Infrared sensors are more common than most people realize. Learn about the differences between active and passive infrared sensors and their applications.
Whether you know it or not, you’ve probably used an infrared (IR) sensor. Most of us have changed the TV channel with a remote control that emits (IR) light, and many of us have walked through security sensors that detect movement via IR radiation. Manufacturers use IR sensors extensively, and you’ve probably seen them at work on automated garage doors. In a standard IR sensor, an emitter shoots invisible light at a receiver some distance away. If the receiver does not pick up the signal, the sensor indicates that an object is in its path.
Passive IR Sensors
Fig 2: PIR sensor hooked up to an LED to indicate operation. Note that we removed the lens assembly in the second image.
What Does PIR Mean?
A passive infrared (PIR) sensor recognizes infrared light emitted from nearby objects. You may assume that “passive” IR sensors mean these devices are less complicated than their active counterparts, but you’d be mistaken. A passive IR sensor’s functionality may be more difficult to understand.
PIR Sensor Functions
First, realize that everything — humans, animals, even inanimate objects — emit a certain amount of IR radiation. How much IR radiation they emit relates to the body or object’s warmth and material makeup. Humans can’t see IR, but we’ve designed electronic detection devices to pick up these signals. PIR sensors are used in thermal sensing applications, such as security and motion detection. They are commonly used in security alarms, motion detection alarms, and automatic lighting applications.
How Do PIR Sensors Work?
Passive infrared (PIR) sensors use a pair of pyroelectric sensors to detect heat energy in the surrounding environment. These two sensors sit beside each other, and when the signal differential between the two sensors changes (if a person enters the room, for example), the sensor will engage. That may mean it triggers an alarm, notifies authorities, or maybe turns on a floodlight. IR radiation focuses on each of the two pyroelectric sensors using a series of lenses constructed as the sensor’s housing. These lenses widen the device’s sensing area.
While the lens setup and sensor electronics are sophisticated technology, these units are easy to use in a practical application. You only need power and ground for the sensor to produce a discreet output that’s strong enough for a microcontroller to use. Typical adjustments include adding potentiometers for sensitivity and tweaking how long a PIR stays engaged once it’s triggered. You can also toggle the sensor between:
- Staying on for a set amount of time after detecting movement.
- Pulsing on and off in a “non-retriggering” mode.
You’ll commonly see PIR sensors in security alarms and automatic lighting setups. These applications don’t require that the sensor detect an object’s specific location, just that objects or people in a particular area are moving around.
Active IR Sensors
Fig 3: An IR receiver detects a reflected signal from a hand to turn off the LED.
While PIR sensors are excellent if you want to detect general movement, they don’t give you any more information on your subject. To know more, you’ll need an active IR sensor. Setting up an active IR sensor requires both an emitter and receiver, but this sensing method is simpler than its passive counterpart. Here’s how an active IR operates on a basic level:
- An IR emitter shoots out a beam of light, facing an in-line receiver.
- If nothing is in the way, the receiver sees a signal.
- If the receiver fails to see an IR beam, it detects that an object is between the emitter and the receiver, and therefore present in the monitored area.
One variation of the standard active IR sensor uses an emitter and receiver facing the same direction. The two sit very close to each other so the receiver can detect an object’s reflection when it enters an area. Here’s another twist: a fixed reflector bounces the signal back. This method replicates the setup of separate emitter and receiver units, but without the need to install a remote electrical component. Each method has its advantages and disadvantages based on the material the sensor will be detecting and other specific circumstances.
Fig 4: An opposed emitter and receiver detect an object (paper) between them.
Active IR sensing is very common in industrial settings. In these applications, an emitter and receiver pair can accurately note whether an object is, for example, in a certain position on a conveyor. You can also find active IR technology in garage door safety sensors, which prevent injury or mechanical failure due to obstructions in a door’s path. Whatever your application, there is a multitude of infrared sensing units available in passive and active configurations to suit your needs.
