Using a PIR Sensor in a Motion Detection System Design

By Lars Thornqvist, FAE, Future Electronics Sweden

READ THIS TO LEARN ABOUT Operation of multi-element PIR sensors Design of motion sensing systems Selection of Cypress PSoC devices for motion detection applications

The Pyro-electric Infra-Red (PIR) sensor is an extremely useful device for detecting the presence of a moving body. This is due to its ability to sense the infrared radiation that every living body emits.

Part of the appeal of the PIR¹ sensor is its ability to reliably distinguish moving bodies from other objects, as well as from stationary bodies. Its basic mode of operation is to detect the difference in heat signature between two ‘segments’ in its field of view. The model of the internal structure of a PIR sensor (see Figure 1) clearly illustrates this operating principle.

 

 

To avoid triggering upon sensing normal temperature variations or disturbances in airflow, a dual-element PIR sensor connects two elements in pairs. These are inverted with respect to each other in terms of polarization. When the two inverted elements are exposed to the same infrared radiation level, they cancel each other out, generating a zero-output signal as a result.

This means that the detected body will have to move into or between the two elements’ field of view to cause the sensor to generate an output signal. In this way, a dual- or quad-element sensor is able to reject false detections accurately and effectively.

By using a dual- or quad-element sensor, it is also possible to detect the direction in which the object is moving. The quad-element sensor in Figure 1 has two outputs: this means that it can indicate in which area the movement is occurring (for instance, on the ceiling or on the floor), and at the same time whether the object is moving in the horizontal or vertical plane. The dual sensor can only indicate movement in one axis; for example, the horizontal plane.

When specifying a sensor, the number of elements is only the first consideration.

Other important parameters that vary between sensors are frequency response, which determines the sensor’s ability to detect low and high speed movements, the angle of the field of view, which will affect the size of the sensor’s coverage area and immunity to RF and background noise.

 

Configuring the Lens

The sensor itself is inefficient if it does not have a lens to focus the radiation. The most commonly used lens type is the Fresnel lens, due to its low losses and small form factor. A Fresnel lens is a compressed plano-convex lens that comprises a set of discontinuous surfaces (see Figure 3).

The grooves on the lens are arranged facing the PIR sensor. This leaves a flat, dust and weather proof surface facing the outside and protecting the otherwise vulnerable sensor.

 

 

The Signal Conditioning Circuit

To be usable, the signal from the sensor has to be amplified and then converted into a digital value for further analysis in software. A typical block schematic for this application is shown in Figure 4.

There are several ways to design a circuit to realize this schematic. The two preferred approaches use either discrete analog components, or, in a more integrated implementation, a mixed-signal programmable array such as a Cypress Semiconductor Programmable System-on-Chip (PSoC) device.

 

 

The Discrete Solution

The most common approach to PIR sensor signal conditioning is to design the amplifier and signal conditioning stage by using discrete components such as operational amplifiers, comparators, diodes, resistors and capacitors. Next in the signal path, a microcontroller with an integrated ADC performs signal identification and also supports connection to a communications interface, such as a radio.

This traditional approach occupies a large PCB footprint. But a PIR sensor produces a very low signal level, so it is essential to keep PCB traces short and the design compact to avoid creating unwanted antennas. These can pick up background noise and RF signals, which can cause the device to trigger falsely. If the PIR sensor is connected to a wireless network (for instance, as part of an intruder alarm system), the danger of this is particularly high.

 

The PSoC Solution

A second approach, which produces a more compact result, is to use a PSoC device from Cypress. The PSoC is a programmable mixed-signal controller with an 8-bit core and a set of analog and digital blocks that can be used to create the functionality needed.

Analog blocks that can be realized in the PSoC include ADCs, DACs, filters, amplifiers and comparators. Integrated digital functions include timers, counters, UARTs, SPI and PWMs. Designers using a PSoC will be able to realize the blocks shown in Figure 4 with far fewer components than in the discrete implementation described above.

Different devices in the PSoC family provide different numbers and types of digital and analog blocks, offer different memory sizes and use different packages. The first step in implementing a PIR sensor with a PSoC device should be to identify the required analog and digital functionality. When the block diagram is defined, an appropriate device with the right number of programmable blocks can be selected.

 

 

Conclusion: Why Use a PIR Sensor

While a number of technologies for motion detection exist, including ultrasonic and microwave radiation sensors, the PIR sensor is popular for its ease-of-setup and high performance. In addition, PIR sensors are inexpensive and draw little power.

Future Electronics expects the rate of adoption of PIR sensors to grow fast, with applications such as surveillance and alarm systems, as well as power saving devices, driving increased usage.

Future Electronics, a broad line component distributor, can supply every component required in a PIR sensor system, including PIR sensors, lenses, discrete components, microcontrollers and PSoC devices. Future Electronics’ FAEs can provide customers with technical support in the implementation of PIR sensor system designs.

 

1 A PIR sensor should not be confused with the thermopile sensor (eg the Melexis MLX90247), the output of which is proportional to static radiation. Thermopiles are more commonly used in temperature measurement devices.