An IR photodiode is an electrical component that converts light into electricity. This type of component is specifically designed to respond to light in the infrared range. Communications, sensors, and security are three applications that often use them.
All photodiodes use the photoelectric effect to generate electrical current from light. When photons of light hit the surface of a diode, electrons are "knocked" out of place. A charge occurs when the electron is moved. Continuous exposure of a photodiode to light will create a constant flow of electricity. The amount of current corresponds to the amount of light hitting the surface of the diode.
Standard photodiodes create electricity in response to visible light. An IR photodiode uses an infrared filter to block visible light, and allow only IR wavelengths of light to pass. This reduces the level of interference from background light, and ensures that current is generated only when infrared light is present. An IR photodiode is technically able to generate continuous electricity by being exposed to infrared light.
Most of these components, however, are not used to generate power but rather to sense when an infrared signal is received. This is useful for remote communication. Televisions, for instance, often use infrared photodiodes to detect when a button is pressed on a remote control. The IR filter helps ensure that sunlight does not interfere with this control. IR photodiodes can also be paired with infrared lasers. This allows communication signals to sent though the air and be detected over very long distances.
IR photodiodes are also useful as sensors in robotics and security applications. Mobile robots often combine a photodiode with an infrared light emitting diode (LED). Infrared light from the LED is invisible to the human eye, but reflects off of solid objects such as walls. When a robot approaches an object, the IR photodiode detects the reflection of infrared light. This allows the robot to stop before hitting a wall or other object.
Security systems use these components to detect intruders. In this application, the invisible nature of infrared light can be a significant advantage. An IR light source can be placed on one side of a doorway or room, with a IR photodiode located on the opposite side. The diode is able to detect the constant beam of infrared light, while intruders cannot see the sensor. If somebody enters the room and breaks the IR beam, the alarm is activated.