Investigating the Mystery of Heart Rate Sensors: Why Apple Watch's Heart Rate Monitor Still Reads When it’s Off-Wrist
Have you ever noticed that your Apple Watch, or any other smartwatch with a heart rate sensor, still displays a heart rate reading even when it's placed off your wrist? This might seem unusual or even concerning, but there's a simple and fascinating explanation behind it. This article aims to demystify the functioning of heart rate sensors and why your Apple Watch may still read a heart rate when itrsquo;s not on your wrist.
Understanding Heart Rate Sensors
The heart rate sensors found in devices like the Apple Watch work using optical technology. They utilize LED lights and photodiodes to detect the blood flowing through your veins. When your heart beats, blood flows to your fingers, and this change in blood flow is detected by the sensor. By measuring the variations in light absorption, the device can determine your heart rate.
The Role of Phase Locked Loop (PLL) Circuit
The heart rate sensor measures heart rate through a process that involves locking onto a heart rate signal. When the sensor is on your wrist, it can lock onto the rhythmic pulse of your heart, providing an accurate reading. But what happens when the sensor is not on your wrist? The primary reason for still receiving a heart rate reading is a circuit known as a phase locked loop (PLL) circuit.
How PLL Circuit Functions in the Absence of a Sufficient Signal
A PLL circuit is designed to search for and lock onto a specific frequency or signal. In the context of a heart rate sensor, it searches for a heart rate to which it will lock up when it finds one. Herersquo;s how this process works:
Search Mode: When the sensor is not on your wrist, the PLL circuit enters a search mode. It scans for any signal that resembles a heart rate but is not based on an actual heartbeat.
Noisy Signals: In this state, the PLL circuit may pick up on a range of signals that may or may not be from a heartbeat. These can include random noise, reflection from surfaces, or other body movements that resemble a heartbeat. The circuit then does its best to lock onto these signals, leading to what may appear as a heart rate reading.
Locking In: Once the PLL circuit finds a signal that it deems sufficiently consistent and rhythmic, it locks on to it, providing a heart rate reading. Because this signal isnrsquo;t based on actual heartbeats, the reading can be inaccurate, often appearing random or fluctuating.
Why This Happens and Is It Normal?
These readings, while anomalous, are a standard part of how the PLL circuit operates. The sensor is designed to provide a constant signal when itrsquo;s not on your wrist, and this can result in readings that are not reliable. This is why you should always wear your Apple Watch on your wrist whenever you want an accurate heart rate reading.
Conclusion
While the phenomenon of an Apple Watch or any other wrist-mounted heart rate sensor providing a heart rate reading when itrsquo;s off your wrist might seem disconcerting, it is actually a byproduct of the devicersquo;s design and the functioning of its PLL circuit. By understanding how these sensors work, you can better interpret the readings and use your smartwatch more effectively.
Frequently Asked Questions (FAQs)
Q: Can these off-wrist readings be trusted?
A: No, these readings are generally not accurate. The sensor is primarily designed to work on your wrist, where it can lock onto your actual heart rate. Off-wrist readings are the result of the sensorrsquo;s search and lock function and should not be relied upon for medical or performance analysis purposes.
Q: How can I get accurate heart rate readings from my Apple Watch?
A: To ensure accurate heart rate readings, always wear your Apple Watch on your wrist, specifically on your dominant hand. This will help the sensor lock onto your actual heart rate signal, providing reliable data.
Q: What is a PLL circuit?
A: A PLL (Phase Locked Loop) circuit is an integrated circuit that takes an input signal and generates an output signal that has a specific phase relationship to the input signal. In the context of heart rate sensors, it is used to lock onto and stabilize a heart rate signal, even when the sensor is not in the optimal position.