Hi lhs hackers. I run a software dev Co. called ioWorld and have a
project requiring a hardware hack that's outside the scope of my
capabilities; I want to create a wireless fob that a smart phone
(Android, to start with) can detect at approx. 100 cm. Any protocol
(inc. Bluetooth) will be ok for the POC/prototype but there is a
strong commercial application for this tech, so ultimately a lo-power
protocol will be preferable.
If you've hacked something similar previously, or know anyone else
that has, I'd be grateful to hear from you and give you all the
details. Please contact me at either: a...@ioWorld.com or on
07930667390
> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
> project requiring a hardware hack that's outside the scope of my
> capabilities; I want to create a wireless fob that a smart phone
> (Android, to start with) can detect at approx. 100 cm. Any protocol
> (inc. Bluetooth) will be ok for the POC/prototype but there is a
> strong commercial application for this tech, so ultimately a lo-power
> protocol will be preferable.
> If you've hacked something similar previously, or know anyone else
> that has, I'd be grateful to hear from you and give you all the
> details. Please contact me at either: a...@ioWorld.com or on
> 07930667390
> On 4 May 2012 13:09, ioAlan <alanhod...@gmail.com> wrote:
>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>> project requiring a hardware hack that's outside the scope of my
>> capabilities; I want to create a wireless fob that a smart phone
>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>> strong commercial application for this tech, so ultimately a lo-power
>> protocol will be preferable.
>> If you've hacked something similar previously, or know anyone else
>> that has, I'd be grateful to hear from you and give you all the
>> details. Please contact me at either: a...@ioWorld.com or on
>> 07930667390
>> On 4 May 2012 13:09, ioAlan <alanhod...@gmail.com> wrote:
>>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>>> project requiring a hardware hack that's outside the scope of my
>>> capabilities; I want to create a wireless fob that a smart phone
>>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>>> strong commercial application for this tech, so ultimately a lo-power
>>> protocol will be preferable.
>>> If you've hacked something similar previously, or know anyone else
>>> that has, I'd be grateful to hear from you and give you all the
>>> details. Please contact me at either: a...@ioWorld.com or on
>>> 07930667390
Hi - firstly apologies for taking so long to reply (my gmail account is not
my primary) - In answer to your question: yes, any type of smartphone but
for the sake of the prototype, I expect Android will probably be easiest.
As for protocols: will NFC work at 100cm if the fob is active?
>>> On 4 May 2012 13:09, ioAlan <alanhod...@gmail.com> wrote:
>>>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>>>> project requiring a hardware hack that's outside the scope of my
>>>> capabilities; I want to create a wireless fob that a smart phone
>>>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>>>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>>>> strong commercial application for this tech, so ultimately a lo-power
>>>> protocol will be preferable.
>>>> If you've hacked something similar previously, or know anyone else
>>>> that has, I'd be grateful to hear from you and give you all the
>>>> details. Please contact me at either: a...@ioWorld.com or on
>>>> 07930667390
> On 4 May 2012 13:09, ioAlan <alanhod...@gmail.com> wrote:
>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>> project requiring a hardware hack that's outside the scope of my
>> capabilities; I want to create a wireless fob that a smart phone
>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>> strong commercial application for this tech, so ultimately a lo-power
>> protocol will be preferable.
>> If you've hacked something similar previously, or know anyone else
>> that has, I'd be grateful to hear from you and give you all the
>> details. Please contact me at either: a...@ioWorld.com or on
>> 07930667390
On Fri, May 4, 2012 at 2:01 PM, Mark Steward <markstew...@gmail.com> wrote:
> NFC won't travel 100cm without magic. Unfortunately, Bluetooth is
> probably the best bet, but security's something to consider.
> Mark
> On Fri, May 4, 2012 at 2:00 PM, Toby Catlin <t...@korfball.com> wrote:
>> On 4 May 2012 13:09, ioAlan <alanhod...@gmail.com> wrote:
>>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>>> project requiring a hardware hack that's outside the scope of my
>>> capabilities; I want to create a wireless fob that a smart phone
>>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>>> strong commercial application for this tech, so ultimately a lo-power
>>> protocol will be preferable.
