impedance network, and the inductor L, capacitor C4, and the primary coil of the transformer T1 form a frequency selection network. Among them, the high frequency is added to the primary coil of the transformer T1 through the capacitor C4, and the low frequency is added to the primary coil of the transformer T1 through the inductor L. The secondary coil of the transformer T1 and the varactor diode BD2 are connected in parallel to form an induction tuning network. The upper end of the potentiometer RW1, the upper end of the inductor L, and one end of the capacitor C4 are connected to the other end of the inductor L1. The other end of the capacitor C4 is connected to one end of the primary coil of the transformer T1. The other end of the primary coil of the transformer T1, the lower end of the potentiometer RW1, and the lower end of the inductor L are connected to the ground. One end of the secondary coil of the transformer T1 is connected to the upper end of the varactor diode BD2, and the other end of the secondary coil of the transformer T1 is connected to the lower end of the varactor diode BD2; The adjustable harmonic filter receives the output signal of the tuner and filters out the interference noise frequency component f in the tuned signal through the resistors R4 and R5 and the electrolytic capacitor E2. Among them, by adjusting the resistance value of the potentiometer RW2, the notch is adjusted to the interference noise frequency. For example, when the frequency generated by the frequency discriminator adjustable harmonic filter is the same as the interference noise frequency, the voltage is zero. When the frequencies are different, the voltage difference is applied to RW2 for adjustment. It includes the resistor R4 and the electrolytic capacitor E2. One end of the resistor R4 and the upper end of the electrolytic capacitor E2 are connected to one end of the secondary coil of the transformer T1. The negative electrode of the electrolytic capacitor E2 is respectively connected to the positive electrode of the electrolytic capacitor E3 and one end of the potentiometer RW2. The other end of the resistor R4 is connected to the positive electrode of the electrolytic capacitor E4. The negative electrode of the electrolytic capacitor E3 is respectively connected to the negative electrode of the electrolytic capacitor E4 and the other end of the potentiometer RW2. The adjustable end of the potentiometer RW2 is connected to the ground through the resistor R5. The adjustable end of the potentiometer RW1 is connected in parallel to the noise frequency signal, and the noise frequency signal is used to control the frequency of the notch filter; The LC frequency selection network is composed of the inductor LN and the varactor diode BD3, and finally the filtered and conditioned optical fiber sensor signal is output through the capacitor C5.

