We all are interested in knowing what is happening at home when we are not in. Either it is about security or critical elements such as gas, fire, or water. Knowing in the moment when something wrong is happening is vital to minimizing the eventual damage. In this chapter, we will focus on the PIR sensor, but the same principles apply to other sensors, such as moisture, gas, or smoke sensors.
PIR is the most common sensor used in the indoor and outdoor alarm systems. It is also used in automated doors and automated lights systems.
The functionality of the PIR sensor is based on radiation emitted by human bodies. Objects generate heat as infrared radiation and those objects include animals and the human body whose radiation is strongest at a wavelength of 9.4 μm.
When a human passes the front of the sensor, the temperature from the PIR sensor point of view changes from the background value to the human value. The sensor detects this change in the infrared radiation and changes its output voltage, signaling the detection.
To increase the sensitivity of the PIR sensor, a Fresnel lens is mounted in front of it. A sensor is in fact a FET transistor with a source pin connected with a pull down to ground. We can see this setup in the following figure:
In the following image, we can see a Fresnel lens made from plastic:
The sensor itself is located under...
Now you can build a security system in no time, but the security is not limited to PIR sensors, you can add other sensors to it, such as CO2 or gas, or sensors for detecting water leaks or smoke, and you can create a dependency between them. For example, if you have a broken pipe in the house and the water leak sensor detects water you will be notified in the Blynk application, but also another ESP8266 that is subscribing to the leak topic can close the main valve so the damage is limited by the time you arrive home.
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