Design of Camera Lens Control Circuit Based on 89C51

Manual aperture lenses are divided into fixed-focus lenses, manual aperture zoom lenses and three-variable lenses. Among them, fixed-focus lenses and manual aperture lenses need to manually adjust the aperture, focus and other parameters of the lens to achieve lens adjustment, so they are less adaptable to automatic systems. The three-variable lens can adjust the aperture, zoom and focus through the internal motor of the lens, realizing full electrical control of the lens parameters, which is convenient for the automatic control system and remote monitoring to adjust the shooting parameters of the lens according to the actual application needs. To meet specific shooting requirements. This article mainly focuses on this type of lens, and takes Computar's H6Z0812M TVZOOMLENS three-variable lens as an example to discuss the design of its control circuit. The control of this lens is mainly achieved by loading +8V~+12V or -8V~12V power supply on three pairs of control signal lines. These three pairs of control signal lines correspond to the adjustment of aperture, zoom and focus parameters respectively, and the power polarity and existence time of each pair of control signals determine the direction and amount of parameter change. For example, if +12V power is input to the aperture control terminal, the aperture becomes larger, and the longer the power-on time, the larger the aperture opens. Conversely, if -12V power is input, the aperture becomes smaller. The longer the power-on time, the smaller the aperture becomes. The lens control circuit discussed in this article mainly provides control voltage signals with precise pulse width, correct polarity and appropriate amplitude for the three input terminals of the three variable lenses according to the control instructions of the system terminal or computer, so as to realize the complete control of the lens parameters by the system controller.

3. Design of three variable lens control circuit

According to the previous introduction, it can be determined that the control circuit of the three-variable lens needs three steps to complete the control function: 1) communicate with the control computer and receive control instructions; 2) analyze the content of the control instructions and generate basic control signals; 3) control the power circuit to generate the control signal required for lens control. Due to the need to complete the functions of data communication and instruction analysis, this paper selects the 51 series microcontroller 89C51 with a serial communication interface as the core to design the control circuit of the lens. The circuit corresponds to the work of the above three steps and is divided into three parts: serial communication circuit, central control circuit, and execution circuit.

3.1 Serial Communication Circuit

The serial interface of the 89C51 microcontroller uses the TTL level mode, that is, 2.4 V or more represents digital 1, and 0.45 V or less represents digital 0, while the general standard serial communication standard RS232 uses a voltage greater than +2V to represent digital 0 and a voltage less than -2V to represent digital 1. Therefore, the serial communication interface between the 89C51 and the control computer must undergo voltage conversion. The general method is to use a dedicated device (such as MAX232, etc.) to complete this conversion, but an additional set of ±12 V power supplies is required, which is not conducive to the safety of the equipment. In addition, since the circuit only needs to receive serial information, this design uses the circuit shown in Figure 1 to complete the level conversion and realize serial communication.

In addition, the optocoupler OP1 in the circuit is mainly used to isolate and transform the +5 V power supply voltage of the 51 microcontroller and the +12 V drive voltage of the lens action; the transistor T1 is used to control the power supply to the relay S1 coil.

3.3 Central control circuit and software design

The central control circuit is shown in Figure 3. The control core of the lens control module is 89C51. It mainly realizes the three functions of receiving control instructions, parsing control instructions and executing control instructions. The software is written in the assembly language of the 51 series microcontroller. The main advantage of using assembly language is that it has fast execution speed, can accurately grasp the action time, and occupies less memory.

Asynchronous serial communication is used between PC and 89C51. The data bit can be up to 8 bits, which are defined as action type and action time. The first 3 bits of the data bit represent 6 action states, including aperture expansion, aperture reduction, image magnification, image reduction, focal length increase and focal length decrease. The last 5 bits of the data bit represent the action time, which can represent 32 different action times in total. According to the three functions to be implemented by the software, the program is initialized first. The two timers/counters of 89C52 are used for baud rate setting and action time timing respectively. The definition of the working modes of the two timers/counters can be completed by setting the working mode control register TMOD. Timer/counter 1 adopts working mode 2 to define the baud rate. Timer/counter 0 adopts working mode 1 for lens action time control.

Then comes the instruction processing part. The instruction is decomposed into two parts: action type and action time through the "logic and ANL" operation. The action type is screened using the comparison transfer instruction CJNE, and the pin setting is completed by assigning values ​​to R1 and R2 in the working register group:

The pin output is performed in interrupt mode. Since changing the state of the double-pole double-throw switch under power may cause "sparking", to avoid this situation, when assigning values ​​to R1 and R2, the double-pole double-throw relay should be changed first and then powered on. The interval between the two steps is 10ms. The action time is in steps of 10 ms. According to the pre-designed instruction protocol, the action time can be controlled in the range of 0 ms to 320 ms, which can meet the requirements of this module.

4 Conclusion

By improving and debugging the software and hardware of this circuit, the expected application effect was obtained, and the qualitative and quantitative control of the lens was realized. The control characteristic curve of the circuit is shown in Figure 4, where the horizontal axis represents the parameter change step size, in units of 10 ms; the vertical axis represents the number of drive levels required for the maximum parameter change range.

The control circuit has a simple structure, reliable control, strong environmental adaptability, and realizes complete control of shooting parameters by intelligent terminal devices. For example, the terminal can expand the aperture of the camera lens when the average brightness of the image is high, so as to make the image in the local shadow clearer. The specific control method can be customized according to actual needs.