How Do IR Diodes & Detectors Work?

Infrared is invisible light just above red in the spectrum.

Infrared (IR) diodes operate by converting electric current into infrared light. Infrared detectors do the opposite; they detect infrared light by turning infrared light into an electric current. The electric current generated by an infrared detector is a signal that indicates infrared light.

Infrared Light

Infrared images can be taken with digital cameras fitted with filters.

Infrared light is electromagnetic radiation similar to visible light, which is measured by wavelength in nanometers (nm). IR is a longer wavelength (above ~720nm) and is invisible to the naked eye. It can be used to form images with common digital sensors such as charge-coupled devices (CCDs) chips found in digital cameras and even cellphones with cameras. Infrared sensors are used to interpret remote television and audio controls. Among their other uses are motion detectors and automatic door openers.

Light-Emitting Diodes

Television remotes use an LED to send an IR signal to televisions.

Light-emitting diodes (LEDs) produce light when they receive an electric current. The energy of electrons in the electric current is converted to light. Different types of LEDs produce different wavelengths of light. Infrared LEDs produce light in the range of infrared. You can "see" infrared light through imaging with a digital camera. If you focus on the transmitter on the front of the remote and press a button, the infrared signal will appear on the digital camera's display screen as a white or pink light.

Infrared Detectors

IR detectors are LEDS wired to produce electric current when exposed to IR light.

Infrared detectors are also LEDs. They are wired differently so that they convert incoming infrared light to an electric current. The current is sent to a device that reads the current to determine the strength of incoming light, or to interpret signals meant to control a television, for example.

Sensitivity

Electronics are manipulated to increase the sensitivity of IR detectors.

IR detectors can be made more sensitive through electronics. Amplifiers make IR detectors extremely sensitive so that even very faint signals are recorded. Sensitivity can be adjusted so a detector is suitable for its anticipated purpose.

Uses

An automatic door opener uses one LED to transmit IR light. A second LED is used as an IR detector. When a person approaches the IR light, the light is reflected off the person back to the detector. The detector sends an electric current to a controller that then opens the door. The sensitivity can be adjusted so that the door opens when a person is at a certain distance from the door. More sensitivity causes the door to be opened when the person is farther away (reflecting less IR back to the detector).

Motion Detectors

Motion detectors can operate without an IR light source. They can sense the infrared generated by an object. In this case, the detector would not have a diode for emitting light, just a diode for receiving IR light. A lens concentrates the IR energy on the detector, and when an object is sensed, an electrical signal is sent out.

References

• Glolab; How Infrared Motion Detector Components Work
• U.S. Department of Energy: Solid-State Lighting--How LEDs Work