First design project

You are having trouble with the ID part because it isn't really electronics -- it's sensors. You need to think about what physical quality of the mailbox changes when there's mail in there, and how to sense it. Some quick things that I can think of are:

Reflectivity. Mail is more (or less) reflective than the bottom of the box. You could blink an LED at the base of the box and look at the return. You could enhance this effect by pasting some reflective tape on the bottom of the box -- but what if you get a sample of reflective tape in the mail?

Weight. Put a scale in there, and detect when there's weight on it. But what about really light mail?

Patterns. Illuminate the box, and take a snapshot of the bottom with a TV camera. Compare the result with a prototype shot of the empty box. But what if the door gets left open, changing the illumination?

Conductivity. Hinge a probe from the top of the box, and see if it makes contact with the bottom. But what if the bottom of the box corrodes?

Have fun.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Party pooper ;-)

It IS an interesting problem. I now live in one of those areas that only has "community mail boxes"... walk all the way down the hill only to find that the mail hasn't been delivered yet... delivery is so random that it blows "random walk" probability theory nuts ;-)

I'm thinking of surreptitiously attaching a flag underneath the community box that gets triggered when the postman opens the back door. I think I could see such a flag with binoculars ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I always wanted to build that for the winter when the driveway becomes slippery. But, as with so many projects it ended up low on the priority list (that the list my wife runs...).

IMHO a nice method to sense mail would be optical. Not reflectivity but

2-3 pulsed LEDs at the top and 2-3 sensors at the bottom. Or vice versa. Pulses can be very brief and on every 10th minute or whatever time you allow for signal delay after mail delivery. These optical paths should cover the whole area where mail lands so that even a little postcard from aunt Ruth will at least cover one path.

You can build it with 74HC logic or use a very low standby current uC such as an MSP430, plus a ISM transmitter. Pulsing of all this is needed because you likely will have to make this run on batteries.

--
Regards, Joerg

http://www.analogconsultants.com

Hello Jim,

For community boxes a little end switch with a "touch flag" should do. It would briefly close while the mail is falling into your box and could then start a little ISM transmitter for a minute or so. This would also avoid a trek when the mail carrier was there but you didn't get any mail that day. As if that ever happened with all the junk mail these days.

That's decidedly too low tech for an electrical engineer household :-)

--
Regards, Joerg

http://www.analogconsultants.com

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I\'d use a couple of orthogonally mounted IRLEDs shining on a pair of
IR receivers such that when there was mail in the box, either lying
on the bottom or resting on one of the walls, at least one of the
beams would be broken.  Hit the LEDs (the transmitters) every once
in a while with modulated carriers, and if the receivers don\'t pick
them both up, you\'ve got mail!

Put a little wireless transmitter with a button inside the box, with a little placard that says, "Dear Postal Delivery Person: If leaving new mail, please press button before proceeding on your route. Thanks!"

Or,

Run a kevlar string from the door of the mailbox, along a line of pulleys on fenceposts, to a bell in the kitchen. ;-)

Good Luck! Rich

Don't US mailboxes have a red flag to indicate new mail?

They have flags, but they're only used to indicate to the mail carrier that there's something to be picked up. Incidentally, if you never have outgoing mail, it seems like you could just use the fact that the mailbox has been opened and closed to indicate new mail. The carrier won't touch the box if there's nothing to put in it.

-- John

My mailbox also has a yellow flag that pops up when the door is opened. I can see the box out of one window, so i don't have to go out with my cane if there is nothing in the box. I rarely mail anything from the house. I write all the checks at once, and mail them when I go to town to to buy groceries.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Leaving mail for the carrier is an invitation for identity thieves. I'm cautious where I deposit mail.

How about an accelerometer to detect the mail dropping into the mailbox?

Leon

How about a hot plate on the box's bottom and a smoke detector?

What I mean is, have you ever designed anything, or built from hobby electronics magazines/books? Tinkered around with anything? I find it strange that someone would master wireless comm but have so little creativity on as what effect to explore for mail detection.

I myself woud just use a microswitch that detects the box being opened. You could power the TX from a battery and run the supply through the switch. Heck, I'd just run a cable all the way to the switch and use a monostable relay to light up a bulb in the house. So much for electronics.

robert

The flag idea is not low tech... it's solar-powered ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Robert Latest a écrit :

How about those ways?

A) assuming the mail box sits on top of a pole:

- measure the free resonating frequency of the pole/mail box

- measure temperature and length of the pole, then deduce the box mail load

B) assuming nothing

- regularly emit a short acoustic click and listen the decay change with mail damping

- emit a light flash and observe the change of reflectivity

- make the box top and bottom isolated metal plates and measure the capacitance change with the air/paper dielectric inside

- monitor the box acoustic standing waves modes

- put some pressure sensitive mat around the mailbox and let the mailman walk on it

- tie your mailbox to a Tesla coil tuned slightly before arcing and have your AM radio turned on (OK, your mail may not get right into the box)

- have some thermocouples measuring the box temperature, a small pilot flame inside and measure the box temperature change. Bonus: you get your mail heating value for free.

- still with the same pilot flame you can simply monitor the box inside light level change

- fill the box with some water and have a mic near it, linked to an "Oh shit!" programmed speech recognition board

- put some medium power heating resistors inside the box that you slowly PWM and measure the box thermal time constant.

And I'm sure I'm forgetting some other interesting ways of doing this.

--
Thanks,
Fred.

That's kind of a sad commentary on society, although there are places in the U.S. where I think it's much more of a concern than others. There still are small towns in the U.S. where few if anyone locks their doors at night.

Hello Jim,

Ok, then it's too environmentalist :-)))

--
Regards, Joerg

http://www.analogconsultants.com

--
Very nice!

Even if you had outgoing mail, you could always reset it after you
closed the door.

--
I wouldn\'t worry about it, since in your case identity theft would
only be detrimental to the thief.

If the long string gets in the way, here's another idea. You need a cat, a dish of cat food, some string and a bell.

Don't feed the cat for a day. Lock it in the mailbox. Leave dish of cat food outside the door. Stretch one end of string just above the dish, attach other end to bell.

When mailman opens mailbox ravenous cat runs up to the dish. It can't avoid moving the string to get at the food. The string rings the bell.

When you hear the bell, go outside, the cat has all its attention on the food so is easy to pick up on the way to the mailbox.

Get mail, lock cat in mailbox.

- YD.

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Remove HAT if replying by mail.