Showing posts with label Passive infrared sensor. Show all posts
Showing posts with label Passive infrared sensor. Show all posts

Saturday, January 17, 2015

Motion Detector Switch

Motion Detector Switch




When you think of switches, you usually think about walking in your home and reaching out to flip a toggle switch in order to turn on the light.  But there is an easier way and it is called a motion detector switch.  It is a specialized switch that detects movement in your home by using infrared or ultrasonic sensors.  I know it sounds space-aged, but it really works.  The very fact that it senses motion is a great selling point, but add the fact that it can also sense sound makes this switch a must-have in my book.

When someone enters a room equipped with  a motion detecor switch that is armed with this technology, it turns on the light connected to it and remains on until there is no motion or sound in the room.  If you've ever left your home and forgot to turn off the lights or got up in the middle of the night to go to the bathroom or downstairs, only to stub your toe before you got to the light switch, you'll really appreciate the motion detector light switch!
There are many variations of these switches that is determined by the manufacturer.  Some of the motion detector switches have a neutral wire connection to power the LED, while others do not.  Be sure to check the wiring schematic that comes with your specific model of switch to ensure you wire it corrcetly.  Normally, the motion detector switch will have three wires.  The black wire connects to the incoming "hot" wire which supplies the power to the switch.  The blue wire is connected to the outgoing wire connected to the light that is called the switch leg. The green wire is always used only for a ground wire.


The motion detector switch is composed of these wires, connected to a large eye that protudes from the front of the switch, usually located on the top of the switch.  It scans 180° from its position on the wall. It is a solid-state electronic switch that can be damaged by incorrect wiring or handling of the product.
There are two variations of this type switch, the active sensor switch and the passive sensor switch. Active sensors, often referred to as radar-based, send out sound waves into the room, like your garage door opener does and waits for the signal to return. You may have had a garage door that always opens when it thunders. That's because a frequency was generated that matched the one that tells your garage door opener to open the door.
Passive sensors, on the other hand, have their own unique way of detecting movement. Called passive infrared sensors (PIR) and sometimes called pyroelectric detectors, they are used frequently in homes and businesses alike. They detect heat from the body of humans and animals alike. The sensor uses aphoto detector which which coverts light in the wavelengths into electrical current that triggers an alarm in the mini compuer housed in the detector. It triggers the switch to turn on. The exception is, the computer ignores slow changes in room temperture due to sunlight.
These type of switches are not limited to the interior of the home. There are also outdoor versions that detect movement from traffic driving up your driveway. These driveway alarms are used to notify soemone in your home that you have company, giving you a heads-up before they reach the door. They are also used in things like deer cams and door entry alarms. The one I have is placed across the room from the door and detects customers as they come in the front door. It then sends a siganl to a remote unit upstairs that sounds an alarm. That way, I don't have to sit and watch the video monitor while I eat lunch. That allows me to be aware of customers, eat lunch and watch TV for a few minutes without worrying about missing someone and causing them to wait impatiently. It's one of the best things I've bought and was relatively inexpensive. Maybe, you need a sensor for your home?
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The Ultrasonic Motion Detector

The Ultrasonic Motion Detector

ultrasonic motion detector circuit diy k20
In class and in the labs you will use an Ultrasonic Motion Detector which measures distance by emitting ultrasonic pulses and determining the length of time it takes for the reflected pulses to return. We can then calculate a distance from the the time and the known speed of sound.  The word "ultrasonic" refers to sound frequencies higher than the response of the human ear (from about 20 Hz to 20 kHz). Bats can detect sound of frequencies up to 120 kHz. [The unit of frequency is the Hertz, where 1 Hz is 1 cycle/sec and 1 kHz is 1000 cycles/sec.]
The speed of sound is related to the wavelength and frequency by the relation:

                                                    c = f l        speed = frequency x wavelength


The speed of sound in air is about 343 m/sec. The speed of sound in water is about 1500 m/sec. So, for example, the wavelength of sound waves of frequency of 1 Hz in air is 343 m. For f = 10kHz, the wavelength becomes 3.4 cm. The wavelength sets the resolution with which we can measure distances.  Therefore, ultrasonic frequencies (very high frequencies) are good for distance measurements. The ultrasonic range finders operate in the range of about 40 kHz.
Ultrasound is also used in medical applications for imaging and surgery. Techniques now exist to generate sound frequencies of a gigahertz (109 Hz) making resolutions of about 1 micron routine.



