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Looking for cheap, simple PIR detector module

Discussion in 'Electronic Design' started by Richard Rasker, Mar 2, 2011.

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  1. Hi all,

    I'm designing the electronics for a flatscreen TV lift system, for tucking
    away a flatscreen TV inside a low design-furniture cabinet (see for an example).

    As safety is an essential requirement for this type of machinery (the lift
    system is easily capable of squashing or even severing fingers), I'm
    looking for a reliable PIR detector module. The idea is to have it "look
    down" from the back of the TV into the machinery, so that the system will
    stop immediately if a body part or (warm-blooded) household pet is
    detected. The PIR detector should run off something between 6 and 12 volts
    (with a preference for 12V), consume no more than 10 mA, and provide a
    momentary low or high level signal upon triggering. A sensitivity setting
    is paramount.

    I tried several cheap PIR detectors which are used for automatic door bells
    or proximity warning, but these have a tendency to trigger too readily;
    maybe they react to the changing heat pattern at the moment the lift starts
    moving (the TV is warm after use, and the movement probably makes the
    detector see quick changes in received heat), or maybe the PWM signal
    (though within permissible EMC levels) makes it go haywire.

    And oh, the cost may not exceed some 20 dollars for a complete module.
    Schematics for PIR detectors are also welcome, but no doubt, existing
    mass-produced modules are cheaper than anything I can make.

    And yes, I know that this will /not/ secure the whole thing against crushed
    fingers when the lid closes; we're contemplating several other measures for
    this. Also, the remote control is IR, with mandatory "button-hold" action,
    so that the operator must always be in sight of the system, and nothing
    happens automatically.

    Thanks in advance,

    Richard Rasker
  2. John - KD5YI

    John - KD5YI Guest

    Much cheaper here:
  3. Phil Allison

    Phil Allison Guest

    "Richard Rasker"
    ** There is no such animal.

    Your idea is dangerous and absurd.

    ** Very stupid and dangerous idea.

    PIR detectors fail, fingers may be too cold to register.

    ** The problem is mechanical in nature.

    Just limit the available torque on any motor to a safe value.

    ** You must make it fail safe by nature.

    Relying on PIR detectors is 100% absurd.

    Time for a re-think.

    ..... Phil
  4. I agree with Phil.
    Limit the torque of the motors. Maybe with auto reversing like
    automotive power windows.
    Perhaps Jim has an IC for that ;D

  5. Phil Allison

    Phil Allison Guest

    "Martin Riddle"

    ** That dull thud you heard we me falling over ......

    ** I suspect the issue is with * lowering* that flat panel screen.

    The motor is then probably opposing the weight of the screen and toque
    limiting is not relevant to finger safety.

    The answer might be to add a counterweight system on a pulley ( like all
    elevators use ) so the motor IS pulling the screen down.

    .... Phil
  6. Phil Allison

    Phil Allison Guest

    "Phil Hobbs"

    ** Nothing guarantees that an infant's finger or feline's paw will actually
    always be any warmer than the surrounding surfaces either.

    ** Plenty of them about.

    Like nearly every video camera set up to provide " security " ....

    .... Phil
  7. Stationary animals or children are no risk: the sliding top of the cabinet
    is closed at all times, except when the lift is raised or lowered. This
    means that it's impossible to get an animal or limb inside without physical
    movement during the time that the machine is active.

    Also, motor current is monitored, and the system stops whenever unexpected
    resistance is detected. I just want to add an extra safety measure which
    will usually stop the machine before body parts get stuck inside.
    I fully agree with this notion. It's not my intention to just stick in a PIR
    device and simply declare the machine safe -- even if I'm satisfied with
    the overall safety, we're going to have it tested by an independent
    certification authority. But I have to start somewhere, and test different
    options, hence my question here. I've also considered ultrasonic devices,
    but these are both more expensive and less relaible than PIR.
    Hmm, thanks, I'll add it to my list of options.

    Anyway, thanks again, best regards,

    Richard Rasker
  8. I agree, and I certainly don't intend to rely on PIR only. Motor torque is
    already monitored, and any unexpected resistance causes the system to stop
    immediately. Some more mechanical measures are in place -- most parts which
    may otherwise pose a risk (e.g. with a shearing action) have covers and
    guides which push objects aside rather than squashing them. Still, there
    are some parts inside that can't be fully shielded this way. And even if
    the motors have a torque trip, a finger or arm will still get stuck at that
    point. We're looking into the option to reverse motion for half a second in
    those cases, but that of course may not introduce new risks.

    Anyway, we going to have to it certified, so I'm not the one to call it
    safe. But if I can significantly improve safety with a simple measure, why
    We're re-thinking it continuously ;-) But thanks for your reply.
    Richard Rasker
  9. Phil Allison

    Phil Allison Guest

    "Richard Rasker"
    ** False argument.

    You CANNOT significantly improve the safety of your contraption with a cheap

    It will have gimmick value at best - and doing that is practically

    If you counter balance the weight of the screen, then only small amounts of
    motor torque are ever needed.

    I think you are an ass.

    Go away.

    ..... Phil
  10. Phil Allison

    Phil Allison Guest

    "John Fields"
    "Phil Allison"
    ** Nothing guarantees that it will be significantly different either - or
    even traverse in a way that registers on a device that was intended to
    detect only whole bodies.

    Most PIR security devices are designed to IGNORE pets.

    None I ever saw were designed to work at such closes ranges.

    The OP's idea is 100% crackpot.

    ...... Phil
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