Capacitive sensor measurement system module circuit design precision amplifier circuit

　　Based on the special environment of offshore oil spill recovery, this paper designs an offshore liquid level measurement system based on AT89S51 single-chip microcomputer. The system adopts the principle of segmented capacitance detection to realize the detection of oil and water dual liquid levels. The hardware circuit of micro capacitance measurement is reasonably constructed. The system can be accurately applied to the measurement of offshore oil level.

　　Square wave generating circuit

　　Figure 4 Square wave generating circuit

　　The multivibrator composed of NE555 is the square wave signal source measured by the diode AC bridge, and its specific circuit is shown in Figure 4. The frequency calculation is shown in Equations (4), (5) and (6).

　　(4)

　　(5)

　　(6)

　　Let , , , then: . Therefore, NE555 can provide a square wave signal source with a voltage amplitude of 5V and a frequency of 500kHz for the capacitance detection circuit.

　　Double T diode AC bridge capacitance measurement circuit

　　The capacitive sensor converts the liquid level into capacitance, and then converts it into a voltage, current or frequency signal that is a linear function of the capacitance through the capacitance measurement circuit. Since the capacitance is only a few PF, a double-T diode AC bridge micro-capacitance detection circuit is used to convert the capacitance change into a voltage output. The measurement principle is shown in Figure 5.

　　Figure 5 Diode T-type network

　　In FIG5 , e is a high-frequency power supply, and D1 and D2 are two diodes with exactly the same characteristics.

　　Precision amplifier circuit

　　Convert 0-0.5mV voltage to 0-5V for A/D conversion. Use OP07 to form a two-stage integrated operational amplifier circuit, the first stage amplification factor is 20, the second stage amplification factor is 50, and its circuit structure is shown in Figure 6.

　　Figure 6 Precision amplifier circuit

　　Single chip microcomputer and A/D interface circuit

　　An A/D converter is needed between the sensor and the microcontroller to convert the input analog voltage signal into a binary digital signal. This article uses the successive comparison A/D converter ADC0809. The specific interface circuit with the microcontroller is shown in Figure 7.

　　Figure 7 ADC0809 and MCU interface circuit

　　In Figure 7, the single-chip microcomputer controls the address selection ports A, B, and C of ADC0809 through P1.0, P1.1, and P1.2. The single-chip microcomputer ALE port outputs a 2MHz clock frequency signal, which is passed through the 74LS74 four-frequency division circuit to generate the 500kHz clock signal required by ADC0809. P3.4 is used as a chip select signal. When it is low, the NOR gate is opened to allow the write signal to pass through and select a channel of ADC0809 for A/D conversion. The conversion end flag triggers external interrupt 0 through the NOR gate to read the conversion data.

　　 Digital tube display circuit

　　A three-digit LED digital tube is used to display the liquid level data. 74HC245 is the display driver chip, its input is connected to the P0 port, and its output is connected to the eight pins of the digital tube. The three-segment digital tube is controlled by P2.0, P2.1, and P2.2 respectively. The display circuit of the AT89S51 microcontroller and the digital tube is shown in Figure 8.

　　Figure 8 Digital tube display circuit