Bench Talk

(Source: Andrey Popov - stock.adobe.com)

Thermal Metal Oxide Semiconductor (TMOS) technology is emerging with promising improvements over the decades-old passive infrared (PIR) sensor technology for human motion and presence detection. At the heart of the sensor is a nano-machined metal-oxide transistor that can detect heat radiation and a digital processing ASIC capable of detecting motion, presence, and temperature. Due to the lower power consumption, integration, and smaller size, the IR sensors based on TMOS technology are well suited for the IoT market, including smart city, smart home, and smart vehicle applications.

In this blog, we examine how TMOS technology builds on conventional PIR sensor technology by detecting human motion and presence using a highly accurate measurable response to minimal infrared radiation, whether the person is moving or still.

One of the historical drawbacks to PIR sensors is the invisible man effect. If you ever used one of those motion-enabled lights that turn on when you enter the room, you would also notice that the light turns off at the most inopportune times. For example, the automatic closet light turns on when you enter the closet, but the light turns off after 30 seconds while you are still in the closet trying to select your shoes. This issue is due to the nature of PIR sensors—they detect motion. If there is no motion in the field of view (FoV), the sensor fails to detect the presence of a human being. Thus, we have the so-called invisible man effect.

In comparison to PIR-based sensors, TMOS technology likewise detects motion as a person enters or leaves the FoV, but it also detects presence (Figure 1). If a person stands still, the PIR sensor does not recognize the person and immediately drops its output signal. This is where TMOS technology shows its advantages.

Figure 1: The difference in presence and motion detection using PIR vs. TMOS (Source: STMicroelectronics)

The TMOS sensor offers two internal algorithms, or flags, called Motion and Presence. The Motion flag, like the PIR sensor, detects the person entering and leaving the FoV. The Presence flag, however, detects that the person remains in the FoV. As long as a person is in the FoV, TMOS will detect their presence and not drop its output signal.

The digital TMOS technology includes an ASIC that converts analog signals into digital signals that can be processed by the ASIC (Figure 2).

Figure 2:ST’s IR Sensor Block Diagram shows how the ASIC converts analog signals into digital signals (Source: STMicroelectronics)

The STHS34PF80 sensor has been designed to measure the amount of IR radiation emitted from an object within its FoV and digitally process the signal in the ASIC, which can be programmed to monitor motion, presence, or an overtemperature condition. The signal processing significantly improves the Signal-to-Noise Ratio (SNR), reduces non-uniformities and false alarms, optimizes power, includes self-calibration, enables automatic operation, and incorporates clever algorithms to optimize and improve performance.

Thanks to its exceptional sensitivity, the STHS34PF80 can detect the presence of a human being at a distance up to 4 meters without needing an optical lens. The IR sensor embeds smart digital algorithms to support presence detection, motion detection, and ambient temperature sudden change detection.

Presence detection is accomplished in the IR sensor by programming the threshold values into the PRESENSE_THS and HYST_PRES registers (Figure 3). In the ASIC section of the sensor, the output of the TMOS sensing element is fed into low-pass filters LPF_P_M and LPF_P. The chip tracks the difference between these low-pass filters and compares the value with the two thresholds of PRESENCE_THS and HYST_PRES, which were configured in the software. The presence detection flag signal (PRES_FLAG) is activated when the difference between the two filtered signals exceeds the threshold value.

Figure 3: Presence detection algorithm in the TMOS sensor diagram (Source: STMicroelectronics)

In addition to the Presence algorithm, the STHS34PF80 can set threshold values for motion and ambient temperature change and alert the system host controller when a threshold is crossed.

Thermal Metal Oxide Semiconductor technology is revolutionizing the field of motion and presence detection by addressing the limitations of traditional passive infrared sensors. The STHS34PF80 is an innovative TMOS technology that is a game-changer for a wide range of Internet of Things (IoT) applications, including smart cities, smart homes, and smart vehicles. With its advanced algorithms for detecting both motion and presence, as well as its high sensitivity and efficient signal processing capabilities, TMOS provides a more robust and reliable sensing solution. As we incorporate an increasingly interconnected world, TMOS technology will play a crucial role in creating smarter, safer, and more responsive environments.

Tom Bocchino is a Product Marketing Engineer and sensor specialist at STMicroelectronics with strategic focus on IoT platforms for building management, smart metering, and sustainable energy. Tom is enjoying the ride on the wave of new applications enabled by MEMS and new sensor technology.