Before we dive into how IR sensors work, let’s first get a quick understanding of what infrared radiation is.
What is infrared radiation?
Infrared (IR) radiation is a type of invisible electromagnetic energy with waveforms longer than visible light but shorter than microwaves, typically ranging from 700nm to 1mm. It is sometimes called thermal radiation because the heat emitted by a person or object can be detected. Warmer objects emit more infrared radiation, which sensors detect as a stronger signal.
IR sensors and IR LEDs: receivers and transmitters
Infrared sensors are electronic devices used to detect infrared radiation. They act as receivers, capturing IR energy and converting it into electrical signals. IR LEDs (infrared light-emitting diodes), on the other hand, emit infrared light. They are transmitters, sending out rays that can be used to sense surroundings, measure heat, or detect motion.
IR sensors and IR LEDs are essential in non-contact, low-power applications. They are found in TV remotes, security systems, thermal imaging devices, and robotics. They are also used as motion detectors, for example, to turn on a light in a dark hallway or trigger an alarm system when an intruder is detected.
How IR sensors work
IR sensors rely on an emitter (IR LED) and a detector (photodiode or photoresistor) to measure infrared light and convert it into electrical signals. There are two main types of sensors:
- Active sensors emit infrared light to detect reflections. The invisible light bounces off an object and returns to a receiver, which measures the proximity or distance of the object. This type of sensor is commonly used in robotics and obstacle detection
- Passive sensors, often called PIR (passive infrared) sensors, detect radiation emitted by objects and do not emit any light. When a warm object moves across the sensor’s field of view, the sensor detects the change in thermal radiation and converts it into an electrical signal. Changes in signal intensity produce voltage changes, which trigger a response, such as turning on an alarm
IR LED technology
An IR LED is a purpose-built LED, typically made from gallium arsenide or aluminium gallium arsenide. The emitted rays are invisible to the human eye but can be captured by IR-sensitive cameras. IR LEDs are used to send signals in systems that rely on wireless communication or to provide the light source for active IR sensors.
The IR sensor detects infrared light, either from heat emitted by a subject or from light emitted and reflected back by an IR LED. IR sensors and IR LEDs are often used together in a range of applications, including motion detection, proximity sensing, temperature measurement, line-following robots, IR communications, night vision, and obstacle detection.
Applications of IR sensors and LEDs
IR technology is low-cost, low-power, and wireless, making it ideal for battery-operated devices and short-range detection. The components are small, supporting miniaturised designs. However, the technology requires a clear path to function effectively. Long distances or physical barriers such as walls reduce performance. Bright light sources, including sunlight, can also interfere with IR sensors, making signal detection more difficult.
From the first IR sensors, designed to study invisible light in the cosmos so astronomers could see objects through dust clouds, to applications in defence for night-time surveillance and target acquisition, IR technology has evolved into a practical tool in everyday life. Today, this low-cost, low-power technology remains widely used, from motion detection and thermal imaging to robotics and communication, proving its enduring relevance amid continuing innovation.
