Detecting when a specific item is near

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Newz2000
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Detecting when a specific item is near

Post by Newz2000 » Fri Jan 30, 2009 12:51 pm

Can anyone suggest an inexpensive and easy to hand-build way for a small micro like a PIC 12F509 or ATtiny12 to detect when either object A or object B are near it? For example the tiny micro would react differently if it has been placed near item A than it would if were placed near item B which is different still than if it were not near either. The micro device needs to be small and battery powered but objects A and B can be larger and even plugged in.

I'm trying to invent a game. Not a video game, but a game that requires kids to run around and expel large amounts of energy.

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jwax
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Re: Detecting when a specific item is near

Post by jwax » Fri Jan 30, 2009 1:45 pm

It would help to know more about the "target". Would that be a child?
Object detection methods depend on the "object" and its characteristics.
Define the object and we'll come up with ways to detect it. :smile:

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Re: Detecting when a specific item is near

Post by Newz2000 » Fri Jan 30, 2009 2:28 pm

I'm thinking the kids will bring the device containing the microcontroller to the object and they could set it on the object or hold it close to the object.

The game is going to be a "find and rescue" game. Devices containing a micro will blink slowly and the kids will have to try and find them in the dark. The blinking will speed up gradually until the device is found by taking it to one of the rescue points. Being rescued could be setting the device on the rescue point or bringing it into very close proximity of it. I'm pretty flexible, cost and simplicity are more important.

I had two ideas...

1. Place the device on a contact pad, each of the contact pads having a different frequency. The micro could then measure the frequency that it touches. I'm not thrilled about this because there'd be exposed electronic pads which I'm afraid would not be durable enough.

2. Make the housing keyed with two buttons on the micro so that the device could fit onto either pad which would then trigger one or the other of the buttons. My concern with this is that kids will be in a rush. Ideally they'd like to just throw the device or set it on the pad without having to line it up.

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haklesup
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Re: Detecting when a specific item is near

Post by haklesup » Fri Jan 30, 2009 2:35 pm

Its mostly in the sensor as the part to interface to a PIC would be simple. The main challenge is in being able to differentiate between object 1 and 2. If you would put transponders on the children then the sensor would need to be something that could detect these transponders independently. Determining range would be a nice upgrade.

For instance say you used a video camera as the sensor and a blue and yellow shirt as the transponder. The video camera would need to process data to detect the shirt and output a signal ( a digital bit for example) to the PIC so it would know something was near. Likewise for the other color but it would be a different bit in your byte. 2 or more bits can be used to denote range.

Once the detect bits are applied to the PIC, its all software after that. Once your algorithm is satisfied, it can output the action you want (another I/O line goes high and activates a lamp etc). Your PIC needs one I/O line for each object your detector can discriminate and additional bits for each output action you want to initiate.

Perhaps a little easier (software wise) would be to put a flashing IR LED beacon which would be detected by an IR (remote control style) receiver. Each beacon would be like a different button on the remote control. (you may be able to hack this from several universal remotes) The last task is to make the IR signal distance sensitive. I don't know how well it would work but filtering the LED so it appeared dim might give some range finding ability.

The remote door lock for my car seems to stop working at about 50 feet. Several key fobs and a remote receiver might also be a possible solution.

A lot depends on the range at which you want the object to be detected and how repeatable you want that to be.

Just break down and get them a Wii. For that matter, take a look around and try to find out how they do it. I hear these systems are a little hackable.

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haklesup
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Re: Detecting when a specific item is near

Post by haklesup » Fri Jan 30, 2009 2:46 pm

Ha, we posted at about the same time.

If I read you right, this is a sort of high tech hide and seek but the hidden is an object (transponder) not a child. I assume they will take turns hiding the object and then 2 or more will hunt for it.

I don't see a very simple and inexpensive way to do this. The only technology I am aware that can do this is RF location and ranging and GPS. RF locating systems are pricey and GPS doesn't really discriminate between objects just their location and you can't move the object after synching. This would be similar to geocaching but at shorter range with a temporary cache.

I don't really see yet why you would need to touch the object to synch to it. That match would likely already be engineered into the game unless you wanted the object to be sensitive to one of the finders.

