Magnetic field sensors and switches are one of the cornerstones of any residential or commercial security system. These devices are used in door and window sensors, as well as many other applications such as tamper detection. Reed switches are a common design in this end equipment, but the simplicity of the device does not come without pitfalls. This document introduces a magnetic field sensor and switch evaluation platform that can be used to showcase the performance of Hall-effect sensors over the reed switch in building security applications. This document provides an overview of a new platform that can be used to evaluate TI's Hall-effect sensors and switches against the commonly-used Reed switch.
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Door and window sensors make up the backbone of any home security system and are specifically intended to monitor which doors and windows are opened and closed within a home or office. These devices are mostly battery operated and communicate with a main security system hub with information as to whether a door or window is open or closed. If an event occurs in which a door or window gets opened or breached when the alarm is on, the sensor sends an alert signal to the main control panel and immediately triggers the main alarm.
If the internal workings of the door or window sensor are examined, there is one device that is clearly integral to the functionality of this device, a ferromagnetic sensitive device. This can be a simple Reed switch or a Hall-effect sensor, but which is better for your design and why?
The subsequent sections of this article provide an overview of a door and window sensor evaluation platform focused on simultaneous comparison of performance and tamper susceptibility among the Reed switch, the DRV5032, and the TMAG5273. Additionally, tests and their respective results are outlined and compared for each device. To allow for a true comparison between the three sensors, each is co-located on the PCB.
The door and window sensor evaluation platform (Figure 2-1) is built upon the CC1312R platform which allows long-range communication with a building security panel throughout a residential or commercial space. Section 2.1 and Section 2.2 provide an overview of the main platform components and the features associated with this design.
Figure 2-2 shows the top view of the door and window sensor evaluation platform.
The following list describes the labels in Figure 2-2.
On the top side of the evaluation platform there are several points of interest for testing. The main MCU is the CC1312R sub-1G wireless MCU. One of the benefits of using a SimpleLink platform aside from the long-range communication is the Sensor Controller Engine (SCE). This allows the main portion of the processor to remain in a low-power sleep mode, only waking up when the processor receives an alert from the SCE indicating activity. This significantly reduces system power consumption and can help increase battery life expectancy.
For the sensors, the Reed switch is placed on top due to the elongated tube that houses the switch itself. All other sensors are placed on the bottom of the board, in proximity to the Reed switch location. One nice feature this board has is the ability to fully remove power from any sensor that does not need to be active, thus removing any quiescent current associated with simply disabling the device and allowing for accurate system-level power consumption data. This is done with the four position slide switches located on the top of the board. A second switch is also located on the top side which allows the user to select the main power source from either the onboard battery or a secondary off-board power source.
Lastly, there is an RGB LED which can be used for debug, but the LED also gives feedback on magnetic field detection with respect to the Reed switch and the DRV5032. The following list summarizes the LED output for different events:
This color scheme can be changed within the Sysconfig file for the Code Composer Studio™ (CCS) project, or disabled if an LED is not desired for evaluation (Detection indication is also given via serial console).
Figure 2-3 shows the bottom view of the door and window sensor evaluation platform.
The following list describes the labels in Figure 2-3.
Most of the evaluation sensors are located on the bottom side of the board, in close proximity to the Reed switch on the top side for a more relevant comparison of performance. The DRV5032 and the TMAG5273 are located on the side next to each other, which also allows for an improved power performance combinational detection system where the TMAG5273 can be disabled, and woken up only when the DRV5032 detects a field presence. This can significantly reduce power by putting the linear Hall-effect sensor to sleep instead of continuously polling during periods of no activity. An accelerometer is also included to compliment the other sensors with respect to tampering, as well as providing a mounting profile for the placement of the board. This can be used to decipher what a normal approach of a magnetic field looks like, thus increasing the effectiveness of tamper detection of the system.
Figure 2-4 shows a summary of different event detection modes that can be realized with this evaluation platform.