Post edited 12:59 pm – October 5, 2010 by Lace
So what we basically have is a unit, say a lockbox or tool box, that needs to automatically lock itself when the user is away or out of range, and needs to automatically unlock itself when the person comes back into range.
RFID is a good start, and I think that technology is a good place to start looking for a solution.
For starters, some background information. There are two main kinds of RFID tags that I know of, called active and passive. A passive tag, the kind that are found in items you buy from stores, as well as the kind in my hand, uses the radio signal from the reader itself for power, which means they have no power source of thier own. These are the cheapest to get started with, but have a limited range.
Active tags on the other hand have their own battery, and transmit their data to the reader. These also typically have a bit more range than passive tags. They are more expensive to get started with but have a greater range.
But you have a third option. You can almost think of it as 'making' your own rfid system. Below are the links to a 315 mhz transmitter and reciever from one of my electronics suppliers.
http://www.sparkfun.com/commer…..ts_id=8947
http://www.sparkfun.com/commer…..ts_id=8945
Here's the way these would work to your advantage. You would couple the transmitter with an inexpensive button cell battery such as the 3 volt 610 mah Lithium battery shown here:
http://www.allelectronics.com/…..ble/1.html
and an inexpensive microcontroller (50 cents to 8 dollars). The reciever and a seperate microcontroller would go in the tool/lockbox and would be powered by a larger battery with higher capacity. It would be constantly listening for a signal with a password, or something similar.
The transmitter, microcontroller, and small battery would be made as compact as possible and made to fit on a keychain, not unlike the keyfob people use to unlock their car. Every 30 seconds or so, it would be programmed to send out the passphrase that the reciever is listening for.
When the man with the transmitter on his keychain (or necklace, or anywhere else it can fit) comes within range of the toolbox with the reciever in it, within thirty seconds or so the password will be sent from the transmitter to the reciever. That will tell the toolbox end of the project to unlock itself.
If the reciever doesn't get the unlock signal every minute or so, then it is designed to relock the toolbox. Easy peasy.
If you need help visualizing it, think of an automobile that is unlockable with a keyfob. Really, what we are doing is mimicking that exact system, but making it unlock a toolbox, and making it send the unlock signal every 30 seconds (or whatever you decide would work best) instead of when you push the unlock button.
If I were going to do this project, that's the route I would take. It's a bit more involved, but you have more control and it will be WAY cheaper to develop AND mass produce.
Keep the questions coming, I'll be here.
-Lace
*edit* And as far as getting started with microcontrollers goes, I would suggest the Arduino, but Randy Storms or Brian Baldridge could probably help you get started with the Basic Stamp as well. I suggest the Arduino, however, because other people have already written code for use with the transmitter and reciever I referenced above, saving you a lot of time trying to start from scratch.
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