Light emitting diode(LED) sources are massively distributed in the computers, display screens and other industrial electronics terminal devices, which not only indicate device status, but can also be used for information transmission. This paper takes a hard drive disk LED indicator embedded on a personal computer as example for statistical analysis. However, due to the fact that we cannot skip the operating system to control the indicators directly, the problem of clock drift is particularly serious. In response, we put forward our scheme of pulse width long-short-key modulation instead of traditional on-off-keying (OOK) modulation, which is optimized by simulation and verified by hardware experiment subsequently. As is indicated in the experiment, our modulation is superior to traditional OOK scheme at the same average data rate. Based on the analysis above, programming interfaces of sending and receiving terminals are designed to transmit real-time messages, files, or voice signal.