Detecting when a specific item is near

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Newz2000
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Re: Detecting when a specific item is near

Post by Newz2000 » Sun Feb 01, 2009 8:11 pm

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).
I'm hoping to make a small object the size of a AA battery or slightly larger. It will be powered by a AA battery if I can get a clean and cheap boost regulator worked out, otherwise 2 AAA. It will pulse a red or green LED at slow intervals (.5s pulse once per 20 sec at first, speeding up to 1hz after several min (5 min?) then after several min more going to constant on). It will contain a PIC 12F509 mcu or similar that will spend most of it's time in deep sleep mode.

Once the device is located it will be taken to a base station of either team A or team B where it will be "rescued". It will be placed in/on/near the base station. If the taken to team A's base station will light solid green, if taken to B's base station light solid red.

The kids will be in a rush to rescue all the objects and get as many as possible to the base station. Therefore the process of "rescuing" the device should be simple. It could be anything, maybe touching the device to the base station or getting it to within a foot of the base station, or maybe the base station is a box and they just toss it in there.

RFID sounds cool except it all sounds a lot more complex than I'd intended. The idea of being able to detect a polarized magnet is may be a better option because there need not be any exposed contacts and just touching it to the surface of the station will suit the need.

If I could get the cost per device without the battery to $2 - $3 it would be a awesome. The PIC 12F509 is about $0.75, I already have scads of LEDs, resistors and capacitors. I think I need an inductor and a FET to boost the voltage from an AA up to the needed voltage for the pic/led (been a while since I've done it so need to refresh my memory on that part) but those aren't expensive at all. The base station can be more expensive but will love to do it as inexpensively as possible.

Bigglez
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Re: Detecting when a specific item is near

Post by Bigglez » Sun Feb 01, 2009 11:55 pm

Newz2000 wrote:I'm hoping to make a small object the size of a AA battery or slightly larger.
It will pulse a red or green LED at slow intervals (.5s pulse once per 20 sec at first, speeding up to 1hz after several min (5 min?) then after several min more going to constant on).
If you use a higher voltage you don't need the booster ($$).
The lowest DC voltage you could use is that of the LED.
A switched-capacitor pump would dump into the LED,
so you could run it from a single cell (1.1V when empty),
and get a two volt pulse to blink the LED.

Regulated DC is not required for the uC. Many work from two
cells, or adapt the capacitor pump to power the uC too.

Other voltage boosters require a coil (inductor). Ready made
one will likely bust your unit cost budget.
Newz2000 wrote:Once the device is located it will be taken to a base station of either team A or team B where it will be "rescued". It will be placed in/on/near the base station. If the taken to team A's base station will light solid green, if taken to B's base station light solid red.
Not sure I follow. Why not 'kill' the found objects (to conserve
power) and turn them off completely?
Newz2000 wrote: Therefore the process of "rescuing" the device should be simple. It could be anything, maybe touching the device to the base station or getting it to within a foot of the base station, or maybe the base station is a box and they just toss it in there.
If the rescued objects are fed through an opening into a
secure box you will have a better shot at IDing them, prevent
them from being "un-rescued" for double counting (I was a kid,
too!), and have a back up hand count if needed.
Newz2000 wrote:RFID sounds cool except it all sounds a lot more complex than I'd intended. The idea of being able to detect a polarized magnet is may be a better option because there need not be any exposed contacts and just touching it to the surface of the station will suit the need.
Never heard of a "polarized magnet". Any (and every)
magnet needs both north-seeking and south-seeking
poles. If a bar magnet was sliced in two, each part
would have a N and S seeking pole. Sometimes objects
have multiple pairs of poles, such as the magnetic
strips along the 'fridge door.

Newz2000
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Re: Detecting when a specific item is near

Post by Newz2000 » Mon Feb 02, 2009 5:45 am

Bigglez wrote:Not sure I follow. Why not 'kill' the found objects (to conserve
power) and turn them off completely?

If the rescued objects are fed through an opening into a
secure box you will have a better shot at IDing them, prevent
them from being "un-rescued" for double counting (I was a kid,
too!), and have a back up hand count if needed.
That they can be stolen or "unrescued" is a feature of the game.
Bigglez wrote:Never heard of a "polarized magnet". Any (and every)
magnet needs both north-seeking and south-seeking
poles. If a bar magnet was sliced in two, each part
would have a N and S seeking pole. Sometimes objects
have multiple pairs of poles, such as the magnetic
strips along the 'fridge door.
I meant polarized hall sensor. I've only used the reed switches. These ones suggested on the list are pretty interesting because they're cheap, solid state and they can detect the different poles of the magnet.

