PIR sensors offer design solutions

Passive infra-red (PIR) sensors are simple, cost effective motion detectors that can be mass-produced for a wide range of applications.

Motion sensors, used to detect moving objects, employ a variety of physical effects of a mechanical, electrical, magnetic, acoustic and optical nature.

Whereas traffic systems use expensive radar and infra-red measurement technology, for speed monitoring for example, it is mainly ultrasound and passive infra-red sensors that are used in consumer and domestic electronics.

Passive infra-red (PIR) sensors are simple, cost effective motion detectors that can be mass-produced for a wide range of applications.

PIR motion sensors use electromagnetic radiation in the low infra-red spectrum in which radiation emitted by human movement falls and is identified using a PIR detector.

The structure of the device can be illustrated as follows: objects that are located within the area of sensitivity and generate electromagnetic radiation in the low infra-red range project images through the focus line of the sensor on to the internal heat-sensitive element.

The signal processing electronics convert the images in to electrical signals.

If the heat image remains the same, the output signal of the sensor remains unchanged.

If the heat image changes, because the object moves within the area of sensitivity, the optical imaging changes accordingly.

This movement results in a change of the output signal that in turn can be evaluated by a window comparator that specifies upper and lower thresholds.

Matsushita manufactures a PIR device called the MP Sensor and has found that lighting controls companies are frequent customers.

Some recent applications include lighting control in pub and restaurant bar drinks fridges that have glass doors to display bottles and cans.

In these cases the refrigeration needs to take place constantly but the interior light does not always need to be on.

However, typically in this type of fridge the light does not go off when you shut the door.

To overcome this the sensor is mounted within its front edge and when the bar staff walk past it at regular intervals the light stays on.

However, once the bar has closed the light will turn off after a preset time.

This saves power both in lighting and in the refrigeration not having to deal with the excess heat given off by the lamps when the bar is closed.

The main advantage for the designer is the MP Sensors' simplicity of integration within the circuit.

With many PIR sensors there are separate components for the sensor, the amplifier and the comparator; and with individual components there is the requirement for circuit design, matching of components and mounting on to a PCB.

The MP Sensor has all these circuits enclosed in one device.

This leaves the designer the task of supplying it with power and connecting the output to a suitable driver circuit that could be anything from a microcontroller input to a transistor so that it can be used to drive a relay or similar device.

Beside there being only one component to place there is another advantage to the circuit designer that may not be immediately apparent.

This is EMI withstand.

As all the components are enclosed in one shielded housing in a TO-5 style case the MP Sensor is significantly more tolerant to external interference from such sources as mobile phones.

The greater shielding helps prevent false triggering and misoperation of the control circuit such as turning lights off in rooms while people are still present.

The MP Sensor is often chosen over other devices for its ease of use and concealment the entertainment industry is no exception.

A large museum was concerned with the amount of energy wasted in keeping display cases and their contents illuminated when no patrons were about.

So the MP Sensor was discretely mounted into the display case frame and only brings the exhibit into life when a visitor approaches.

Another use within this field has been to activate and track patrons along a series of plasma video displays to continue the information exchange while continuing along an exhibit.

This particular application used a black spot type MP Sensor that was visible only as a black disk within the black screen bezel.

Due to high performance and low cost, the MP Sensor is well suited to energy and security management applications like: access control - door intercom systems; intelligent light management - switching lights on and off in corridors, garages, stairwells, display windows and exhibition rooms; automatic control of air conditioning and ventilation systems; and control of lighting in and around ATMs, vending machines and public telephones.

The advantages of this type of PIR sensor lies particularly in their energy-saving functions, small size, ability to be mass-produced at low cost and their imperviousness to interference from electromagnetic fields.

The MP Sensor has proven to be first choice among electronics designers due to the ease of circuit integration and its stable operation.

In addition, mechanical and product designers have enjoyed the benefits of small size of both the sensor and the associated electronics.

Matsushita reckons there is no better choice on the market for discrete PIR implementation.

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