>>> If you've hacked something similar previously, or know anyone else
>>> that has, I'd be grateful to hear from you and give you all the
>>> details. Please contact me at either: a...@ioWorld.com or on
>>> 07930667390
> On Fri, May 4, 2012 at 2:01 PM, Mark Steward <markstew...@gmail.com> wrote:
> NFC won't travel 100cm without magic. Unfortunately, Bluetooth is probably the best bet, but security's
> something to consider.
> On 4 May 2012 13:09, ioAlan <alanhod...@gmail.com> wrote:
> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
> project requiring a hardware hack that's outside the scope of my
> capabilities; I want to create a wireless fob that a smart phone
> (Android, to start with) can detect at approx. 100 cm. Any protocol
> (inc. Bluetooth) will be ok for the POC/prototype but there is a
> strong commercial application for this tech, so ultimately a lo-power
> protocol will be preferable.
> If you've hacked something similar previously, or know anyone else
> that has, I'd be grateful to hear from you and give you all the
> details. Please contact me at either: a...@ioWorld.com or on
> 07930667390
Nope. The first two are based on full size SD cards, I rather doubt that any phone
using those is still made.
The third link refers to needing to get a new SIM to make use of the NFC hardware
that the phone already has.
If a phone doesn't have NFC built in then there is no convenient way to add it. For
the latest Android devices it should be possible to use USB to connect to an NFC
interface, but that is rather messy.
> will NFC work at 100cm if the fob is active?
Possibly, it depends on the allowable size of the tag as the antenna size has a big
effect on range. An RFID reader that I was working on a few years ago had about
5cm range with a keyfob and 10cm with a credit card sized tag. Using a 25cm
coil in the reader extended that to 30cm, active tags (which I didn't have available) should do considerably better.
>> will NFC work at 100cm if the fob is active?
> Almost certainly not.
> Can the phone be activly scanning for the tag in some way? If so you could
> use qrcodes and have the phones user point the phone at them.
That sounds very battery-intensive, so wouldn't seem to suit the OP's low-power requirement -- also not hugely effective at 100cm distance without large codes :-)
I expect then battery life in the fob could be an issue if boosted to circa
100cm range? In any case, if you're interested in this, would you like to
see the full spec and the intended application for the use of this tech?
> Nope. The first two are based on full size SD cards, I rather doubt that
> any phone
> using those is still made.
> The third link refers to needing to get a new SIM to make use of the NFC
> hardware
> that the phone already has.
> If a phone doesn't have NFC built in then there is no convenient way to
> add it. For
> the latest Android devices it should be possible to use USB to connect to
> an NFC
> interface, but that is rather messy.
> will NFC work at 100cm if the fob is active?
> Possibly, it depends on the allowable size of the tag as the antenna size
> has a big
> effect on range. An RFID reader that I was working on a few years ago had
> about
> 5cm range with a keyfob and 10cm with a credit card sized tag. Using a 25cm
> coil in the reader extended that to 30cm, active tags (which I didn't have
> available) should do considerably better.
On Wed, May 9, 2012 at 2:38 PM, alan hodes <alanhod...@gmail.com> wrote:
> I expect then battery life in the fob could be an issue if boosted to
> circa 100cm range? In any case, if you're interested in this, would you
> like to see the full spec and the intended application for the use of this
> tech?
> On Wed, May 9, 2012 at 2:30 PM, Nigel Worsley <nig...@googlemail.com>wrote:
>> Nope. The first two are based on full size SD cards, I rather doubt that
>> any phone
>> using those is still made.
>> The third link refers to needing to get a new SIM to make use of the NFC
>> hardware
>> that the phone already has.
>> If a phone doesn't have NFC built in then there is no convenient way to
>> add it. For
>> the latest Android devices it should be possible to use USB to connect to
>> an NFC
>> interface, but that is rather messy.
>> will NFC work at 100cm if the fob is active?
>> Possibly, it depends on the allowable size of the tag as the antenna size
>> has a big
>> effect on range. An RFID reader that I was working on a few years ago had
>> about
>> 5cm range with a keyfob and 10cm with a credit card sized tag. Using a
>> 25cm
>> coil in the reader extended that to 30cm, active tags (which I didn't
>> have available) should do considerably better.
