Interfacing PIR sensor to 8051 microcontroller

PIR sensors are widely used in motion detecting devices. This article is about interfacing a PIR sensor to 8051 microcontroller. A practical intruder alarm system using PIR sensor and 8051 microcontroller is also included at the end of this article. Before going in to the core of the article, let’s have a look at the PIR sensor and its working.

PIR sensor.

PIR sensor is the abbreviation of Passive Infrared Sensor. It measures the amount of infrared energy radiated by objects in front of it. They does not  emit any kind of radiation but senses the infrared waves emitted or reflected by objects. The heart of a PIR sensor is a solid state sensor or an array of such sensors constructed from pyro-electric materials. Pyro-electric material is material by virtue of it generates energy when exposed to radiation.Gallium Nitride is the most common material used for constructing PIR sensors. Suitable lenses are mounted at the front of the sensor to focus the incoming radiation to the sensor face. When ever an object or a human passes across the sensor the intensity of the of the incoming radiation with respect to the background changes. As a result the energy generated by the sensor also changes. Suitable signal conditioning circuits convert the energy change  to a suitable voltage output. In simple words the output of a PIR sensor module will be HIGH when there is motion in its field of view and the output will be LOW when there is no motion.sensor to 8051 microcontroller

DSN-FIR800 is the PIR sensor module used in this project.Its image  is shown above.  It operates from 4.5 to 5V supply and the stand by current is less than 60uA. The output voltage will be 3.3V when the motion is detected and 0V when there is no motion. The sensing angle cone is 110° and the sensing range is 7 meters. The default delay time is 5 seconds. There are two preset resistor on the sensor module. One is used for adjusting the delay time and the other is used for adjusting the sensitivity. Refer the datasheet of DSN-FIR800 for knowing more.

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