The HC-SR501 sensor is a highly versatile component often used in automated lighting, security systems, and other devices that are sensitive to motion. Depending on how this is set up, HC-SR501 can operate in wide distances or various ranges, as well as on extreme temperatures.

How Does the HC-SR501 Motion Sensor Work?

To understand its mechanism, one needs to know more about PIR, which is its core technology. This acronym stands for passive infrared.

In layman’s terms, infrared is the radiant energy that heated objects can emit. It has different wavelengths or frequencies. It is invisible to the naked eye but can be felt, even by humans. A perfect example is burning coal. Most often, you cannot see the smoke but you can feel its heat. 

Objects produce infrared radiation when the waves travel through the atmosphere and hit them. They excite the molecules in the objects, compelling them to produce heat in the process. It turns out, though, warm bodies of humans and even animals also produce infrared. The likes of HC-SR501 then capitalize on this heat to help detect changes in infrared, which could mean there’s a body in motion.

Parts of HC-SR501 PIR Motion Sensor

Despite being small, motion sensors come with many components. Two of the essential ones are a pyroelectric sensor and a special lens called Fresnel

The pyroelectric sensor has a pair of positive and negative electrodes and another pair of rectangular slots at the front. The latter allows infrared radiation to pass through. The electrodes, on the other hand, determine if there’s any fluctuation in the IR waves. 

Both electrodes work by canceling each other out, so it should not create any signal or output when there’s no movement. Otherwise, the output may be either high or low, depending on the level of infrared waves the module can detect.

Benefits HC-SR501

HC-SR501 is equipped with LHI778, a type of PIR sensor. The BISS0001 then helps regulate motion detection. Overall, the unit’s role is to constantly keep track of the infrared output changes. 

It has a broad range (between 3 to 7 meters), which implies that the sensor is highly sensitive. The average distance that other sensors can detect is less than 5 meters. Note, though, that physical barriers and the amount of heat an object or body produces can affect the sensitivity. 

This unit is also versatile. Users can adjust or customize the time it is active or remain high after it can detect motion. The longest period one can set is 5 minutes, while the shortest is 3 seconds. 

There are also solder pads available, so users can attach more components. These include RT components for thermistors. Through RT, the motion sensor can also work even in extreme temperatures. Another component is RL, which makes sure the unit can work even on low-lighting conditions. 

HC-SR501 can operate as a standalone unit or may be part of a module with a microcontroller for more comprehensive automation. Doing the latter will then allow Arduino users to customize more of its functions.

HC-SR501 Features

  • Comes with 6 pinouts for trigger selection jumpers, ground input, and time-delay & sensitivity adjustments
  • Operates at 5V to 20V
  • Has a 65mA power consumption
  • High motion-sensing time between 3 seconds and 5 minutes
  • 2 solder pads for additional components
  • Works at temperatures between 15 and 70 degrees Celsius
  • Sensing range from 3 to 7 meters

This article has only covered the basics and we hope it sheds some light on the HC-SR501 PIR motion sensor and its potential applications in the real world. Keep an eye on more related topics in the coming weeks. To buy this product or other related proximity sensors, use our manufacturer's part number tool at the top of this page for real-time price data and inventory availability. For bulk orders, go to our Bill of Materials Tool and upload your list for free! 

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Author: Sherly Mendoza
Published: March 19, 2021