Does Zigbee need to be installed on a smartphone or is it something that
they already have? I've looked up what Zigbee is, and from the description
(low power, short range, secure etc) it would seem ideal for the intended
application.
On Wed, May 9, 2012 at 2:52 PM, Mark Steward <markstew...@gmail.com> wrote:
> Use Bluetooth. It comes built into phones, and is designed for low power.
> If you really want to make something, use Zigbee.
> Mark
> On Wed, May 9, 2012 at 2:38 PM, alan hodes <alanhod...@gmail.com> wrote:
>> I expect then battery life in the fob could be an issue if boosted to
>> circa 100cm range? In any case, if you're interested in this, would you
>> like to see the full spec and the intended application for the use of this
>> tech?
>> On Wed, May 9, 2012 at 2:30 PM, Nigel Worsley <nig...@googlemail.com>wrote:
>>> Nope. The first two are based on full size SD cards, I rather doubt that
>>> any phone
>>> using those is still made.
>>> The third link refers to needing to get a new SIM to make use of the NFC
>>> hardware
>>> that the phone already has.
>>> If a phone doesn't have NFC built in then there is no convenient way to
>>> add it. For
>>> the latest Android devices it should be possible to use USB to connect
>>> to an NFC
>>> interface, but that is rather messy.
>>> will NFC work at 100cm if the fob is active?
>>> Possibly, it depends on the allowable size of the tag as the antenna
>>> size has a big
>>> effect on range. An RFID reader that I was working on a few years ago
>>> had about
>>> 5cm range with a keyfob and 10cm with a credit card sized tag. Using a
>>> 25cm
>>> coil in the reader extended that to 30cm, active tags (which I didn't
>>> have available) should do considerably better.
Zigbee's a lower power and simpler protocol to Bluetooth - it's more suited
to simple sensors than, say phones or computers. Phones don't speak it,
though, so you'll need to build a device that can interface directly with
the phone (SD card/audio socket/Bluetooth) as well as a corresponding
Zigbee board on the fob.
On Wed, May 9, 2012 at 3:08 PM, alan hodes <alanhod...@gmail.com> wrote:
> Does Zigbee need to be installed on a smartphone or is it something that
> they already have? I've looked up what Zigbee is, and from the
> description (low power, short range, secure etc) it would seem ideal for
> the intended application.
> On Wed, May 9, 2012 at 2:52 PM, Mark Steward <markstew...@gmail.com>wrote:
>> Use Bluetooth. It comes built into phones, and is designed for low
>> power. If you really want to make something, use Zigbee.
>> Mark
>> On Wed, May 9, 2012 at 2:38 PM, alan hodes <alanhod...@gmail.com> wrote:
>>> I expect then battery life in the fob could be an issue if boosted to
>>> circa 100cm range? In any case, if you're interested in this, would you
>>> like to see the full spec and the intended application for the use of this
>>> tech?
>>> On Wed, May 9, 2012 at 2:30 PM, Nigel Worsley <nig...@googlemail.com>wrote:
>>>> Nope. The first two are based on full size SD cards, I rather doubt
>>>> that any phone
>>>> using those is still made.
>>>> The third link refers to needing to get a new SIM to make use of the
>>>> NFC hardware
>>>> that the phone already has.
>>>> If a phone doesn't have NFC built in then there is no convenient way to
>>>> add it. For
>>>> the latest Android devices it should be possible to use USB to connect
>>>> to an NFC
>>>> interface, but that is rather messy.
>>>> will NFC work at 100cm if the fob is active?
>>>> Possibly, it depends on the allowable size of the tag as the antenna
>>>> size has a big
>>>> effect on range. An RFID reader that I was working on a few years ago
>>>> had about
>>>> 5cm range with a keyfob and 10cm with a credit card sized tag. Using a
>>>> 25cm
>>>> coil in the reader extended that to 30cm, active tags (which I didn't
>>>> have available) should do considerably better.
