Franck-Hertz based on P89LPC932 MCU

1 Introduction

The Franck-Hertz experiment is a very important experiment in modern physics. Under certain conditions (mainly a certain first grid voltage UG1K, blocking voltage UG2P, and filament voltage UF), the existence of atomic energy levels is confirmed based on the energy change characteristics of electrons in collisions by studying the relationship between the anode current IP and the second grid voltage UG2P. Therefore, the key point in the Franck-Hertz experiment instrument is to control the magnitude of the UG1K, UG2P, and UF voltages and generate a variable voltage UG2K and measure the magnitude of the current IP and voltage UG2P. In early instruments, mechanical potentiometers were used to adjust the UG1K, UG2K, UG2P, and UF voltages, which could only be operated manually; and ordinary operational amplifiers were used for weak current amplification, which had poor stability. In order to improve the intelligence and stability of the instrument, a design scheme for a Franck-Hertz experiment instrument based on the P89LPC932 microcontroller is proposed here.

2 Overall design of the instrument

This design is based on the single-chip microcomputer P89LPC932. The A/D converter uses the TLC2543 with 12 bits, 11 input channels and SPI serial interface. The IP current (10-9~10-7A) amplified by the amplifier circuit composed of the high-performance operational amplifier OPA128 and the UG1K, UG2K, UG2P, and UF voltages divided by resistors are measured. The measurement results are displayed by two identical 4-digit 7-segment LEDs respectively. At the same time, they are saved in the 64 K-byte E2PROM device CAT24C256. Under the control of the SPI interface of the single-chip microcomputer, the digital potentiometer MAX5481 generates a 0~100 V variable UG2K voltage with a resolution of about 0.1 V through the circuit. Under the control of the I2C interface of the single-chip microcomputer, the digital potentiometer DS1844 generates 3-way 64-step variable voltages UG1K, UG2P, and UF through the circuit. The USB-serial converter PL2303H connects the instrument to the computer through a pseudo USB interface for data transmission. Figure 1 is the overall block diagram of the instrument.

3 Basic Circuit

The core of the instrument is the P89LPC932 microcontroller, which has rich interfaces, including 768 bytes of RAM data memory, 8 KB erasable Flash program memory, and supports ISP download; 400 kHz byte mode I2C communication port, SPI communication port and enhanced UART serial port, which makes it easy to connect and use devices with I2C and SPI interfaces. Its internal 512 bytes of E2ROM can be used to save the initial state of the power-on and the safe working conditions of the FH tube (Frank-Hertz tube filled with argon).

3.1 A/D conversion circuit

The A/D conversion circuit uses TLC2543, which has 11 input terminals, 12-bit resolution, 10μs conversion time, internal sample and hold and clock circuit, and uses SPI interface. The single-chip microcomputer P89LPC932 is connected to it through 4 pins of the SPI interface to control the writing and conversion operation of its control word. The control word specifies the analog channel to be converted by TLC2543, the length of the converted output data, and the format of the output data. The conversion result is read out by the single-chip microcomputer through the SPI interface. The UG1K, UG2K, UG2P, and UF voltages are divided by resistors and the Ip current is amplified and added to the 5 input terminals of TLC2543 for measurement.

3.2 UG2K voltage generation circuit

In the early design, the Franck-Hertz experiment instrument used a mechanical potentiometer to manually adjust the voltage to obtain the change. In order to reduce the design workload and make full use of the mature analog circuit part of the instrument, this design uses a digital potentiometer instead of a mechanical potentiometer.

The digital potentiometer MAX5481 is a 10-bit (1 024-step) non-volatile, linearly variable, programmable voltage divider (its two fixed-end resistors are 10 kΩ), which realizes the function of a mechanical potentiometer and uses an SPI interface. In terms of hardware, the microcontroller P89LPC932 is connected to the device through four pins of the SPI interface; in terms of software, the increase, decrease and setting of the potentiometer are controlled by writing control words. Since the load capacity of the MAX5481 is limited (the current range on the resistor is from hundreds of microamperes to milliamperes), an amplifier circuit is generally required to expand its load capacity.

3.3 UG1K, UG2P, UF voltage generation circuit

The digital potentiometer DS1844 is used to generate UG1K, UG2P, and UF voltages. The device contains 4 independent, 6-bit (64-step) linearly variable, programmable voltage dividers and uses an I2C interface. In terms of hardware, the microcontroller P89LPC932 is connected to it through two pins of the I2C interface; in terms of software, the increase, decrease, and setting of the potentiometer are controlled by writing control words. Due to the limited load capacity of DSl844, an amplifier circuit is also required to expand its load capacity. Figure 2 shows the UG1K voltage generation circuit. VT4 is connected to a common collector amplifier circuit, which can output a 64-step voltage UG1K. Similarly, the other two voltage dividers of DS1844 and the same drive circuit are used to output 64-step UG2P and UF voltages.

3.4 IP current amplifier circuit

The current IP generated in the FH tube is very small (10-9～10-7A), and an amplifier circuit composed of a high-performance op amp OPA128 is designed to amplify the current, as shown in Figure 3. This amplifier circuit is the key to the stability of the instrument performance, and it is necessary to select an operational amplifier with a small bias current. OPA128 is selected here because it uses an "electrometer-level" op amp with FET input, an input bias current of no more than 75 fA, a maximum offset voltage of 500μV, a maximum offset voltage drift of 5μV/℃, and an input impedance of 1013Ω. This circuit can amplify the current of 10-9A and convert it into a voltage output of 1 V, which is sent to the conversion A/D for conversion and measurement.

3.5 Other circuits

In the manual operation mode, the results of the instrument measurement need to be displayed directly. Two identical current and voltage display modules are designed here. These two modules use 4 74LS164 devices and 4 7-segment LED digital tubes to form a static display. Their connection with P89LPC932 is relatively simple. Only 4 I/O interfaces are needed to simulate the serial port 0 mode for display drive. In the automatic operation mode, it is necessary to collect data first and then play back the results. For this purpose, the CAT24C256 E2ROM device with I2C interface and 64 K bytes is used, which can be directly connected to the 2 pins of the I2C interface of the single-chip microcomputer P89LPC932. In the online operation mode with the computer, considering the convenience of the USB interface, the USB serial port conversion device PL2303H is selected, so that the instrument can be connected to the computer through the pseudo USB interface to transmit data. PL2303H only needs to be connected to the TXD and RXD pins of the UART of the single-chip microcomputer P89LPC932, and read and write the UART-related registers. The computer driver is provided by the manufacturer, which is actually mapped to an RS232 interface programming. Thereby simplifying the programming design of the host computer.

4 Conclusion

Since the instrument is designed with P89LPC932, TLC2543, OPA128, MAX5481, DS1844 and PL2303H, the stability of the instrument is guaranteed. This design takes simplicity and practicality as the starting point, and reasonably configures the components, thereby improving the overall performance of the instrument and simplifying the circuit design and debugging.