How can you make a sensor invisible?

Many sensors appear in plain sight, such as in cameras or home security systems’ presence/motion detectors. However, some are disguised in other products, such as a rearview mirror (camera) in a car or a smartphone. Specifically, the iPhone has 15 or more sensors embedded in its cell phone package design. Designers continue to find out-of-sight, out-of-mind places for ways to gather more data.

Eliminating the telltale signs of the sensor and its electronics, like the wires to provide power, is often the starting point. So, battery-powered designs are often the first step toward making a sensor invisible. Making it small enough to avoid detection is another common design criteria.

Figure 1. An invisible microphone to sense confidential information was embedded in a replica of the Great Seal of the United States and given to the U.S. by the Soviets. The sensing device was located near the eagle’s mouth. (Image: Mental Floss)

In the world of spies, invisible sensing is essential for bugs. One of the classic and well-known examples of non-detectable sensing occurred right after World War II in the U.S Embassy in Moscow. While small microphones and cameras are common today, the inventor of the theremin musical instrument, a Russian, created a wireless microphone to detect private conversations in the embassy. The sensor was a gift from the Soviets of a large carved wooden replica of the Great Seal of the United States. Transmitting a specific radio frequency activated the sensing device without requiring any other power source. A Soviet van parked near the embassy transmitted the signal. The invisible sensor was not discovered for seven years.

Figure 2. A lightbulb form factor provides an intriguing hiding place for several sensors. (Image: Walmart)

Sensing in a light bulb

Today, an interesting example of invisible sensing occurs in a common light bulb. Since the system is installed in a normal E27 bulb base, its power comes from a wired 110V240V source instead of battery power. A high-definition (1080P HD, 2-megapixel CMOS) camera, light and motion sensors, and a microphone and speaker are embedded in the 2.8 x 6.1-inch light bulb form factor. Data communications occur over 2.4 GHz and 5GHz dual-band Wi-Fi networks.

Sensors in wearable products

Wearable products for health, wellness, and daily activity monitoring need invisible sensing. Common examples of wearables include smartwatches, fitness trackers, smart glasses, and other wearable health monitoring form factors. One company offers sensing in a shoe insole form factor, and its company name, Inviza Health, is derived from the word ” invisible. The product is called InvizaSoles.

“The idea behind InvizaSoles was delivering health diagnostics to wearers that was essentially undetectable or invisible to observers and wearers,” said Robert Andosca, Founder, President, and CEO/CTO of Inviza Health.

The insoles use advanced optical sensors to measure various physiological parameters with medical-grade accuracy and see through the wearer’s socks. Physiological parameters that the insole can measure include heart rate (HR), resting heart rate (RHR), heart rate variability (HRV), respiratory rate (RR), percentage of blood oxygen saturation (SpO2), and body temperature.

The smart remote patient monitoring (RPM) insoles are also equipped with force sensors that assist the wearer in maintaining balance and detecting any potential issues related to gait or stability. A 9-axis inertial measurement unit (IMU) with acceleration, gyration, and a compass/magnetometer sensing helps capture data related to movement. In addition, GPS technology integrated into the insoles enables location determination.

Sensor data can be processed through edge computing within the insoles or at the mobile app level. Battery power for the sensors and other electronics can last up to 90 days before requiring wireless charging. Increased user activity can extend the charge life through a piezoelectric footstep power generator.