How Do IR Sensors Work?

What is Infrared Light?

Door sensor
A black optical sensor
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Infrared is light that has a wavelength longer than visible red light. The ranges of infrared include near infrared, mid infrared and far infrared, spanning wavelengths from about 710 nanometers (near infrared) to 100 micrometers (far infrared).

All objects emit light according to their temperature--this is called "black body radiation." The hotter the object, the shorter wavelength of light it emits. The Earth emits infrared light at a peak of about nine to 10 micrometers--and so do warm-blooded animals like humans. This light can be used to detect motion or warmth.

LED IR Detectors

IR (infrared) sensors detect infrared light. The IR light is transformed into an electric current, and this is detected by a voltage or amperage detector.A property of light-emitting diodes (LEDs) is that they produce a certain wavelength of light when an electric current is applied--but they also produce a current when they are subjected to the same wavelength light.

A pair of IR LEDs can be used as motion detectors. The first IR LED is wired to emit LED and the second LED is wired to transmit a signal when it receives an IR input. When an object comes within range of the emitted IR, it reflects the IR back to the receiving LED and produces a signal. This signal can be used to open sliding doors, turn on a light or set off an alarm.

See For Yourself

IR detectors (and emitters) can be found almost everywhere. If you have a computer mouse with a red LED or laser--it is using IR light. Try using this mouse on a damp mouse pad--water almost completely absorbs the IR and the mouse won't work as well.

TV and stereo remotes also use IR signals--the TV has an IR detector that interprets the signal from the remote. Most digital cameras are sensitive to IR light. Turn on your camera and point the TV remote at the camera. Press a button on the remote and you will see a pinkish or purplish light coming out of the remote on the LCD display of the camera. That is the IR signal from the remote.