Pick up your TV remote and click a few buttons. Even though your eyes see nothing, the TV remote shines light across the room to a small sensor on the front of the television. This sensor reacts to the light from the remote and triggers circuits that raise the volume or change the channel. Infrared detectors are found in a variety of devices from motion detectors to video games and night vision scopes.
Just as sound increases in pitch from low bass to high whistles, light goes from cool blue to warm red. As a sound continues to rise in pitch, it becomes harder to hear. At some point, you no longer hear it, but dogs, with more sensitive hearing, continue to respond. Light works in the same way. Your eyes see the visible spectrum from blue to red, but additional wavelengths continue to shine both below and above the visible spectrum. Infrared, the light warmer than red, can be detected by electronic sensors including most cell phone cameras. Turn on your cell phone camera and point it at a TV remote and see if your camera lights up when your friend clicks the remote.
Electronic devices react to light either by generating electricity or by varying their ability to conduct current. Certain diodes -- devices that act like one-way electrical valves -- generate electrical current when exposed to light. In the same way, some transistors react to light and use this input instead of voltage to control the flow of current through the device. Some sensors act as simple power switches while others can measure levels of brightness. Many sensors can be etched onto a single chip surface to form the light receptors used in digital cameras. Sensors can also be controlled to only respond to infrared light, either by altering chemistry or by using filters.
Although Bluetooth and WI-FI have replaced infrared communications in many computer devices, remote controllers and video games still rely heavily on infrared communications. The most common of these devices is the TV remote, which converts each button press on the remote into a distinct signal transmitted using an infrared light-emitting diode. The IR sensor on the TV receives the signal, interprets the code sequence, then switches circuits within the TV to perform the requested function.
Seeing in the Dark
Just as fire emits visible light, warm surfaces like buildings, roads and people emit some infrared energy. Arrays of IR detectors can act as cameras to capture infrared radiation and convert this signal into visible images, allowing people to see in the dark. In addition to night vision, IR cameras locate energy leaks in buildings, monitor materials during manufacture and are used in satellites to study earth geology. IR detectors and cameras also have many applications for security and law enforcement including detecting motion in and around buildings, monitoring low-light video and detecting weapons hidden under clothing.
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