On Friday, May 4, 2012 1:09:47 PM UTC+1, ioAlan wrote:
> Hi lhs hackers. I run a software dev Co. called ioWorld and have a > project requiring a hardware hack that's outside the scope of my > capabilities; I want to create a wireless fob that a smart phone > (Android, to start with) can detect at approx. 100 cm. Any protocol > (inc. Bluetooth) will be ok for the POC/prototype but there is a > strong commercial application for this tech, so ultimately a lo-power > protocol will be preferable.
> If you've hacked something similar previously, or know anyone else > that has, I'd be grateful to hear from you and give you all the > details. Please contact me at either: a...@ioWorld.com or on > 07930667390
oooh I wanted to make some small magnetic bluetooth temperature sensors but
was put off by the price of arduino bluetooth shields. Do you have a link
for anymore?
thanks
toby
On 9 May 2012 15:26, tom <bollocks...@gmail.com> wrote:
> I'd go with bluetooth, we found some cheap 3.3v modules on ebay a while
> back, i think they were about 6 quid each in total
> On Friday, May 4, 2012 1:09:47 PM UTC+1, ioAlan wrote:
>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>> project requiring a hardware hack that's outside the scope of my
>> capabilities; I want to create a wireless fob that a smart phone
>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>> strong commercial application for this tech, so ultimately a lo-power
>> protocol will be preferable.
>> If you've hacked something similar previously, or know anyone else
>> that has, I'd be grateful to hear from you and give you all the
>> details. Please contact me at either: a...@ioWorld.com or on
>> 07930667390
On Wed, May 9, 2012 at 3:28 PM, Toby Catlin <t...@korfball.com> wrote:
> oooh I wanted to make some small magnetic bluetooth temperature sensors
> but was put off by the price of arduino bluetooth shields. Do you have a
> link for anymore?
> thanks
> toby
> On 9 May 2012 15:26, tom <bollocks...@gmail.com> wrote:
>> I'd go with bluetooth, we found some cheap 3.3v modules on ebay a while
>> back, i think they were about 6 quid each in total
>> On Friday, May 4, 2012 1:09:47 PM UTC+1, ioAlan wrote:
>>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>>> project requiring a hardware hack that's outside the scope of my
>>> capabilities; I want to create a wireless fob that a smart phone
>>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>>> strong commercial application for this tech, so ultimately a lo-power
>>> protocol will be preferable.
>>> If you've hacked something similar previously, or know anyone else
>>> that has, I'd be grateful to hear from you and give you all the
>>> details. Please contact me at either: a...@ioWorld.com or on
>>> 07930667390
> On Wed, May 9, 2012 at 3:28 PM, Toby Catlin <t...@korfball.com> wrote:
>> oooh I wanted to make some small magnetic bluetooth temperature sensors
>> but was put off by the price of arduino bluetooth shields. Do you have a
>> link for anymore?
>> thanks
>> toby
>> On 9 May 2012 15:26, tom <bollocks...@gmail.com> wrote:
>>> I'd go with bluetooth, we found some cheap 3.3v modules on ebay a while
>>> back, i think they were about 6 quid each in total
>>> On Friday, May 4, 2012 1:09:47 PM UTC+1, ioAlan wrote:
>>>> Hi lhs hackers. I run a software dev Co. called ioWorld and have a
>>>> project requiring a hardware hack that's outside the scope of my
>>>> capabilities; I want to create a wireless fob that a smart phone
>>>> (Android, to start with) can detect at approx. 100 cm. Any protocol
>>>> (inc. Bluetooth) will be ok for the POC/prototype but there is a
>>>> strong commercial application for this tech, so ultimately a lo-power
>>>> protocol will be preferable.
>>>> If you've hacked something similar previously, or know anyone else
>>>> that has, I'd be grateful to hear from you and give you all the
>>>> details. Please contact me at either: a...@ioWorld.com or on
>>>> 07930667390
I've used those. They work well, and can be used without a breakout (you just need rx, tx, vss and vdd -- the hardest part was making a voltage divider for tx on an otherwise 5v circuit).
