Stealth-mode LED controls itself

摘要

Since the LED's invention more than 30 years ago, its emission efficiency has steadily increased, and, although it may surprise you, the increased conversion efficiency works in two directions. Certain bright, efficient LEDs, such as Hewlett-Packard's (www.hp.com) HLMP-EG30-NROOO, a red emitter molded in clear encapsulation, also exhibit significant photovoltaic action. The circuit in Figure 1 shows how you can put an LED's photovoltaic characteristics to work. Using the same components, older, red LEDs also function but with lower light output in this circuit. This Design Idea circuit describes an LED that controls itself by determining whether it's on or off without the assistance of any light sensor other than its own characteristics. When you darken the LED, it turns on, and, when you illuminate it, it turns off. The circuit's main components comprise LED D{sub}1 micropower operational amplifier IC{sub}1, one-shot IC{sub}(2A), and transistor switch Q{sub}1 to control current through the LED.