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Function of photocell on operation of PIR?

Discussion in 'Electrical Engineering' started by Bill Woods, Apr 26, 2005.

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  1. Bill Woods

    Bill Woods Guest

    I have a PIR motion detector designed to sense if someone is walking
    towards my house.

    In addition to a dial for adjusting PIR sensitivity, this motion
    detector unit has a photocell mounted to face downwards. The motion
    detector unit is designed for 24 hour operation so clearly the
    photocell is NOT for sensing darkness for "night time only"

    So what does the photocell do?

    Unfortunately I can't test it reliably under light and dark
    conditions because it is a wirefree detector unit which transmits
    pulses every few seconds and gives very sporadic responses.

    I figure the photocell probably alters the PIR's sensitivity
    charactersistics in some way. Is this right?

    But would the unit be made more sensitive or less sensitive in
    brighter daylight?
  2. Its more than likely that the motion sensor and the photocell are wired
    in series, and the output taken from the centre of them.

    The resistance varies WILDLY with ambient light, and this arrangement
    allows a basic comparison between them.

    Then AC couple that to detect the d/dt of the lot and you are in business.
  3. AlanBown

    AlanBown Guest

    All of the fixtures that I have seen that are motion only, do not have a
    photo cell. Sort of dumb turning on a light in the day time. My porch
    fixture has a photo cell and motion detector. I do not have a adjustment on
    the photocell. I try to keep the sensitivity down so that anyone on the
    sidewalk does not trigger it. A couple of steps into the yard and it will
    turn on. I leave the fixture on all of the time so in the evenings when it
    starts getting dark the motion sensor takes over and will turn the light on
    according to my settings.
  4. Bill Woods

    Bill Woods Guest

    On Tue 26 Apr 2005 17:12:29, AlanBown wrote:
    I too thought a daytime lamp was silly but it works rather well:

    When the PIR is tripped, the unit makes a loud sound. If you plug
    a lamp into the mains socket on the unit then power is sent to
    that socket. You might have a red bulb in your study and get a
    visual alarm that someone is approaching. Could be useful if you
    didn't want them to hear the audio alarm.

    Sounds a bit like mine. What do you find the photocell does to
    the PIR's ability to sense people in different conditions of
    ambient light?
    The unit I describe sounds more of an alarm.

    Your unit lights a lamp and seems more as a warning to intruders
    or a courtesy to visitors.
  5. R.Lewis

    R.Lewis Guest

    The photocell is nothing to do with varying the sensitivity of the device -
    it is there to detect 'darkness'.
    Commonly these things are arranged so that the pir triggers a lamp of some
    for some fixed time, to illuminate a path for example, but such illumination
    is not required in daylight.
    The position and direction of view the photocell (most frequently actually
    either a photo-resistor or a photo-transistor) depends on the combined
    optical characteristics of the housing, lensing, and photo device. Pointing
    down is the most common way of getting the best average ambient illumination
    level. Pointing up almost never is.
  6. Bill Woods

    Bill Woods Guest

    On Tue 26 Apr 2005 21:55:12, R.Lewis wrote:
    I think you may have skipped too lightly over where I say that the
    motion detector unit is designed for 24 hour operation so clearly
    the photocell is not for sensing darkness for "night time only"
  7. He never said it was a PIR operated light.

    In fact I assumed it was part of an alarm system.

    are for
  8. This sounds like a unit that I have. It can be set for motion activation
    and/or darkness. For some strange reason, the manufacturer didn't see
    the need to incorporate a daytime inhibit on the motion detector.
  9. R.Lewis

    R.Lewis Guest

    Why would you need a pir that did not detect in the daylight?
    You may need a pir that does not turn on a light in daylight hours but why
    inhibit the pir function?
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