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Re: Detecting when a specific item is near

Post by Bigglez » Fri Jan 30, 2009 2:49 pm

Newz2000 wrote: an inexpensive and easy to hand-build way
...
to detect when either object A or object B are near it?
How big is the object?
Is the object battery powered?
How long does the object stay alive?
Is this a contact or a proximity detector?
How close does the object have to be to the proximity detector for detection?
What happens if two (or more) objects arrive at the detector?
How long does the detector have to identify the object?
Is the object tamperproof?
What is the cost budget for an object?
What is the cost budget for a detector?

A little more info from you would narrow the choices and set
the performance for a reliable solution (on a given budget).

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Re: Detecting when a specific item is near

Post by dyarker » Fri Jan 30, 2009 5:13 pm

Short range RFID tags in the objects to be found? No contacts to line up, just get it close. Microprocessor in rescue station to record/signal arrival.

Cheers,
Dale Y

desy2820
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Re: Detecting when a specific item is near

Post by desy2820 » Fri Jan 30, 2009 5:19 pm

I'd be tempted to go with either magnets or RFID. A Linear Hall-Effect sensor can detect the difference between the north and south poles of a magnet. Object A could have one or more magnets inside it with the north pole facing out. Object B could use the south pole of the other magnets. This will obviously work better if the objects are non-magnetic. Sensing distance will probably depend on the number of magnets and their strength. You must also be able to fix the magnets to the objects so that the correct pole faced out, without moving or coming loose.

RFID is more complicated, but would also allow for expansion, ie, more than two objects or types of objects. Non-metallic objects will proably work better here too.

Coded IR beacons may also be an option, but you'll need to experiment. Line-of-sight issues, sunlight and flourescent lighting may drown out the signals, making this idea less than useful. Use a gated, 38 kHz signal to send an ID, the same idea as a remote control for your TV, but reversed. The sender is your object, with the reciever in your handheld. The object would blink out a code for the receiver to ID it.

Would dropping the objects into a base or container be an option? Build a base, but use the base opening's size and shape to differentiate the objects. An IR sensor set (emitter and detector) could be used to create a beam across the opening to allow counting...and timing. Make it physically impossible to put the wrong object into the base. If you use a beam, make sure the kids hand's can't accidently trigger it.

I hope this helps, or at least gives you food for thought.

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Re: Detecting when a specific item is near

Post by Newz2000 » Fri Jan 30, 2009 9:14 pm

desy2820 wrote:I'd be tempted to go with either magnets or RFID. A Linear Hall-Effect sensor can detect the difference between the north and south poles of a magnet.
RFID is more complicated, but would also allow for expansion, ie, more than two objects or types of objects. Non-metallic objects will proably work better here too.
desy2820 wrote:Coded IR beacons may also be an option, but you'll need to experiment. Line-of-sight issues, sunlight and flourescent lighting may drown out the signals, making this idea less than useful.

I hope this helps, or at least gives you food for thought.
Very good suggestions. After posting the last time I thought of the 38khz IR but I like the hall sensor even better. I'd not heard of the ones that could detect between north and south, all I'd heard of were the glass tube kind.

Can anyone suggest a part? Hand solderable surface mount sized would be awesome, something like SOT23, but I'm very flexible. I see on mouser there are scads of parts available, lots of them in the under $1 price range. It'll take me ages to sort through these. I'm looking at this page: http://www.mouser.com/Search/Refine.aspx?N=10834224 but there's no way to filter. Can you suggest a search word that will give me parts with this feature?

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CeaSaR
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Re: Detecting when a specific item is near

Post by CeaSaR » Fri Jan 30, 2009 9:32 pm

If you just click on SOT23 and APPLY FILTERS, you'll end up with 4 to choose from. To deselect, click the reset underneath.

Happy searching!

CeaSaR
Hey, what do I know?

Newz2000
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Re: Detecting when a specific item is near

Post by Newz2000 » Fri Jan 30, 2009 9:47 pm

CeaSaR wrote:If you just click on SOT23 and APPLY FILTERS, you'll end up with 4 to choose from. To deselect, click the reset underneath.
Yeah, it doesn't have to be sot23, just somethign hand solderable and preferably smt. I've never heard of these kind that can detect the north and south poles. I'm just curious if anyone can suggest a part or something to look for.

