Introduction to related knowledge of anemometer
The illuminance meter is a commonly used light source illuminance measurement instrument in the field of photoelectric testing. Most of the mature products on the market today have a single function, which can only meet the needs of a single illuminance test, and cannot be applied to the occasion of real-time monitoring and coordinated adjustment of the illuminance of multiple light sources. For this kind of application, a portable low-power illuminance meter is introduced in this paper. This illuminance meter can not only complete the illuminance test under the condition of a single machine, but also form a test network through wireless communication to monitor the illuminance information of multiple light sources in real time. The test data is transmitted to the central control in real time
The computer provides accurate data for real-time illuminance adjustment. At the same time, the instrument also has a self-calibration function, which can easily adjust the calibration parameters, improve the measurement accuracy, and has high test accuracy and stable working status.
features.
1 Hardware Design
The main function of the illuminance meter is to test the illuminance of the light source and display the test results. In the networking test mode, the lux meter also needs to transmit the test results to the main control computer through the wireless network. To meet the above functional requirements, the hardware of the equipment is composed of sensors, signal processing circuits, AD conversion circuits, core controllers, human-computer interaction interfaces, communication interfaces and other modules, hardware composition and data flow.
The illuminance sensor of the illuminance meter uses a silicon photocell, which is a photoelectric conversion device with high sensitivity, which can linearly convert the luminance information into a current signal, and the illuminance information can be calculated by detecting the current value. The main function of the signal processing circuit is to convert the current signal output by the sensor into a voltage signal that can be recognized by the AD conversion circuit
Since the output of the silicon photovoltaic cell is a weak current signal, a large measurement error will be introduced by the direct conversion method of parallel resistors. Therefore, the design scheme of using an operational amplifier to build an I-V conversion circuit is selected in the design. The circuit schematic diagram is shown in the figure. Show.
In practical applications, the relevant indicators of the operational amplifier will also affect the test results, so it is very important to select a suitable operational amplifier. In this design, the AD8571 operational amplifier is selected. This operational amplifier has the advantages of single-ended power supply, high gain, and bias current as low as 20pA. It is mainly used in the field of precision current measurement and fully complies with the illuminance meter needs.
The AD conversion circuit converts the analog signal output from the signal processing circuit into a digital signal. In order to ensure the resolution and test accuracy of the illuminance meter, and take into account the low power consumption, the AD conversion chip AD7472 with 12-bit resolution is used in the design. The signal-to-noise ratio under input conditions is as high as 70dB, which fully meets the needs of use, and its current is only 50nA in sleep mode, which meets the
Low power consumption design concept, its reference voltage is provided by AD78o standard 2.5V.
The keyboard and liquid crystal display module serve as the human-computer interaction interface to provide users with convenient operation methods and status display. Users can use the buttons to complete the startup, standby, and test mode settings of the illuminance meter, and obtain real-time testing of the illuminance meter through the liquid crystal display module. Results and job status information.
