Infrared LED

An infrared LED is a light-emitting diode whose light lies just beyond red, in the infrared, so we can't see it. Each time current passes through it, it gives off a burst of infrared, which is how a remote sends its commands.

The light comes from a tiny chip of gallium arsenide, a semiconductor. Layers with slightly different impurities are grown on a gallium arsenide wafer, forming a junction where electrons fall into holes and release their energy as light, usually at a wavelength around 940 nanometers. Metal contacts are added and the wafer is sawn into chips smaller than a grain of sand. Each chip is glued into a little reflector cup on a lead frame, and a gold bond wire links its top to the other leg. Then the whole thing is cast in clear epoxy resin, molded into a domed lens that aims the light forward.

In a remote control, the LED flashes in a coded pattern for each button. You can watch it work: many phone cameras pick up infrared, so the LED glows faintly on screen when a button is pressed. Infrared LEDs were first developed in the early 1960s.