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haklesup
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Re: Detecting when a specific item is near

Post by haklesup » Fri Jan 30, 2009 10:30 pm

I was looking at the RFID suggestion and my initial reaction was the range would be rather short for a game like that and how would I measure signal strength and directionality but then I thought of a number of ways to (possibly) do it. However, the digital nature of the reciever/decoder made not only for an expensive parts list but for critical design of things like antennas and RF circuits. Not impossible but a definate challenge.

Hall sensors act over rather short range unless you plan to make huge electromagnets for the objects, you need to overpower the earth's magnetic field (~0.5Gauss) at the range you want to use it. If your object can disturb a compass needle (even a tiny bit) this may work. By pulsing a solonoid at different frequencies, you could potentailly tell objects apart and by shielding the hall sensor in one plane with a piece if NuMetal, you might impart directionality. You get directionality from a static magnetic field in the objects but can't tell A from B.

Further thought led to analog. Perhaps even more fun for kids would be a directional antenna and a headphone setup. The object could be an AM noise source buzzing away at a frequency eqal to an unused station (650kHz +/- tunable Oscillator and a small antenna). The reciever is a simple AM transistor radio with an external antenna/adjustable attenuator modification. You've probably seen a similar setup for tracking animals in the wild or something like that but they likely use a different radio band and signal (greater range too). Fine tuning the power at the object and attenuation at the reciever should dial in an effective useful range. This idea needs a little work because I'm not sure how directional AM antennas are. Another signal may work better. If FM works better, one of those FM modulator gadgets might work if you rig an oscillator to it instead of an iPod.

That makes for a complete solution or a front end for a digital user interface. For example, instead of using headphones, one could measure signal strength and digitize the result instead. I don't think you need to go that for for a kids game. Designing and making an antenna with appropriate gain, sensitivity and directionality with an adjustable attenuator will be hard enough. Anyway, google "Radio Direction finding" for more than I know on the subject.
I've never heard of these kind that can detect the north and south poles.
Starting with your Mouser search result, I started clicking on the datasheet links in tha table. I got to this pretty much right away.
http://www.rohm.com/products/databook/s ... 2gul-e.pdf
Looks like you want 2 devices. One that detects N and another for S. Its called a Unipolar Hall Effect Device. This leads me to think there are Bipolar devices too but its late so I'll leave that to the OP. My universal advice on device selection is to read lots of data sheets, app notes and look for reference designs.

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haklesup
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Re: Detecting when a specific item is near

Post by haklesup » Sat Jan 31, 2009 8:54 am

Is this what you want to emulate. Ad says it has ~600 foot range

http://www.brickhouse-childsafety.com/locator.html?

http://www.brickhousesecurity.com/vbsik.html

desy2820
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Re: Detecting when a specific item is near

Post by desy2820 » Sat Jan 31, 2009 11:04 am

I have to confess that I have no practical experience with these devices, I just did some research.

Here are the links I used. Once you have a better idea about the parts you want to use, you can figure out who carries them.
The main source seems to be Allegro Micro: http://www.allegromicro.com/en/Products ... linear.asp

Here is an app note that looks like it could help: http://www.allegromicro.com/en/Products ... n27702.pdf

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Re: Detecting when a specific item is near

Post by Bigglez » Sat Jan 31, 2009 11:41 am

<Bump Bump>
To the OP, please attempt answers to these questions before you take
any action. Failure to consider these points will result in a failed project,
one with disappointing outcome, or a waste of everyone's time.
Bigglez wrote: How big is the object?
Is the object battery powered?
How long does the object stay alive?
Is this a contact or a proximity detector?
How close does the object have to be to the proximity detector for detection?
What happens if two (or more) objects arrive at the detector?
How long does the detector have to identify the object?
Is the object tamperproof?
What is the cost budget for an object?
What is the cost budget for a detector?

A little more info from you would narrow the choices and set
the performance for a reliable solution (on a given budget).

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