I agree that the boost regulator probably adds unnecessary complexity. I remembered an appnote from microchip that showed how a pic could use oscout as a voltage doubler and even power itself off the boost voltage. However it's 2(Vin - 2 * Vdrop) where Vdrop is the voltage dropped across a diode. When Vin is 1.5v that doesn't leave you much left to double. :-)

I'll see what the current draw of the device is and pick my power after that. Considering the light duty cycle and low power requirements I'll be strongly tempted to use two AG3s.

I'd still love to stand on the backs of giants regarding the hall sensors if anyone has personal experience using them.

Bigglez
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Re: Detecting when a specific item is near

Post by Bigglez » Mon Feb 02, 2009 9:08 am

Newz2000 wrote:That they can be stolen or "unrescued" is a feature of the game.
Let's see if I've got now:
(1) Game starts, all objects are alive and hidden.
(2) First flash is at twenty seconds interval
(3) Flashes accelerate over 5 minutes
(4) Any object rescued is counted by the team's 'home plate' (box)
(5) Objects rescued but still blinking may be stolen
(6) Objects that are rescued are set to LED always on
(7) Objects are either Red or Green flashers

A dual colour LED would offer four states (off, Red, Green, Amber)
the amber state can be a flasher too (saves power over an always
on LED).
Newz2000 wrote:I meant polarized hall sensor.
Some field experiments are needed. I've only seen these used
in a DC current probe.
Newz2000 wrote:However it's 2(Vin - 2 * Vdrop) where Vdrop is the voltage dropped across a diode. When Vin is 1.5v that doesn't leave you much left to double.
Not if synchronous rectification is used. Instead of Si diode drops
the PMOSfets act as very good low voltage, low resistance switches.
Newz2000 wrote:I'll see what the current draw of the device is and pick my power after that. Considering the light duty cycle and low power requirements I'll be strongly tempted to use two AG3s.
Aren't those button cells costly? Having said that I do see the LR44
(slightly larger size) in play alot for toys, musical cards, etc.

Newz2000
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Re: Detecting when a specific item is near

Post by Newz2000 » Mon Feb 02, 2009 9:28 am

Bigglez wrote:Aren't those button cells costly? Having said that I do see the LR44
(slightly larger size) in play alot for toys, musical cards, etc.
AG3 packs of 100 for $10 on ebay (those are the 1.5v low capacity) to 50 for $10 (the 1.55v high capacity). Lithium chemistry (3v) CR2032 are probably a better deal and are 100 for $15. I guess I could stick one CR2032 which is 3v 220mAh easier and cheaper than two 1.55v

My first suggestion on the AG3 is because I need to buy some for a favourite toy that needs replacements. I can get a pack of 100 from HK with shipping for about $1 more than 3 from Radio Shack. :-/

Regarding the bi-colour LEDs, I have scads of 0805 green and red LEDs. I have no bi-colours so I will probably use what I have.

My big unknowns are the sensor and the form factor so I thought I'd tackle those first and deal with the LEDs and power afterwards.

Newz2000
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Re: Detecting when a specific item is near

Post by Newz2000 » Mon Feb 02, 2009 11:47 am

Wow, finding the right part is time consuming. Here's what I've figured out so far:

* One lunch break isn't enough, two probably isn't either, especially if you actually intend to eat during your lunch break
* For hall sensors, there are two different kinds of bipolars:
* single output - detects north or south but only has one output so either "yes there's a magnet" or "no there isn't"
* dual output - detects north or south - if north then output "a" if south then output "b" goes high, also called "polarity discrimination"

I found one I like. The price is good, the size is good, the package is ok. Only problem? Smallest quantity for purchase seems to be 3,000 pieces. :-( (It's the ROHM BU52014HFV) There is a similar part that meets all my requirements except for the hand solderable need. It has the small package with the solder pads on the bottom.

Anyone suggest if this part would be hand solderable, and if so, some tips? It's 1.1mm square. I've never tried soldering dust before.

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haklesup
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Re: Detecting when a specific item is near

Post by haklesup » Mon Feb 02, 2009 3:13 pm

only problem? Smallest quantity for purchase seems to be 3,000 pieces
Are you sure, Mouser lists it for $0.75 qty 1. Maybe you were looking at a part number for a Reel. I just googled the part number by itself. DigiKey also has 3000 in stock and may only want to sell the reel. There are other distributors too
http://rohmelectronics.us/sales/usa/statesa-g.asp#CA

Try sampling it
http://www.beganto.com/supplier/rohm.html

Don't forget to check the manufacturer for app notes
So happens they have a product focus page for Hall ICs
http://rohmelectronics.us/Hall_ICs.html

And it looks like the AN has a solution to the rotating LEDs of another thread on this board.
http://rohmelectronics.us/downloads/Ads ... icDial.pdf

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