[0017] The reverse excitation circuit receives the amplitude of the detection signal, filters it through a loop filter, charges it reversely, and applies it to the gate of MOS transistor M1, changing the drain voltage of MOS transistor M1. The drain voltage is superimposed with -5V and applied to potentiometer RW1, changing the impedance of potentiometer RW1. In this way, by filtering the received front-end modulated frequency signal with the adaptive filtering coefficient of the FIR filter, and taking attenuation + variable impedance acquisition + primary frequency selection of the transformer and secondary induction tuning for the back-end signal processing, and finally filtering through an adjustable harmonic filter and an LC frequency selection network, the noise interference can be effectively filtered. It includes capacitor C6, resistor R6, and inductor L4. One end of capacitor C6, one end of resistor R6, and one end of inductor L4 receive the amplitude of the detection signal. The other end of capacitor C6 is connected to the ground. The other end of resistor R6 is connected to one end of grounding capacitor C7. The other end of inductor L4 is respectively connected to one end of inductor L3 and the negative electrode of electrolytic capacitor E4. The other end of inductor L3 is connected to the gate of MOS transistor M1. The drain of MOS transistor M1 is connected to one end of inductor L2. The other end of inductor L2 and one end of grounding capacitor C8 are connected to power supply -5V. The source of MOS transistor M1 is respectively connected to the negative electrode of diode D1, one end of grounding resistor R7, and the normally open contact of switch JP1. The positive electrode of diode D1 is connected to the ground. The normally closed contact of switch JP1 is connected to power supply -5V. The common end of switch JP1 is connected to the adjustable end of potentiometer RW1. When the present invention is specifically used, the front-end digital filter receives the modulation signal of the fiber optic sensor, filters it with the adaptive filtering coefficient of the FIR filter, removes noise, and extracts useful information, and converts it into an analog signal through a DA converter. Among them, the steps for determining the coefficient of the adaptive filtering coefficient are as follows: obtaining the desired frequency characteristic signal, obtaining the input signal; extracting the frequency characteristic components of the input signal; estimating the frequency deviation by the correlation method; estimating the FIR length N according to the frequency deviation, setting N as (1 - n) * 2 / 0.1. To avoid overfitting and underfitting, the FIR length N and the corresponding weights are determined by the frequency deviation and the frequency accuracy, and when the frequency deviation trend becomes smaller, the weight is adjusted forward, and when it becomes larger, it stops, determining the weight adjustment range, and fine-tuning within the weight adjustment range. The number of fine-tuning times is set to 1 / 2 of the frequency accuracy; estimating the corresponding weights of each convolutional layer, initially set as evenly divided weights, that is, (1 / N) * 100%; in step 7, the input at each moment is multiplied by the weight coefficient by a multiplier, and finally the accumulator performs superposition to output the filtered signal; the weight value correction process is improved by the additional momentum method. The specific method is: adding a part of the weight adjustment amount of the previous or several previous times to the weight adjustment amount calculated according to the current error as the actual weight adjustment amount of this time; The backend analog filter receives an analog signal. The intensity of the optical signal is controlled within a certain range by an attenuator, and then the impedance is changed by a tuner to obtain the primary frequency selection of the transformer, selecting a specific frequency range. The secondary of the transformer is inductively tuned, and finally, it is filtered by an adjustable harmonic filter to filter out the detected noise frequency signal, and then filtered by an LC frequency selection network to filter out the signals outside the specific frequency range and output; Adopting the filtering of the adaptive filtering coefficient of the front-end FIR filter in this way, and using attenuation + impedance change to obtain + primary frequency selection of the transformer and secondary inductive tuning of the transformer at the backend, and finally filtering by an adjustable harmonic filter and an LC frequency selection network, the noise interference can be effectively filtered.

Claims

1. An adaptive filter for an optical fiber sensor, comprising a front-end digital filter and a back-end analog filter, characterized in that: The front-end digital filter receives the modulation signal of the optical fiber sensor, adopts the filtering of the adaptive filter coefficient of the FIR filter to remove noise, extract useful information, and converts it into an analog signal through a DA converter, wherein the FIR filter internally includes a FIR controller, an adder, a multiplier, and an accumulator; The back-end analog filter receives the analog signal, controls the intensity of the optical signal within a certain range through the attenuator, then obtains the optical signal through the tuner variable impedance, the transformer primary frequency selection, the transformer secondary induction tuning, and finally filters the optical signal through the adjustable harmonic filter and the LC frequency selection network. The steps of determining the coefficients of the adaptive filter coefficients are: Step 1, obtaining the desired frequency characteristic signal; Step 2, obtaining an input signal; Step 3, extracting the frequency characteristic component of the input signal; Step 4, estimate the frequency deviation; Step 5, estimating the FIR length N according to the frequency deviation; Step 6: Estimate the corresponding weights of each convolutional layer; Step 7: The input at each moment is multiplied by the weight coefficient by the multiplier, and finally the accumulator performs superposition and outputs the filtered signal.

2. The optical fiber sensor adaptive filter according to claim 1, characterized in that: In order to avoid overfitting and underfitting, the FIR length N and the corresponding weight are determined by the frequency deviation and the frequency accuracy. The weight is adjusted positively when the frequency deviation trend becomes smaller, and stopped when it becomes larger. The weight adjustment range is determined, and fine-tuning is performed within the weight adjustment range. The number of fine-tuning is set to 1 / 2 of the frequency accuracy.

3. The optical fiber sensor adaptive filter according to claim 2, characterized in that: It also includes the introduction of momentum factor and the use of additional momentum method to improve the weight value correction process. The specific approach is: superimpose a part of the weight adjustment amount of the last or previous times on the weight adjustment amount calculated according to the current error as the actual weight adjustment amount this time.