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Passive infrared sensor


Passive infrared sensor


A passive infrared sensor (PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR-based motion detectors.


How Infrared motion detector components work

  Infrared Radiation : 

Infrared radiation exists in the electromagnetic spectrum at a wavelength that is longer than visible light. It cannot be seen but it can be detected. Objects that generate heat also generate infrared radiation and those objects include animals and the human body whose radiation is strongest at a wavelength of 9.4um. Infrared in this range will not pass through many types of material that pass visible light such as ordinary window glass and plastic. However it will pass through, with some attenuation, material that is opaque to visible light such as germanium and silicon. An unprocessed silicon wafer makes a good IR window in a weatherproof enclosure for outdoor use. It also provides additional filtering for light in the visible range. 9.4um infrared will also pass through polyethylene which is usually used to make Fresnel lenses to focus the infarared onto  sensor elements.

 Pyroelectric Sensors :

The pyroelectric sensor is made of a crystalline material that generates a surface electric charge when exposed to heat in the form of infrared radiation. When the amount of radiation striking the crystal changes, the amount of charge also changes and can then be measured with a sensitive FET device built into the sensor. The sensor elements are sensitive to radiation over a wide range so a filter window is added to the TO5 package to limit detectable radiation to the 8 to 14mm range which is most sensitive to human body radiation.
Typically, the FET source terminal pin 2 connects through a pulldown resistor of about 100 K to ground and feeds into a two stage amplifier having signal conditioning circuits. The amplifier is typically bandwidth limited to below 10Hz to reject high frequency noise and is followed by a window comparator that responds to both the positive and negative transitions of the sensor output signal. A well filtered power source of from 3 to 15 volts should be connected to the FET drain terminal pin 1.
pir1.jpg (11520 bytes)
The PIR325 sensor has two sensing elements connected in a voltage bucking configuration. This arrangement cancels signals caused by vibration, temperature changes and sunlight. A body passing in front of the sensor will activate first one and then the other element whereas other sources will affect both elements simultaneously and be cancelled. The radiation source must pass across the sensor in a horizontal direction when sensor pins 1 and 2 are on a horizontal plane so that the elements are sequentially exposed to the IR source. A focusing device is usually used in front of the sensor

The figure below shows the PIR325 electrical specifications and layout in its TO5 package. Note the wide viewing angle without an external lens.
pirspec.jpg (34297 bytes)
This is a typical application circuit that drives a relay. R10 and C6 adjust the amount of time that RY1 remains energized after motion is detected. Download PDF drawing.
appckt.jpg (32281 bytes)

  Fresnel Lens_____________

A Fresnel lens (pronounced Frennel) is a Plano Convex lens that has been collapsed on itself to form a flat lens that retains its optical characteristics but is much smaller in thickness and therefore has less absorption losses.
plano.jpg (9982 bytes)
Our FL65 Fresnel lens is made of an infrared transmitting material that has an IR transmission range of 8 to 14um which is most sensitive to human body radiation. It is designed to have its grooves facing the IR sensing element so that a smooth surface is presented to the subject side of the lens which is usually the outside of an enclosure that houses the sensor.
The lens element is round with a diameter of 1 inch and has a flange that is 1.5 inches square. This flange is used for mounting the lens in a suitable frame or enclosure. Mounting can best and most easily be done with strips of Scotch tape. Silicone rubber can also be used if it overlaps the edges to form a captive mount.
The FL65 has a focal length of 0.65 inches from the lens to the sensing element. It has been determined by experiment to have a field of view of approximately 10 degrees when used with a PIR325 Pyroelectric sensor.
fresnel.jpg (35285 bytes)


This relatively inexpensive and easy to use Pyroelectric Sensor and Fresnel Lens can be used in a variety of science projects, robots and other useful devices.
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