I need to trigger an automatic door opener when my wheelchair is within about 3 feet. I would like some sort of radio transmitter on the wheelchair and a receiver near the door. Here are my requirements:
- it must work at about 3 feet. I can't find an affordable RFID reader that works at this range.
- the transmitter must operate on a battery for at least two months continuously. I like Bluetooth, but can't find a low power transmitter. What do they use on those automatic pet doors?
- it must provide a simple contact closure, or have a serial or usb interface and Linux drivers. Zigbee?
- I'm not too concerned about security. There are easier ways to get into my house than spoofing a radio signal.
- it must not require too much soldering. I have friends that can help with this, but within reason.
I've seen various radio transmitter modules on the web, but don't know how much power they require, and how to interface a receiver to my computer. I've figured out how to do this with Bluetooth, but I have the power issue.
Would you be able to operate a separate switch control? I'm thinking of the transmitter-receiver units used for garage doors and car alarms. They are about $50 for both units. The transmitter would need a 9VDC battery or perhaps a watch battery and last years. You can also get multiple button versions. Most are very easy to interface with by opening up the unit and soldering on your preferred switch.
Another option is to reverse the transmitter-receiver location. The transmitter would be at the door and trigger the low power receiver on your chair to respond with a signal.
I've not read any of your other posts, if you've had any. What do you consider to be cost effective? Are the doors that you are triggering to open non-secure (inside) doors or entry ways to your home? I'm thinking perhaps to have the sensor on the floor near the door on both sides. It could sense your chair near the door. This could be anything from a magnetic sensor (very poor security) to RFID, optical scanner, or whatever might work for you.
The automotive and garage door units are cheap and very low power. Most have security features to prevent spoofing. Another idea that just occured to me is having the transmitter on your chair trigger with perhaps a 1/2 turn of one of the wheels. Then all you need is to be approaching the door and retriggering it probably isn't a problem. That also prevents keeping the transmitter on all the time. Many of these transmitter-receiver pairs exchange codes when they recognize each other. That way anyone that did record a signal while you were away from the house probably couldn't use it to get in later.
A local accessibility group (perhaps if you have a customized chair or vehicle from them) could help with a cost effective solution.
There is also the possibility of using a bar code scanner to read a code off the chair. Scanners are perhaps $20-30 and easily interfaced to a computer.
I'll check out my reference books at work for ideas. You might also ask this question in comp.arch.embedded. They can at least address what Zigbee is good for and give you a few more ideas. They might even be able to locate a used RFID device for you.
A system like this would have to have some serious interlocking, I would think, to prevent the door from closing inadvertently as well as keep it from "fluttering" -opening and closing repeatedly - if you happen to be sitting at the edge of the receiver range. It may mean you can't sit out on the porch without the door going crazy. That's assuming a mode of operation that's typical of some doors, but may notbe true of yours.
As mentioned, Velcro and a garage door opener eliminate those issues although they do require enough mobiity to operate. Someone could modify one for you if that's a problem, so that the opener could be triggered with a non-traditional activation methods.
Have you checked Ebay for used commercial equipment? Marc H. found a place on the web that was selling video MUX's for about 1/10 the cost of a new one. (Thanks, Marc!) Good deals abound if you know where to look and how to get support from other than the vendor, who's probably just a liquidator.
Some that I have seen use a magnet inbedded in the pet collar. A small neodymium magnet can put out a field detectable from several feet away. Hall-effect sensors allows contactless sensing of an external magnet or a ferrous object.
Whether you could or would even want to use that for *your* door is another question. It is, however, how many pet doors work since they want the "transmitter" part to be as cheap and passive as it can be.
Other forms of pet doors use IR in addition to magnets:
Thanks for your thoughts, David. I have looked on ebay for commercial RFID readers that work with active tags, but the listings have very little information and google searches don't help much. There isn't strong hacker interest in active RFID tags yet. I saw a recent post about the $12 G2 active tags that work with wifi. It would be great if I could hack some Linksys WRT54G routers to work with these, but I doubt G2 will release much information about them since they want to sell systems.
As long as I can calibrate the sensitivity relatively accurately, I don't think accidental door openings will be a problem. I can stay away from the door.