> On Wed, May 9, 2012 at 3:28 PM, Toby Catlin <t...@korfball.com> wrote:
> oooh I wanted to make some small magnetic bluetooth temperature sensors but was put off by the price of arduino bluetooth shields. Do you have a link for anymore?
> thanks
> toby
> On 9 May 2012 15:26, tom <bollocks...@gmail.com> wrote:
> I'd go with bluetooth, we found some cheap 3.3v modules on ebay a while back, i think they were about 6 quid each in total
> On Friday, May 4, 2012 1:09:47 PM UTC+1, ioAlan wrote:
> Hi lhs hackers. I run a software dev Co. called ioWorld and have a > project requiring a hardware hack that's outside the scope of my > capabilities; I want to create a wireless fob that a smart phone > (Android, to start with) can detect at approx. 100 cm. Any protocol > (inc. Bluetooth) will be ok for the POC/prototype but there is a > strong commercial application for this tech, so ultimately a lo-power > protocol will be preferable.
> If you've hacked something similar previously, or know anyone else > that has, I'd be grateful to hear from you and give you all the > details. Please contact me at either: a...@ioWorld.com or on > 07930667390
On Wed, May 9, 2012 at 3:21 PM, Mark Steward <markstew...@gmail.com> wrote:
> Zigbee's a lower power and simpler protocol to Bluetooth - it's more
> suited to simple sensors than, say phones or computers. Phones don't speak
> it, though, so you'll need to build a device that can interface directly
> with the phone (SD card/audio socket/Bluetooth) as well as a corresponding
> Zigbee board on the fob.
> Mark
> On Wed, May 9, 2012 at 3:08 PM, alan hodes <alanhod...@gmail.com> wrote:
>> Does Zigbee need to be installed on a smartphone or is it something
>> that they already have? I've looked up what Zigbee is, and from the
>> description (low power, short range, secure etc) it would seem ideal for
>> the intended application.
>> On Wed, May 9, 2012 at 2:52 PM, Mark Steward <markstew...@gmail.com>wrote:
>>> Use Bluetooth. It comes built into phones, and is designed for low
>>> power. If you really want to make something, use Zigbee.
>>> Mark
>>> On Wed, May 9, 2012 at 2:38 PM, alan hodes <alanhod...@gmail.com> wrote:
>>>> I expect then battery life in the fob could be an issue if boosted to
>>>> circa 100cm range? In any case, if you're interested in this, would you
>>>> like to see the full spec and the intended application for the use of this
>>>> tech?
>>>> On Wed, May 9, 2012 at 2:30 PM, Nigel Worsley <nig...@googlemail.com>wrote:
>>>>> Nope. The first two are based on full size SD cards, I rather doubt
>>>>> that any phone
>>>>> using those is still made.
>>>>> The third link refers to needing to get a new SIM to make use of the
>>>>> NFC hardware
>>>>> that the phone already has.
>>>>> If a phone doesn't have NFC built in then there is no convenient way
>>>>> to add it. For
>>>>> the latest Android devices it should be possible to use USB to connect
>>>>> to an NFC
>>>>> interface, but that is rather messy.
>>>>> will NFC work at 100cm if the fob is active?
>>>>> Possibly, it depends on the allowable size of the tag as the antenna
>>>>> size has a big
>>>>> effect on range. An RFID reader that I was working on a few years ago
>>>>> had about
>>>>> 5cm range with a keyfob and 10cm with a credit card sized tag. Using a
>>>>> 25cm
>>>>> coil in the reader extended that to 30cm, active tags (which I didn't
>>>>> have available) should do considerably better.
> I expect then battery life in the fob could be an issue if boosted to circa 100cm range?
Possibly, but not necessarily. All the tag needs to do most of the time is listen for a carrier,
the main receiver can then be powered up to see if there is any real data being sent. If so,
send the appropriate reply and wait for the reader to disappear before going back to sleep.
> In any case, if you're interested in this, would you like to see the full spec and the
> intended application for the use of this tech?
Yes please, that would make it much easier to suggest the right solution.
>> Zigbee's a lower power and simpler protocol to Bluetooth
> As good as the protocol seems, it sounds too complex - at least for the sake of building a prototype.
