PIR sensor

Discussion in 'General Electronics Chat' started by Dritech, Apr 7, 2012.

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  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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  2. crutschow

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    Not readily. The sensor optics and electronics are designed to detect only motion (notice its name on ebay). A static scene, even if it contains a warm object, generates no signal. You would need a different sensor and optical design to do what you want.

    An alternate is to add a long time delay on turning off the backlight, longer than anyone is likely to stay in the room.
     
  3. Dritech

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    Sep 21, 2011
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    So are all PIR sensors like this or is it the module that is designed to turn off when there is no motion??

    If i remove the sensor, is it possible to do a circuit that will keep the output high even when you stay still ?
     
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  4. crutschow

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    I don't know of any that don't require motion to respond, but that doesn't mean someone doesn't make one. But it would likely involve using an IR imaging sensor and optics, which would be significantly more expensive then a PIR motion sensor.

    What do you mean "remove the sensor"? If you remove the sensor the circuit won't work. :confused:
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the reply.

    What i mean is that i remove the sensor and use it with other components to create a circuit that will keep the output high even when there is no motion.
     
  6. crutschow

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    The sensor generates a very small signal when movement is detected. The difference in a static scene with and without a human in it would give a signal which such a tiny difference that it would likely be buried in the noise and be difficult or impossible to reliably detect with a PIR sensor.
     
  7. vpoko

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    Jan 5, 2012
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    Edit 4/8/2012 - This answer not appropriate for type of sensor being discussed, see further down in thread. -vpoko
    These sensors aren't in the thermal imaging region of the spectrum, which has a wavelength of longer than 8µm. They operate in the near-infrared regime and are sensitive to wavelengths of 1-2µm. As such, they can't tell a warm body from a cold object and can only detect differences in a scene, aka motion.
     
    Last edited: Apr 8, 2012
  8. Dritech

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    Thanks for the replies. I will include a delay as crutschow suggested.
     
  9. crutschow

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    Where did you get that info? The PIRs typically use a pyroelectric detector which are sensitive to a broad range of IR radiation from about 5-14μm. The human body IR output peaks at around 10μm. But the detector circuits are designed to respond to only changes in signal due to any motion of a warm body and thus do not respond to stationary heat sources.
     
  10. PaulEE

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    Dec 23, 2011
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    OP-

    I recently bought a Parallax PIR sensor and took it apart to understand how it works. It does the following...

    The dome that usually resides above the IR sensor is covered in small fresnel lenses. This magnifies moving lines of infrared heat flux - even at far distances.

    The sensor outputs an alternating signal of varying magnitude and frequency, depending upon the movement and distance/speed parameters, as well as the size of the object and amount of infrared radiation being emitted, etc.

    (inside the control chip)

    This alternating signal is amplified/integrated - and compared to an internal reference.

    If the input value causes a breach of either threshold of the internal window comparator, a logic signal is sent to the control circuit, which is composed of two timers and some logic that outputs a "high" signal for some duration - depending on one of the two timer's periods. The other timer runs the logic circuit clock.

    So, to answer your question with far more detail than you may have been after, yes - the object does indeed need to move if the sensor is a PIR motion sensor. To sense static objects, an IR TX-RX pair are usually used for near-field (a few feet from sensor I/O) transmit - reflect - receive sensing actions. Think automatic sliding doors - they won't close on you if you're in the way...
     
    Last edited: Apr 8, 2012
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  11. vpoko

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    I might be completely wrong about this, in which case I apologize for leading anyone astray with my info. According to this page (http://dmohankumar.wordpress.com/2011/08/29/pir-sensor/) silicon PIR sensors are sensitive around 1050nm, while indium gallium arsenide sensors are sensitive from 950nm to 2600nm. More broadly, I've always thought there was a difference between near-IR and far-IR, with near-IR sensors only picking up IR light that's created by light sources and reflected off of objects like the human body. I remember having a camcorder some years ago with night vision mode (with an active IR blaster). While I could see people because of the reflected light from the blaster, as soon as I covered it the only thing I could see was someone pressing buttons on a TV remote control aimed at the camcorder. I figured these PIR sensors worked the same way.
     
  12. crutschow

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    That's true for the type of sensors you are referencing. But the PIR sensors you buy for turning on lights or alarms uses pyroelectric detectors which are thermal detectors (generate a voltage due to temperature delta) not semiconductor quantum type detectors (which directly detect the presence of light). Pyroelectric detectors thus are sensitive to the broad band of IR wavelengths that generates heat when absorbed.
     
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  13. vpoko

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    I stand corrected, thanks.
     
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