See my other post about commercial RFID readers.
Thanks for the information about the pet doors. I didn't know they were passive. Maybe I can hack one.
It's a power wheelchair, so I do have access to 24 volt power (two 12 volt gel batteries in series). It sounds like there are no radio transmitters that can operate continuously off a small battery, so my Plan B is to attach a Bluetooth device to my chair power.
Is there a < $50 module that converts 24 vdc to 5 vdc?
There are a number of component-level voltage regulators that would do the trick nicely, if someone in your house is handy with a soldering iron. Beyond that there plenty of pre-packaged DC/DC voltage converters. A wire to each side of the battery, and the other side has regulated +5vdc. "dc voltage converter" as a google phrase reveals plenty, a search of some surplus sites
How about a sip & puff switch? How do you control the chair?
I would not use a small battery powered device. You might find yourself "locked out" by a dead battery. Ideally, you need something like a barcode scanner powered by the household wiring with the barcode on the chair.
Not really a "module", but a lm7805 voltage regulator (three pin - Vin, Vout, GND) will give you a steady 5v. Depending on current requirement, they can be had for about $0.25, and at most are about $1.
I'm not sure how well these things can be calibrated, give the amount of trouble my local Target has with supposedly deactived tags going off at the exit sensor.
I've been looking at some unit at Ebay, particularly this kit:
Dimensions: (58mm X 34.5mm or 2.3?x 1.4?)
Read Range up to 75mm (embedded antenna), or 250mm (external antenna)
Build in Multi Interface with Processors SPI (up to 1Mbps) for high-speed application Or RS232 with 9600-57600bps for long distance application
5V DC or Battery power for mobile operations. Power consumption as low as
Multi-protocol firmware enables read and write of ISO standard and Philips Mifare Tag/Card.
LED and BUZZER output for industry application.
The kit is listed at a starting bid of $50. You'd have to add an external antenna and even with it, the range is only a foot. However, it may be possible to mount the antenna near the door and low to the ground on one side. Two different tags, each mounted on opposite sides of the chair, could be used to determine if you are leaving (right tag detected - close the door behind you, left tag detected, your are outside approaching the door).
I'd look at my entryway to see if I could figure out whether I could conceal the antenna close enough to the passing wheelchair that it came within a foot. At my house, there's no way to approach the door without going through a narrow porch opening made of brick. If I mounted an RFID antenna there, it would almost impossible for chair-mounted tag to NOT get within range.
Where do you want to mount the reader or its antenna? I am assuming inside the house, on the other side of the door and that's why you need a three-foot range. If you can rethink that requirement and find some way to mount the unit to be close to the tag, you could use the much cheaper passive RFID tags and eliminate the siphoning circuit.
I'd probably design a sensor swing arm that would almost act as a inverse "curb feeler" and would deflect when the wheelchair edge hit it. The swing arm would then follow along the edge of the chair and would swing back like a saloon door to its original position once the chair has passed by. Mount the RFID tag at the height of the sensor arm and it should get close enough to activate it.
If used commercial equipment is anything like the incredibly marked down MUXes Marc found, they only appear every few months or so. It might take some serious searching via their email notification service to find the right device. Anyone looking for a 100 Wasp employee tags will find them on Ebay at about 1/10 the cheapest price I have seen anywhere else. You're right, though, the descriptions are deliberately brief. Most likely to avoid any issues about the item not being as represented. You have to do the research, model number by model number.
That's pretty low security! Perhaps lower than you want to go. Not just any moving thing can get into your house, true. But anything moving thing with a magnet could.
Yes. That's the easiest part of the project. And with the enormous capacity of wheelchair batteries compared to, say a 9 volt transistor battery, siphoning off power from the chair batteries would be the way I'd go.
There's an outfit:
That has a lot of pricing info. As you already know, active RFID stuff clocks in at the "thousands of dollars" price point although they have some readers listed for under $500 that will do the range you want. But not much under that and it's not clear what else you might need to make it work.
It's very possible that if you contacted the PR department of one of the big active RFID players and explained what you are doing that they would contribute to the cost of the project. They might be more than willing to help if they could get it written up in the trades. They might also see it as a potential new market for active RFID.