It is, simpler than Bluetooth isn't the same thing as simple! The underlying radio format is IEEE 802.15.4
which does almost all of what you will need for just getting the ID of a tag. Range is a bit naff, but
that is probably an advantage for your application.
On Wed, May 9, 2012 at 4:25 PM, Nigel Worsley <nig...@googlemail.com> wrote:
> I expect then battery life in the fob could be an issue if boosted to
>> circa 100cm range?
> Possibly, but not necessarily. All the tag needs to do most of the time is
> listen for a carrier,
> the main receiver can then be powered up to see if there is any real data
> being sent. If so,
> send the appropriate reply and wait for the reader to disappear before
> going back to sleep.
> In any case, if you're interested in this, would you like to see the full
>> spec and the
>> intended application for the use of this tech?
> Yes please, that would make it much easier to suggest the right solution.
I have two suggestions regarding this that will permit close range
proximity measurements, the first is to use an audible tone, and the second
is a simple, RF transmitter/receiver pair operating on one of the lower ISM
bands such as the one around 433MHz. In the first case a [reasonably] high
pitched sound could be emitted by the fob (high enough as to be quite
unnoticable), just loud enough to be detected with a smartphone's
microphone within 1m. This would only really work in the open air though.
The second option would require some specialist hardware - the simplest,
most compatible way would be to downmix the RF carrier to an audio
frequency voltage, which is then sampled by the microphone jack. Once
again the power of the RF transmitter could be titrated so that it only
works over a few metres, and because there is an inverse square law for EM
signals the distance could probably be quite accurately judged. The
advantages of using 433MHz are that the power requirements are much lower
than 2.4GHz (lower frequency EM waves carry/require less energy), and they
generally penetrate walls etc far better. You could even have the
transmitting dongle send a series of bytes to identify itself every so
often (for example every 10 seconds), and track multiple devices with the
same receiver.
On Wed, May 9, 2012 at 5:35 PM, alan hodes <alanhod...@gmail.com> wrote:
> I'll contact you off-group
> On Wed, May 9, 2012 at 4:25 PM, Nigel Worsley <nig...@googlemail.com>wrote:
>> I expect then battery life in the fob could be an issue if boosted to
>>> circa 100cm range?
>> Possibly, but not necessarily. All the tag needs to do most of the time
>> is listen for a carrier,
>> the main receiver can then be powered up to see if there is any real data
>> being sent. If so,
>> send the appropriate reply and wait for the reader to disappear before
>> going back to sleep.
>> In any case, if you're interested in this, would you like to see the
>>> full spec and the
>>> intended application for the use of this tech?
>> Yes please, that would make it much easier to suggest the right solution.
> I have two suggestions regarding this that will permit close range proximity
> measurements, the first is to use an audible tone, and the second is a
> simple, RF transmitter/receiver pair operating on one of the lower ISM bands
> such as the one around 433MHz. In the first case a [reasonably] high
> pitched sound could be emitted by the fob (high enough as to be quite
> unnoticable), just loud enough to be detected with a smartphone's microphone
> within 1m. This would only really work in the open air though. The second
> option would require some specialist hardware - the simplest, most
> compatible way would be to downmix the RF carrier to an audio frequency
> voltage, which is then sampled by the microphone jack. Once again the power
> of the RF transmitter could be titrated so that it only works over a few
> metres, and because there is an inverse square law for EM signals the
> distance could probably be quite accurately judged. The advantages of using
> 433MHz are that the power requirements are much lower than 2.4GHz (lower
> frequency EM waves carry/require less energy), and they generally penetrate
> walls etc far better. You could even have the transmitting dongle send a
> series of bytes to identify itself every so often (for example every 10
> seconds), and track multiple devices with the same receiver.
The problem you'll have with this (as I've found with some commercial
medium-range RFID tags) is that attenuation of RF is predictable in
free space but affected hugely by solid bodies. So it's difficult to
distinguish between a tag that's out of range and one that's in range
but on the wrong side of the wearer's body.
Depending on your application, you might be able to get around this by
noting the occasional glimpses of a nearby (but shielded) tag, but
this makes low power difficult as you need to scan often.
