ZigBee-Based Wireless Motion Sensors Combine IR with RF

Motion sensing using Infrared (IR) technology is a multimillion-dollar market that today is dominated by small-scale, short-range applications such as lighting control. These applications operate in a closed, indoor environment where it is convenient to use behind-the-wall wiring to connect the sensor to the device being actuated. Motion sensing applications that operate outdoors pose at least one additional challenge: signal transmission between the sensor and the controlled device.

ZigBee (IEEE 802.15.4) is an ideal wireless technology for these applications because it has good range, which, if needed, can be extended by repeaters as part of ZigBee's mesh networking capability. Mesh networking enables ZigBee to be used for simple applications, such as garage door openers, as well as for relatively sophisticated intrusion detection systems in which multiple passive infrared (PIR) motion sensors can communicate with a main security console over a ZigBee network.

The most efficient way to design an outdoor system – or any other system that is not amenable to wired connections – is to use ZigBee modules from companies such as Digi International, RFM, and Panasonic. In the basic design presented in this article, two of Digi International's XBee modules are combined with a 3.3-V voltage regulator from National Semiconductor and a type AMN14111 PIR from Panasonic. All of these products are available on the DigiKey line card.

Digi International's XBee (DigiKey part number XB24-AWI-001-ND) and XBee PRO embedded RF modules support the ZigBee PRO feature set, which makes them interoperable with other manufacturers' ZigBee devices. Programmable versions of the XBee-PRO (DigiKey part number XBP24-AWI-001-ND) module are available for designers with limited RF experience to easily customize ZigBee applications.

Digi International also helps accelerate the design cycle with I/O line passing that uses digital I/O pins (DIO) to pass digital signals from one module to another without additional hardware. These signals are transmitted in addition to any serial UART data that the module sends. Digi International's XBee modules have eight DIO lines for digital data.

The sample circuit has a short bill of materials because it uses only a single control loop connected to a single sensor. In cases that use multi-point intrusion detection systems, for example, additional circuits can be added as more sensors are required. The ZigBee protocol is efficient at eliminating crosstalk and other types of interference. This design also requires a 9-V battery, a transistor and two resistors.

This design focuses primarily on the interaction between the motion sensor and the XBee slave module. A Panasonic AMN14111 PIR (DigiKey part number 255-2901-ND) sensor resides on the same PC board as the module and other components, including built-in antennas for wireless communication with the master module. For most applications, such as a garage door opener, the PIR should have a digital output and a detection range of at least 10 meters.

The Panasonic PIR in this design has digital output and a range of at least 10 meters, along with an operating voltage of 5 V. The difference between the PIR's 5 V operating voltage and the 3.3 V operating voltage of the XBee module is resolved by adding a 3.3 V regulator. In this design, National Semiconductor's LM1117MPX-3.3/NOPB (DigiKey part number LM1117MPX-3.3CT-ND) linear LDO voltage regulator was selected. The LM1117 also is available in an adjustable version, which can set the output voltage from 1.25 to 13.8 V with only two external resistors. A transistor pulls the digital input of the ZigBee module to ground when the PIR sensor detects motion, applying the 3.3 V supply voltage to the ZigBee module using a 2.2 kΩ pull-up resistor. Power is provided by the 9-V battery.

The I/O line passing feature mentioned earlier is employed in the communication between the master and slave modules. A digital input signal on the DIO1 pin (pin 19) of the XBee slave module can drive a digital output signal (DIO1) of the XBee master module. Similarly, an analog input signal on AD0 of the slave module (pin 20) can control a PWM output signal of the master XBee module.

XBee OEM modules are smaller than 3 x 1.5 x 0.5 inches high and have through-hole connections to PC boards. Even after adding the other components, the sensing module is still quite compact and can be built into a small water-resistant case for outdoor use.

The complete PIR sensor/transmitter module can be placed anywhere within a range of 30 meters from the receiver when an XBee module is used. For longer range applications, XBee Pro modules should be selected.

The modules are programmed using Digi International's X-CTU software, which operates only on Windows platforms but allows designers to do all of the necessary test and configuration tasks including loopback range testing. It also provides important performance data, such as received signal strength indication (RSSI).

Digi International also offers several development kits ranging from its XB24-DKS starter kit (DigiKey part number XB24-DKS-ND) to its highest end XB24-PDK kit (DigiKey part number XB24-PDK-ND).

Other companies supply ZigBee modules designed for outdoor sensing applications. RFM's ZMN2430HPA-R (DigiKey part number 583-1129-ND), for example, is a low-cost solution for point-to-point, point-to-multipoint, and mesh wireless systems appropriate for applications ranging from cable replacements to sensor networks. Panasonic's ENW-C9A08A3EF (DigiKey part number P13683TR-ND) includes sensor applications in its specification sheet and offers several firmware options. Modules can be configured from simple point-to-point proprietary devices to complex mesh networks.