In the previous article, we had talked about the outer interface of the HC-SR501 PIR Motion Sensor module. There are many more similar modules for example HC-SR505 with advanced facilities. But the basic principles of working remain the same. We can divide the module into significant segments as;
◉ Pyroelectric Sensor
◉ Arrays of Fresnel Lens
◉ BISS001 PIR Chip
◉ Retriggering Setting
◉ Delay time & Sensitivity Adjust
◉ Capacitors & Resistors
▶ IR Reception: Heat generating objects also generate infrared radiation. The sensor after calibration, it can detect the change in IR. However, the pyroelectric sensor is sealed within a metal can for protection purposes. There is an opaque window (Si or Ge made) in the metal can that permits the IR to penetrate with small attenuation but visible light can't. Fresnel lens arrays cover the metal can to increase the range.
▶ IR Processing: The pyroelectric sensor is made of crystalline material that generates surface electric charge when exposed to heat in the form of IR. There is a built-in FET connected to the sensitive material. The change of surface electric charge can be detected by the FET. The Source of the FET is connected to a pulldown resistor to ground. The output of the FET goes through a signal conditioning circuit for amplification and comparison process.
▶HC-SR501 Circuit: In this module, the process of signal conditioning is done by the usage of capacitors and resistors. And also the BISS001 Microchip processes the signal for the desired output. There's another voltage regulator ic is installed to stabilize the input voltage and there's a current limiting protection diode.
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| HC-SR501 Circuit Diagram |
▶ Retriggering Setting: The pins are visible in the schematic as JP1. This setting defines the output nature of the PIR. For this, we need to consider a circuit;
Now, the module can operate in two modes;
1) Repeatable (H) Mode: In this mode, the output, Dout will be high and the LED will turn on. The output will be high no matter the object is still in the range or not. The LED will turn off after a predefined time. This time can be controlled by the delay time adjust potentiometer. This is the default mode.
2)Non-repeatable (L) Mode: Now the LED will continue to glow as long as the object is in the coverage area. When the object leaves the area, the LED turns off after a pre-defined time. Similarly, the time and sensitivity can be set by the delay time adjust and sensitivity adjust potentiometer respectively.
▶ Sensitivity Adjust: This potentiometer adjusts the sensitivity of the module. Clockwise rotation of the potentiometer increases the sensitivity.
▶ Delay Time Adjust: PIR has two kinds of time delay as Tx (How long the LED will lit) and Ti (How long the LED will be off if there is no movement). These Tx & Ti can be adjusted according to the provided datasheet of the BISS001 chip. Following the datasheet,
Tx= 24576* R10* C6
Ti= 24* R9*C7
Component numbers are referred to as the datasheet schematic.
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| BISS001 datasheet schematic |
Now the problem may arise when the older version or different manufacturer products are encountered. There is no existing potentiometer. In this case, the resistance values are fixed and surface mounted in the PCB. Therefore Tx and Ti have to be determined from the datasheet or from manual calculation.
Reference Articles:
Image Sources
* EDAboard.com
* ozcott.com
* ResearchGate.com
* semanticscholar
* CircuitDigest
Previous blog link: Passive Infrared Sensor (PIR) Part-01: How It Works
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