Interpretation of the overall circuit design of the smart Bluetooth virtual oscilloscope system

　　In electrical maintenance work, an oscilloscope is sometimes used to view various voltage waveforms. Ordinary oscilloscopes are large and bulky, and most importantly, they have no electrical isolation. Be careful not to get an electric shock when operating them. With the development of electronic technologies such as single-chip microcomputers and the popularity of smart phones, it is a good idea to design a virtual oscilloscope that can be used with mobile phones. It is low-cost and easy to carry and use.

　　The hardware design circuit includes four parts: signal preprocessing, single-chip microcomputer, Bluetooth serial port module and power supply. The circuit schematic is shown in Figure 1.

　　

　　Figure 1: Bluetooth virtual oscilloscope circuit schematic

　　Signal preprocessing: The voltage range of the AD conversion of the microcontroller is 0~3.3V. In order to measure the AC signal, the input signal ground terminal is connected to the 1.65V adjusted by RP3, so that the voltage of -1.65~+1.65V can be measured. In actual work, electricians measure voltages ranging from a few volts to hundreds of volts. For this reason, a resistor voltage drop network R1, R2, R4, R5, RP1 and RP2 are added, so that a voltage of 60V can be measured. With an oscilloscope probe with a x10 block (10 times magnification), the maximum voltage can be measured is 600V. LM358 is a dual op amp connected in voltage follower mode. Its main function is to increase the input impedance so that the oscilloscope input impedance is 1MΩ. Otherwise, when the oscilloscope probe is placed in the x10 block, the measured value will be biased. The op amp can also limit the signal amplitude input to the microcontroller when the input signal is too large to protect the microcontroller. There are 2 signal input terminals in total, forming a dual-trace oscilloscope, which can compare the phase relationship of the 2-way signal.

　　Microcontroller: Philips' microcontroller P89LPC938 is selected. It is a low-power FLASH microcontroller based on the 80C51 core. It has a 7.373MHz oscillator, reset circuit, 8-channel 10-bit AD converter, 8K bytes of Flash program memory, and 768 bytes of RAM data memory, which makes the peripheral circuit very simple. In order to improve the AD conversion speed and the accuracy of the communication baud rate, an external 12MHz crystal oscillator is used. P2.0 (AD07) and P2.1 (AD06) of the microcontroller are connected to the voltage output of the dual op amp LM358, and P1.0 (TXD) and P1.1 (RXD) of the microcontroller are connected to the serial port of the Bluetooth module.

　　The main parameters of the Bluetooth serial port module: compatible with Bluetooth V2.0 specification, CLASS 2 standard, maximum transmission distance of 10M, support SPP service (serial port). The multi-functional Bluetooth module can change the master-slave settings, the default is the slave device, can be connected to the computer or mobile phone Bluetooth, and can also change the communication baud rate. The default is 9600bps, which can be changed to a larger baud rate, such as 115200bps, to speed up the communication speed. The default pairing password is 1234. Before searching for the Bluetooth module each time, power on the Bluetooth module and search again. When searching again, power on the Bluetooth module first, otherwise it may not be searched. The connection between the mobile phone Bluetooth and the Bluetooth serial port module is the focus and difficulty of program design. This connection process is to create a Bluetooth client, which is divided into 4 main steps: searching for devices, searching for services, obtaining connection URLs, and establishing connections.