Abstract: To provide a power supply for a test instrument operating at high temperature environment, a scheme of using a photodiode as the switch of the power supply was developed, which had the advantages of short response time, high sensitivity and so on. The photodiode received the infrared ray emitted by the high-temperature emitter, the power was switched on after optical-electric conversion, and the desired results were achieved. This paper introduced the concept of the optically controlled switch and its implementation. The simplicity and feasibility of the design was verified by experiment results, and it is suitable for some operations not accessible by human being.