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Thousands of groups around the world use variety of sensors in the experiments carried out. The most common are portable, easy to place movement sensors to use and maintain.
PIR sensors:
PIR passive infrared, infrared means passive, that is to say that they do not emit any power for detection but is “nourish” of the emitted by bodies or objects in their detection range.
Consists of an encapsulated with materials which produce electrical charge to receive the infra-red light, all object emits the radiation, unless you are at 0 degrees kelvin (- 273,15 ° C), but these sensors are designed for the detection of the infrared Strip issued by the human body. Its peak is at 10 pm as well which are sensors that respond to long wave.

The operation is quite simple, the sensor after a time that va depending on the model of 10 to 60 seconds has created a ‘map’ emission go environment, when this is broken by the presence of a body, activates an output and this is used to activate an alarm indicator light or any other use, a flashlight in this case if no emitted in the spectrum of IR wave should not affect its operation. If the body after the new mapping remains immobile will be not detected.

On the other hand the sensor is covered by a FRESNEL lens that helps focus on the sensor information that has to receive this.

The maximum range of these sensors is 6 meters usually, although according to the budget the most advanced can cover 180 degrees and more than 12m, must be taken into account that does not detect objects or behind corners.
There are mixed assemblies that include a transmitter receiver ultrasounds that are not activated until both detected.
Sensor with LDR
Basic operation, this type of photoelectric sensors respond simply to change of light intensity, increase or decrease its electrical resistance also varies, used in many usual applications in the domestic arena.

They are used in detectors of shadows, lowering the light intensity sensor by the passage of an object with opacity it does skip one alarm, also there are low-cost devices, with the appearance of motion detectors containing inside instead of PIR sensors a circuit employing a resistance of this type.
Conditions of use for optimal these LDR issued to them light source must be controlled, if for example put it facing a wall and focus to this our headlight reflection will surely have a false positive, therefore needed more control over tests performed with these devices.
Conclusions:
-The use of detection devices must be subject to a protocol of placement and monitoring of the experiment, measuring detection limits and tests of operation or interaction with the rest of the team, mainly with emitters of light, (IR spotlights, flashlights, etc.). testing has to be reflected back contrasting with possible outcomes.
-The low cost of these sensors makes them a tool accessible, reliable operation, and useful (even for delimitations perimeter security in field experiments).

Sources: knowledge of industrial electronics and Wikipedia.

Raul Barranco García.