Looking for cheap, simple PIR detector module

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

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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

--
http://www.linetec.nl
Reply to
Richard Rasker
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I hate when people go off topic when I ask for help in a newsgroup, but if you'll forgive me this once...

I'm using a PIR sensor from Radio Shack. It's from Parallax (Parallax part number 555-28027). it's about $10. It's not adjustable for sensitivity as far as I can tell, but it's cheap. It gives continuous triggering or single trigger. It has a Fresnel lens on the sensor to give a wide angle of detection. You might be able to remove/replace this lens and get the directionality/sensitivity you want. Mouser carries this same detector for about $10, and others for a LOT more (hundreds of $).

Another solution might be to use a beam-break method of "intrusion" detection. A simple laser pointer, a couple of mirrors, and an inexpensive photodiode could make up a light curtain barrier. There are also commercial beam-break or light curtain detectors. Banner Engineering might be a good place to start. There are sensors of this type available as surplus -

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as an example.

Reply to
lektric.dan

Another solution might be something like:

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WT18-3N110S08 Photoelectric proximity switches, infrared, background suppression by SICK

Reply to
lektric.dan

Really bizarre. And 7500 euros! But if it impresses dates...

John

Reply to
John Larkin

(see

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an example).

Much cheaper here:

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Reply to
John - KD5YI

"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

Reply to
Phil Allison

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

Cheers

Reply to
Martin Riddle

"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

Reply to
Phil Allison

(see

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an example).

Using a pyroelectric for this is a recipe for Kitty Squash. They're charge transducers, rather than current, so it won't even see a stationary animal. The reason for the segmented Fresnel lens on a porch light sensor is that it forms a dozen or so images that cross a split (2 element differential) sensor as you walk by. That makes an AC signal that the back end has no trouble recognizing.

No motion, no signal.

That's the worst kind of safety system--the kind that lulls you into a false sense of security while actually doing bupkiss.

You can use a chopper, maybe on the drive motor shaft, to turn a static temperature difference into an AC signal that you can detect with a pyroelectric.

Cheers

Phil Hobbs

PS: Philips used to make pyroelectric vidicons whose sensitive material has the most jaw-cracking name in technology: deuterated triglycine fluoroberylate. Try saying that three times fast.

--
Dr Philip C D Hobbs
Principal
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Reply to
Phil Hobbs

"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

Reply to
Phil Allison

--
It doesn't have to be warmer, it just has to be different from ambient
when its image traverses the transducer.
Reply to
John Fields

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I agree with Phil. Do not attempt this with PIR. It is not the correct technology for what you want to do. Meausure the lock-rotor current on the motor instead, and shut it off if stalled. Or, use a beam break, but not sure how you would implement that.

Reply to
mpm

king

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Ah, that's nothing. I just got back from a plastics convention, and discovered there are nearly 60,000 engineering plastics available -- many of which have names only a mother could love.

Note: Plastics info + EE =3D dangerous conbination. It's worth sticking to what ones knows!

Reply to
mpm

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Not necessarily. Some commercial PIR's used for security alarms also have pulse count capabilities (to reduce false positives). In that case, an infant might have to poke their fingers in several times to trip the PIR.

Actually, the case for PIR false positives should be enough to preclude their use in this application.

-mpm

Reply to
mpm

--
I wasn't talking about downstream processing, my point was that the
transducer will generate an output when its surface is traversed by a
 Click to see the full signature
Reply to
John Fields

[snip]

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

--
http://www.linetec.nl
Reply to
Richard Rasker

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 not?

We're re-thinking it continuously ;-) But thanks for your reply.

Richard Rasker

--
http://www.linetec.nl
Reply to
Richard Rasker

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With something as heavy as a TV, is sensing motor current all that reliable?

Can't you integrate a pressure sensor into the lid. Maybe a piezo cable running the circumference of the lid and located at the bottom. Once the lid has slipped past the opening in the box, you can ignore the sensor assuming the clearance between the lid and box is small, i.e. too small for something to slip inside.

There is a youtube video of a kitten stealing pens out the cabling hole of a computer desk. Never underestimate the ability for something or someone to put themselves in the wrong place. Who knows...you might sell your product in Arizona.

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Reply to
miso

** False argument.

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

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

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

Reply to
Phil Allison

"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

Reply to
Phil Allison

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