With the continuous development of electronic technology, the functions of various instruments and meters have gradually become intelligent. For example, intelligent digital display counters use the single-chip microcomputer for counting, display and control processing. Therefore, the performance requirements for the power supply are also higher. Compared with low-frequency transformers, switching power supplies have many advantages. The switching power supply can be directly mounted on the control board, with high efficiency, high DC voltage accuracy, small size, reliable performance, eliminating installation troubles and making the product lighter. Power scheme: 220V mains enters the instrument. It is converted into DC12V to the control relay through the AC-DC power module. The external sensor is powered, and then regulated to DC5V by 78M05 to supply power to the MCU and the display.