4. The optical fiber sensor adaptive filter according to claim 1, characterized in that: The attenuator includes an inductor L1 and a capacitor C1, one end of the inductor L1 and one end of the capacitor C1 are connected to the analog signal, the other end of the inductor L1 is respectively connected to the other end of the capacitor C1, the right end of the diode BD1, and one end of the capacitor C3, the cathode of the diode BD1 is connected to the detection signal amplitude, and the left end of the diode BD1 is connected to one end of the grounded capacitor C2; The tuner comprises a potentiometer RW1, an inductor L, and a capacitor C4, wherein the potentiometer RW1, the inductor L, and the capacitor C4 constitute a variable impedance network, and the inductor L, the capacitor C4, and the primary coil of the transformer T1 constitute a frequency selection network, wherein high frequency is added to the primary coil of the transformer T1 through the capacitor C4, and low frequency is added to the primary coil of the transformer T1 through the inductor L, and the secondary coil of the transformer T1 and the variable capacitance diode BD2 are connected in parallel to constitute an inductive tuning network, the upper end of the potentiometer RW1, the upper end of the inductor L, and one end of the capacitor C4 are connected to the other end of the inductor L1, the other end of the capacitor C4 is connected to one end of the primary coil of the transformer T1, the other end of the primary coil of the transformer T1, the lower end of the potentiometer RW1, and the lower end of the inductor L are connected to the ground, one end of the secondary coil of the transformer T1 is connected to the upper end of the variable capacitance diode BD2, and the other end of the secondary coil of the transformer T1 is connected to the lower end of the variable capacitance diode BD2; The adjustable harmonic filter receives the tuner output signal and realizes the interference noise frequency component in the notch filter tuning signal, including a resistor R4 and an electrolytic capacitor E2, one end of the resistor R4 and the upper end of the electrolytic capacitor E2 are connected to one end of the secondary coil of the transformer T1, the negative electrode of the electrolytic capacitor E2 is respectively connected to the positive electrode of the electrolytic capacitor E3 and one end of the potentiometer RW2, the other end of the resistor R4 is connected to the positive electrode of the electrolytic capacitor E4, the negative electrode of the electrolytic capacitor E3 is respectively connected to the negative electrode of the electrolytic capacitor E4 and the other end of the potentiometer RW2, the adjustable end of the potentiometer RW2 is connected to the ground through the resistor R5, and the adjustable end of the potentiometer RW1 is also connected to the noise frequency signal, and the noise frequency signal is used to control the frequency of the notch filter; The LC frequency selection network is composed of an inductor LN and a variable capacitance diode BD3, and finally outputs a filtered and conditioned optical fiber sensor signal through a capacitor C5.

5. The optical fiber sensor adaptive filter according to claim 4, characterized in that: The variable impedance network is realized by adjusting the resistance value of the potentiometer RW1, which is a voltage-controlled potentiometer and is reversely adjusted by the negative feedback of the amplitude of the detection signal; The negative feedback reverse regulation of the amplitude of the detection signal adopts a reverse excitation circuit; The reverse excitation circuit includes a capacitor C6, a resistor R6, and an inductor L4. One end of the capacitor C6, one end of the resistor R6, and one end of the inductor L4 receive the detection signal amplitude. The other end of the capacitor C6 is connected to the ground, the other end of the resistor R6 is connected to one end of the grounding capacitor C7, the other end of the inductor L4 is respectively connected to one end of the inductor L3 and the negative electrode of the electrolytic capacitor E4, the other end of the inductor L3 is connected to the gate of the MOS tube M1, the drain of the MOS tube M1 is connected to one end of the inductor L2, the other end of the inductor L2 and one end of the grounding capacitor C8 are connected to the power supply -5V, the source of the MOS tube M1 is respectively connected to the negative electrode of the diode D1, one end of the grounding resistor R7, and the normally open contact of the switch JP1, the positive electrode of the diode D1 is connected to the ground, the normally closed contact of the switch JP1 is connected to the power supply -5V, and the common end of the switch JP1 is connected to the adjustable end of the potentiometer RW1.

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